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Reviewed on May 08, 2019 , Jake Thauvette, Marc Henney, Sam Vafaei, Yannick Khong

Sennheiser HD 660 S
HEADPHONES REVIEW

Usage Ratings - Version 1.2

Test Benches:

  • 1.2: Winter 2018
  • 1.1: Summer 2017
  • 1.0: Winter 2017
  • 0.9: Winter 2016
5.7
Mixed Usage
What it is: This is the combination of the different use cases to evaluate how versatile the headphones are. Therefore an everyday headphone should be well-rounded enough to adapt to most situations and environments without significant losses in sound quality, design ergonomics or isolation.
Score components:
7.6
Critical Listening
What it is: The level of audio fidelity a headphone can reproduce. Therefore a balanced and true representation of bass, mids, treble, soundstage and imaging, as well as a comfortable listening experience, is essential for critical listening.
Score components:
4.5
Commute/Travel
What it is: How well the headphones handle the loud environments involved in commuting or traveling. Therefore your listening experience should be comfortable, hassle-free and as isolated from noise as possible.
5.6
Sports/Fitness
What it is: How well-adapted the headphones are, to use while doing sports or strenuous exercise. Therefore the headphones should not be too cumbersome and deliver a stable and comfortable listening experience.
5.0
Office
What it is: How well the headphones can deliver a comfortable and isolated listening experience in an office-like environment. They should not leak much and should block the noise of a busy office.
5.7
Gaming
Type : Over-ear
Enclosure : Open-Back
Wireless : No
Noise-Cancelling : No
Mic : No
Transducer : Dynamic

The Sennheiser HD 660 S are good open-back critical listening headphones. They have a good audio reproduction and are comfortable enough for long listening sessions. However, they are quite a bit tighter than other Sennheiser headphones we've tested, like the Sennheiser HD 650. They won’t be great for any other use as their open design doesn’t block any noise and leaks a lot, so they're best used in a quiet listening room where you can really benefit from their sound quality.

Test Results
Design 6.4
Sound 7.7
Isolation 2.4
Microphone 0
Active Features 0
Connectivity 4.8
Pros
  • Good audio reproduction.
  • Comfortable fit.
  • Well-built design.
Cons
  • High leakage and low isolation, by design.
  • Can be tight on some heads.

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6.4

Design

Score components:

The Sennheiser HD 660S are decently designed open-back headphones. Their cups are large and comfortable, but they are quite a bit tighter than the similar Sennheiser HD 650 and Sennheiser HD 600, which some may not like during long listening sessions. These headphones are for critical listening, offer no controls, and aren’t foldable for you to carry them around. On the upside, they are well-built, but the headband frame is a bit thin and seems fragile. They have a 1/4” connector by default but come with a 1/8” adapter as well so you can use them with multiple sources, like your phone.

Style

The HD 660S are good looking headphones and have a similar design to that of the HD 650 and HD 600, but with a black matte finish. The large oval cups are padded with microfiber-like fabric, which gives them a premium look. The cups are open and the grill design shows off the drivers. They aren’t very eye-catching like some other similar models, but they look like sleek and high-end headphones.

7.5 Comfort
What it is: Adjustability and degrees of freedom, pressure, stiffness and weight.
When it matters: When the headphones are going to be used for long durations.
Score components: Subjectively assigned
Weight : 0.6 lbs
Clamping Force
What it is: The force that the headphones exert on your head, once you have them on. This is purely a measurement of the force applied, which does not take into account the earpad's surface area and the resulting pressure you will feel, on or around your ears.
When it matters: The tighter the headphones, the more force they put on your head. This can get uncomfortable or cause pain and soreness during long listening sessions.
:
1.2 lbs

The HD 660S are comfortable headphones, but they feel tight on the head due to their high clamping force. They are tighter than the similar HD 650, but on the upside, the microfiber-like fabric used on the cup padding is soft and feels nice on the skin. The cups are very large and will be suitable for most ear sizes and shapes without a problem. The headband design is the same that of the HD 650, which is more comfortable than the HD 600 and distributes the headphones’ weight effectively.

0 Controls
What it is: The control scheme of the headphones, the number of functions provided, button layout and ergonomics as well as the quality of tactile feedback.
When it matters: If you want to control volume, pause your music or make phone calls without directly interacting with your audio device.
Ease of use : N/A
Feedback : N/A
Call/Music Control : No
Volume Control : No
Microphone Control : N/A
Channel Mixing
What it is: Being able to mix audio channels directly on the headphones.
When it matters: This is most useful when using a separate chat software so that you can mix in-game audio and chat audio depending on your needs.
:
N/A
Noise Canceling Control : N/A
Talk-Through
What it is: A transparency feature that uses the mics of the headphones to let you hear what is doing on around you without removing them.
When it matters: If you want to be aware of what is going on around you without removing your headphones or while still listening to your audio. This is typically a feature for the noise canceling headphones and passively isolating in-ears that block a lot of noise.
Good value: Yes or adjustable.
:
N/A
Additional Buttons : N/A

These headphones do not have any controls.

6.2 Breathability
What it is: How hot the headphones get when you wear them for an extended period of time.
When it matters: If you often have long listening sessions or use your headphones while doing physical activities like running or working out.
Score components:
Avg.Temp.Difference : 6.4 C

The open-back design of the HD 660 S make them decently breathable. They still create a good seal around your ears that will trap some heat under the ear cups, but the grill design will help with airflow. Overall, these still won't be a good option for sports.

5.4 Portability
What it is: The volume of space occupied by the headphones when folded into their most compact format.
When it matters: If you're often on the move and need to carry your headphones in a bag, purse , or pocket.
Score components:
L : 7.7 "
W : 7.1 "
H : 3.6 "
Volume : 197 Cu. Inches
Transmitter required : N/A

These headphones are fairly large and their footprint is quite big. You can’t fold them into a more compact format and the cups don’t lay flat either, which would have been easier to slide them in a bag. They also don't offer a dedicated case or pouch to carry the headphones in, which is slightly disappointing, but you shouldn’t really be on the go too often with open-back headphones.

0 Case
What it is: The provided carrying options to protect your headphones when transporting them.
When it matters: To prevent damaging your headphones, if you often carry them in your bag or pocket.
Score components: Subjectively assigned
Type : No case
L : N/A
W : N/A
H : N/A
Volume : N/A

They don’t come with a case or pouch to carry and protect the headphones.

7.5 Build Quality
What it is: Durability, material quality, cheap/expensive feel.
When it matters: When the headphones are going to be used by multiple users (classes/studios), by children, in tough conditions, on a daily basis, or for exercise.
Score components: Subjectively assigned

The HD 660S are well-built headphones, but their build has a few minor flaws. The headband frame is a thin metal sheet that feels somewhat durable, but is not that flexible. The joints where the headband meets the yolks feels fairly fragile and seems to be the weak point of the build. On the upside, the plastic used feels decently solid and they also have detachable cables, which are easily replaceable, making them more durable.

7.5 Stability
What it is: How the headphones are designed to prevent them from slipping off your ears or falling off your head.
When it matters: If you plan on using the headphones while doing sports or other physical activities that requires a lot of movement.
Score components: Subjectively assigned

These headphones are quite stable on the head because of their tight fit and their large cups. They don’t sway around much, but still aren’t designed to be used for sports. Also, even if their cable is detachable, it needs quite a bit of force to disconnect, meaning the headphones could be yanked off your head if it was hooked on something.

Cable
Detachable : Yes
Length : 9.8 ft
Connection : 1/4" TRS

These headphones come with a 9.8ft 1/4” TRS detachable cable, a 0.17 inch (4.4 mm) balanced cable. They also come with a 1/4” to 1/8” adapter.

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7.7

Sound

What it is: How accurately the audio is reproduced. The tests are performed with the headphones' most commonly used features enabled (noise-cancelling, wireless, etc.)

The Sennheiser HD 660 S are good sounding open-back over-ear headphones and have a similar sound profile to that of the HD 650. Their bass is very good, consistent, and punchy, the mid-range is well-balanced and clear, and their treble is very good. However, their bass lacks a bit of thump and is a bit muddy; they also sound a bit mid-rangy and forward. Also, their treble lacks a bit of detail and might sound slightly veiled. Overall, they are great versatile headphones for a wide variety of music genres, but their sub-bass might be lacking a bit for bass-heavy music.

8.0 Bass
What it is: Frequency Response from 20Hz-250Hz
When it matters: When the material is heavy on bass frequencies, such as those of kick drums and bass guitar.
Score components:
Std. Err.
What it is: The amount of deviation (weighted standard error) in bass frequency response (20Hz-250Hz) as compared to a target response that would sound perfectly balanced to most people.
When it matters: When a balanced and neutral (reference) bass performance is desired.
Good value: <4dB
Noticeable difference: 0.1dB
:
2.66 dB
Low-Frequency Extension
What it is: The lowest frequency at which the frequency response reaches -3dB of the target response.
When it matters: Shows how extended the bass is.
Good value: <40Hz
Noticeable difference: 5Hz
:
41.77 Hz
Low-Bass
What it is: The average amount of over/under-emphasis in frequency response from 20Hz-60Hz.
When it matters: Kick drums and low frequency effects get their 'thump' from this range. Mostly felt than heard.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
-4.44 dB
Mid-Bass
What it is: The average amount of over/under-emphasis in frequency response from 60Hz-120Hz.
When it matters: Melodic bass instruments have most of their fundamental frequencies in this range. This is where the 'body' and 'punch' of the bass sits.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
-0.16 dB
High-Bass
What it is: The average amount of over/under-emphasis in frequency response from 120Hz-250Hz.
When it matters: Most instruments get their warmth and full-ness from this range. When over-emphasized, mixes tend to get muddy and boomy.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
2.86 dB

The bass of the Sennheiser HD 660S is great. LFE (low-frequency extension) is at 42Hz, which is good. Low-bass, responsible for the thump and rumble common to bass-heavy genres, is lacking by more than 4dB. This will be noticeable but subtle, and results in a low amount of thump and rumble. Mid-bass, responsible for the body of bass guitars and punch of kick drums, follows well our neutral target. High-bass, however, is overemphasized by almost 3dB, adding a bit of muddiness to the overall sound.

7.8 Mid
What it is: Frequency Response from 250Hz-2KHz
When it matters: When the material is heavy on mid-range frequencies. This is the case for the majority of audio content.
Score components:
Std. Err.
What it is: The amount of deviation (weighted standard error) in mid frequency response (250Hz-2.5KHz) as compared to a target response that would sound perfectly balanced to most people.
When it matters: When a balanced and neutral (reference) mid performance is desired.
Good value: <4dB
Noticeable difference: 0.1dB
:
2.99 dB
Low-Mid
What it is: The average amount of over/under-emphasis in frequency response from 250Hz-500Hz.
When it matters: Most instruments have their fundamentals or low harmonics in this range. Over-emphasis in this range sounds muddy and cluttered. Under-emphasis, thins out the vocals and lead instruments.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
3.22 dB
Mid-Mid
What it is: The average amount of over/under-emphasis in frequency response from 500Hz-1KHz.
When it matters: This range is occupied mostly by upper harmonics. Over-emphasis sounds forward and boxy. Under-emphasis pushes instruments to the back of the mix.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
2.69 dB
High-Mid
What it is: The average amount of over/under-emphasis in frequency response from 1KHz-2KHz.
When it matters: Most instruments, especially vocals, get their intensity and clarity from this range. Over-emphasis sounds honky and harsh, under-emphasis sounds weak and distant.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
2.99 dB

The mid-range is great. The response throughout the range is very even and flat, but consistently over out neutral target by about 3dB. This results in a clear and well-balanced reproduction of vocals and lead instruments, but with a bit of excess emphasis on the mid-range that can make them sound slightly forward.

8.6 Treble
What it is: Frequency Response from 2KHz-20KHz
When it matters: When the material is heavy on high-range frequencies, such as voice, cymbals, and any other material where brightness, brilliance and airiness is desired.
Score components:
Std. Err.
What it is: The amount of deviation (weighted standard error) in treble frequency response (2.5KHz-20KHz) as compared to a target response that would sound perfectly balanced to most people.
When it matters: When a balanced and neutral (reference) treble performance is desired.
Good value: <4dB
Noticeable difference: 0.1dB
:
2.74 dB
Low-Treble
What it is: The average amount of over/under-emphasis in frequency response from 2KHz-5KHz.
When it matters: Almost all instruments rely on this range for their presence, detail, and articulation. Over-emphasis can sound harsh and painful. Under-emphasis hurts the comprehensibility of vocals and lead instruments.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
0.38 dB
Mid-Treble
What it is: The average amount of over/under emphasis in frequency response from 5KHz-10KHz.
When it matters: This is the sibilance range. Cymbals, vocals, and lead instruments rely on this range for brightness and presence. Over-emphasis sounds piercing and painful, under-emphasis sounds dull and lispy.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
-1.94 dB
High-Treble
What it is: The average amount of over/under-emphasis in frequency response from 10KHz-20KHz.
When it matters: This range gives brilliance and airiness to the sound. Over-emphasis sounds hissy, under-emphasis sounds closed-up and lifeless.
Good value: +/-3dB
Noticeable difference: 0.1dB
:
-3.18 dB

The Sennheiser HD 660 S have a very good treble performance. Low-treble and mid-treble are fairly flat, but there’s a dip centered around 5kHz which results in a treble that lacks a bit of brightness and brilliance. This will be mostly noticeable on vocals, lead instruments, and cymbals.

Raw Frequency Response
What it is: The average uncompensated frequency response of the headphone. For in-ears and earbuds, this corresponds to the average of 5 measurements/re-seats on the dummy head (HMS). For over/on-ear headphones, this corresponds to the average of 5 measurements/re-seats on the HMS (Head Measurement System) for the mid and treble ranges, and 5 measurements/re-seats on 5 human subjects for the bass range.
When it matters: This is for those who want to see the raw and uncompensated frequency response of the headphone. Some of the more advanced users, are able to read and evaluate headphone frequency response in its raw form and without compensation. This will be especially useful to them if they have their own headphone compensation/target curve, which may differ from the compensation curve/target response used by RTINGS.com.
8.0 Frequency Response Consistency
What it is: The amount of deviation of each frequency response pass, from the average frequency response.
When it matters: Shows how consistently the headphones perform after re-positioning them.
Score components:
Avg. Std. Deviation
What it is: The average amount of deviation in frequency response of 5 re-seats, from the average frequency response.
When it matters: Shows how consistently the headphones perform after re-positioning them.
Good value: <0.5
Noticeable difference: 0.1
:
0.4 dB

The HD 660S have good frequency response consistency. Due to their open-back and ear pad design, these headphones have near perfect consistency in delivering their bass. The maximum amount of deviation throughout the bass range was less than 1dB. However, they are prone to inconsistencies in the treble range, and depending on the positioning and ear shape, there could be as much as 7dB of variation in the treble response, which will be noticeable.

9.2 Imaging
What it is: Imaging qualities are inherent to the audio content, the headphones have to 'reproduce' them rather than 'create' them. They determine how accurately the objects are positioned in the stereo image, and how transparent the imaging is.
When it matters: When accurate positioning of the objects in the stereo image, and clear and transparent imaging is desired.
Weighted Group Delay
What it is: The average amount of group delay calculated based on a perceptual weighting filter. Group delay indicates how long it takes for each frequency to reach their maximum amplitude. This is a monaural quality and can be perceived even with one ear.
When it matters: Headphones with lower group delay have more transparent imaging and a tighter bass. Headphones with higher group delay in the bass range tend to have a wimpy and loose bass, and headphones with higher group delay in the treble range tend to have a less transparent imaging.
Good value: <0.5
Noticeable difference: 0.1
:
0.2
Weighted Amplitude Mismatch
What it is: The Left/Right balance of our test unit, that is, the amount of amplitude difference between the left and right drivers. This is not a design test, but a marker for manufacturing tolerance and ergonomics.
When it matters: When a properly balanced stereo image and low manufacturing tolerance is desired. A poor score indicates a noticeable difference in level between the left and right drivers.
Good value: <1.5
Noticeable difference: 0.1
:
0.17
Weighted Frequency Mismatch
What it is: The amount of difference (Std. Err.) between the frequency response of the left and right drivers of our test unit. This is not a design test, but a marker for manufacturing tolerance and ergonomics.
When it matters: When an even and stable stereo image, as well as a low manufacturing tolerance, is desired. A poor score indicates there may be 'holes' in the stereo image at certain frequencies.
Good value: <2
Noticeable difference: 0.1
:
1.23
Weighted Phase Mismatch
What it is: The amount of difference (Std. Err.) between the phase response of the left and right drivers of our test unit. This is not a design test, but a marker for manufacturing tolerance and ergonomics.
When it matters: When an even and stable stereo image, as well as a low manufacturing tolerance is desired. A poor score indicates there may be inaccuracies in the stereo image reproduction at certain frequencies.
Good value: <16
Noticeable difference: 3
:
2.29

The imaging performance is excellent. Weighted group delay is at 0.2, which is very good. The GD graph also shows that the entire group delay response is below the audibility threshold and the spikes under 20Hz won’t be audible. This ensure a tight bass and a transparent treble reproduction. Additionally, the L/R drivers of our test unit were very well-matched. This is important for the accurate localization and placement of objects (voices, instruments, video game effects) in the stereo image. However, these results are only valid for our unit and yours may perform differently.

5.2 Soundstage
What it is: Soundstage qualities are not inherent to the audio content, the headphones have to 'create' them rather than 'reproduce' them. They determine whether the sound is perceived to be coming from inside or in front of the head, how open and spacious the soundstage is, how much the headphones acoustically interact with the environment, and how strong the phantom center is.
When it matters: When an accurately produced, large and spacious soundstage, similar to that of a stereo loudspeaker setup is desired.
PRTF Accuracy (Std. Dev.)
What it is: The standard deviation of the PRTF (Pinna-related transfer function) of the headphones compared to a reference loudspeaker's PRTF at 30°. This quality is monaural and can be perceived even with one ear. This test does not apply to in-ears and earbuds, due to the lack of pinna interaction.
When it matters: An accurate pinna activation is mainly responsible for how natural and speaker-like the soundstage is perceived to be. The less error in the shape of the PRTF, the more natrual and speaker-like the perception of the soundstage will be. High amounts of error may indicate a soundstage that is unnatural or odd.
Good value: <2.5
Noticeable difference: 0.1
:
2.89 dB
PRTF Size (Avg.)
What it is: The average amplitude of the PRTF (Pinna-related transfer function) of the headphones compared to that of a reference loudspeaker's PRTF at 30°. This quality is monaural and can be perceived even with one ear. This test does not apply to in-ears and earbuds, due to the lack of pinna interaction.
When it matters: This value is responsible for the perceived size of the soundstage. The higher the value, the larger the perceived size of the soundstage. However, values above the reference (5.0dB) could result in a soundstage that is perceived as unnatural or odd.
Good value: >3.7
Noticeable difference: 0.1
:
2.37 dB
PRTF Distance
What it is: The depth of the "10KHz notch" of the headphone's PRTF, which is caused by phase cancellations at the concha. This quality is monaural and can be perceived even with one ear. This test does not apply to in-ears and earbuds, due to the lack of pinna interaction.
When it matters: This value is mainly responsible for the perceived distance and elevation of the soundstage. A small distance value may result in a soundstage that is perceived to be located inside the head. Larger values may help pull the soundstage out from inside of the head and bring it to the front.
Good value: >13
Noticeable difference: 1
:
1.79 dB
Openness
What it is: How open the headphones are, and how open and spacious they sound. This quality is monaural and can be perceived even with one ear. This test differentiates between acoustically and electronically produced crosstalk and only takes the acoustically generated crosstalk into account. This value is the inverse of the Noise Isolation test score, and could be indirectly related the acoustic impedance of the headphones.
When it matters: When a headphone with a sense of an open, and spacious soundstage is desired. A value of 10 indicates a fully open headphone, and a value of 0 indicates a fully closed headphone.
Good value: >7.5
Noticeable difference: 0.5
:
8.7
Acoustic Space Excitation
What it is: How loud the headphones are, and how much they excite their environment acoustically. If the headphones are loud and open enough, the sound leaking from the headphones will be affected by the environment (reflections/reverb) before reflecting back into the open headphones and to the listener's ears. This quality is monaural and can be perceived even with one ear. This value is the inverse of the Leakage test score.
When it matters: Headphones with higher excitation values, similar to openness, tend to have soundstages that are perceived as more open and spacious.
Good value: >7.5
Noticeable difference: 0.5
:
8.6
Correlated Crosstalk
What it is: How strong and solid the phantom center is. This is mostly a stereo quality and its effects on mono content are minimal. This test is sensitive to the phase of the crosstalk and whether it is produced acoustically or electronically.
When it matters: When a true reproduction of the stereo image is desired. A value of 0 indicates no crosstalk, or that the existing crosstalk is not correlated enough to affect the phantom center. A negative score means the crosstalk is out of phase with the original signal, resulting in a slightly wider stereo image at the expense of creating a 'hole' at the center of the stereo field. A positive score means the crosstalk is in phase and positively affecting the phantom center.
Good value: >1dB
Noticeable difference: 0.3dB
:
0.0 dB

The Sennheiser HD 660 S have a sub-par soundstage. The PRTF graph shows that these headphones don't interact with the pinna that much, and therefore show very little activation. They don't show a dip around the 10kHz area either. This means that although these are open-back headphones and they may feel more open and spacious sounding that closed-back headphones, their soundstage won't be perceived to be large or located outside of the listener's head.

8.6 Total Harmonic Distortion
What it is: The subtle, unwanted frequencies (harmonics) produced alongside the intended frequencies.
When it matters: When clean and pure sound reproduction is desired, though its effect is not as noticable as frequency response.
Score components:
Weighted THD @ 90
What it is: The overall amount of harmonic distortion measured at 90dB SPL. To make the score more perceptually relevant, more weight is given to the higher frequencies.
When it matters: How pure the sound is at moderate listening levels.
Good value: <0.100
Noticeable difference: 0.1
:
0.33
Weighted THD @ 100
What it is: The overall amount of harmonic distortion measured at 100dB SPL. To make the score more perceptually relevant, more weight is given to the higher frequencies.
When it matters: How pure the sound is at loud listening levels.
Good value: <0.300
Noticeable difference: 0.1
:
0.521

The harmonic distortion performance of the HD 660S is very good. The THD in the bass range is within good limits and there isn’t a big jump under heavier loads, which is also good. The overall amount of harmonic distortion in the mid and treble ranges are also very low, regardless of the volume.

2.4

Isolation

Score components:

By design, the Sennheiser HD 660S don’t block ambient noise and leak a lot to achieve a more open sound. These headphones are not designed to be used while commuting or in public environments. A lot of noise will seep into your audio and you will disturb people around you even if you are listening at moderate volumes.

2.8 Noise Isolation
What it is: How much outside noise is blocked out by putting the headphones on.
When it matters: If the headphones are going to be used in a noisy environment (airplane, subway, etc.)
Score components:
Isolation Audio
What it is: The simulated noise isolation of the headphones, demonstrating how much outside noise is blocked out by putting the headphones on. This recording is created using an EQ and is not an actual recording. For headphones with ANC (active noise cancellation), the playback simulates the isolation with ANC enabled.
When it matters: When the headphones are going to be used in a noisy envinronment (airplane, subway, etc.)
:
Overall Attenuation
What it is: The overall amount of environmental noise reduction in dB.
When it matters: In loud envinronments like planes, trains, offices, etc.
Good value: <-20dB
Noticeable difference: 1dB
:
-3.45 dB
Bass
What it is: The overall amount of noise isolation in the bass range (20Hz-250Hz).
When it matters: When the outside noise is bass-heavy, like in airplanes.
Good value: <-15dB
Noticeable difference: 1dB
:
0.02 dB
Mid
What it is: The overall amount of noise isolation in the mid-range (250Hz-2.5KHz).
When it matters: When the environment's noise is mid-heavy, like in an office.
Good value: <-15dB
Noticeable difference: 1dB
:
0.86 dB
Treble
What it is: The overall amount of noise isolation in the treble range (2.5KHz-20KHz).
When it matters: When the environment's noise is treble-heavy. Although uncommon, areas with sharp sounds fall under this category.
Good value: <-30dB
Noticeable difference: 1dB
:
-10.94 dB
Self-Noise
What it is: The amount of noise created by the active electronics of the headphones (if applicable), measured from 300Hz-20KHz. Applies mostly to wireless and noise-cancelling headphones.
When it matters: If too loud, it could become distracting when listening to quiet material like podcasts and audiobooks.
Good value: <16dB
Noticeable difference: 1dB
:
0 dB

The Sennheiser HD 660 S have poor noise isolation due to their open-back design. In the bass range, where the rumble of airplane and bus engines sit, they achieve no isolation. In the mid-range, important for blocking out speech, they also don’t block any noise. In the treble range, occupied by sharp S and T sounds and A/C system noise, they isolate by about 11dB, which is inadequate.

1.7 Leakage
What it is: The amount of sound bleeding out of the headphones.
When it matters: When the listener doesn't want people around them (in office, recording studio, etc.) to hear what is being listened to.
Score components:
Leakage Audio
What it is: The simulated sound leakage heard 1 foot away from the user, while the user is listening to a 100dB SPL signal. This recording is created using an EQ and is not an actual recording.
When it matters: When you don't want people to hear what you are listening to.
:
Overall Leakage @ 1ft
What it is: The amount of sound leakage heard 1 foot away from the user, while the user is listening to a 100dB SPL signal.
When it matters: When you don't want people hear what you are listening to.
Good value: <35dB
Noticeable difference: 1dB
:
63.28 dB

The leakage is poor. The significant portion of their leakage is spread between 300Hz and 20kHz, which is a very broad range. This means the leakage will be fuller sounding compared to that of closed-back headphones and in-ears. The overall level of the leakage is quite loud too. With the music at 100dB SPL, the leakage averages at 63dB SPL and peaks at 78dB SPL, which is way louder than the noise floor of an average office.

0

Microphone

What it is: The microphone section shows the quality of speech capture and transmission by the mic, as well as how well the microphone under test handles noisy environments.
When it matters: For your speech to be transmitted to and understood properly by the listener, the microphone needs to have a good recording quality. If the environment the microphone is being used in is noisy, a microphone with a good noise handling performance would be needed as well.
Score components:
Integrated
What it is: The microphone integrated in the ear cup or ear bud of a wireless headphone.
When it matters: For calls, gaming and voice over IP software or for any other use of the microphone.
Good value: Yes
:
N/A
In-line
What it is: The microphone inside the in-line remote of audio cables for wired and wireless headsets.
When it matters: In-line microphone are usually better than integrated mics. If you need better recording quality and noise handling for calls, gaming and voice over IP software then use the audio cable of your wired or wireless headphone if it has an inline microphone.
Good value: Yes
:
N/A
Boom
What it is: A typically better microphone, that's also adjustable and extends so that the mic is closer to your mouth.
When it matters: Much better recording quality and noise handling than in-line or integrated mics. Primarily used for gaming and voice over IP software.
Good value: Yes
:
N/A
Detachable Boom
What it is: A boom mic that is detachable from the headset.
When it matters: If you want to use your headphone outdoors without the bulk and hassle of the Boom mic.
:
N/A

These headphones do not come with a microphone. For wired headphones with a good in-line microphone, check out the Bose SoundTrue Around-Ear II or the Bose QuietComfort 25.

0 Recording Quality
What it is: Microphone recording quality shows how natural, neutral, extended and intelligible speech would be with the device under test, in a quiet environment.
When it matters: A microphone with a good recording quality ensures that the person listening to you would hear a full, clear, and easily understandable speech. Therefore, it is important whenever a good quality of speech transmission and intelligibility is needed.
Score components:
Recorded Speech
What it is: Actual audio recording of the headphone's microphone, recorded while placed on the dummy head, with speech being played back through the dummy head's mouth simulator.
When it matters: When a clean, full, and intelligible speech transmission is required.
:
N/A
LFE
What it is: Low-frequency extension shows how deep the bass response of the microphone is, and therefore, how deep and full your voice would sound to the listener. It is the lowest frequency at which the frequency response reaches -3dB of the target response.
When it matters: LFE is not a big factor in speech intelligibility and even speech recorded with a mic that has an LFE of 500Hz could still be easily understood. Therefore, it is mostly important if you are concerned with how deep and full your voice would be heard.
Good value: <150Hz
Noticeable difference: 30Hz
:
N/A
FR Std. Dev.
What it is: Frequency Response Standard Deviation shows how accurately and balanced sound is captured by the microphone at each frequency. FR Std. Dev. is calculated between LFE and HFE, and the rest of the spectrum is ignored.
When it matters: A good frequency response is desired when a natural and neutral speech quality is desired. As opposed to HFE which is more a metric for speech intelligibility, frequency response could be considered as a metric for a natural and neutral sound.
Good value: >3.5dB
Noticeable difference: 0.5dB
:
N/A
HFE
What it is: High-frequency extension is the highest frequency at which the frequency response reaches -3dB of the target response. It shows how extended the treble response of the microphone is.
When it matters: HFE is one the most important factors in speech intelligibility. The higher the HFE, the brighter, more open, and more extended the speech quality will be which makes it a lot easier to understand by the listener.
Good value: >8KHz
Noticeable difference: 1KHz
:
N/A
Weighted THD
What it is: The unwanted frequencies (harmonics) produced alongside the intended frequencies, which cause deformation of an output signal compared to its input.
When it matters: When clean and pure sound reproduction is desired, though its effect is not as noticable as frequency response.
Good value: <1.5
Noticeable difference: 1.0
:
N/A
Gain
What it is: Shows how much louder the microphone can go above our reference loudness level. The gain value is reported relative to our reference level, which is 94dB at a distance of 5cm from the mouth.
When it matters: A microphone with a high gain is important when the input signal (speech) is very quiet. For example when whispering, or talking on the phone in a library.
Good value: >18dB
Noticeable difference: 3dB
:
N/A

The Sennheiser HD 660S have no microphone, so the recording quality has not been tested.

0 Noise Handling
What it is: How well the microphone is able to separate speech from background noise, so that the transmission would include more voice and less noise.
When it matters: When a clean and intelligible speech transmission is desired in a noisy situation like talking on the phone on a busy street or on the bus.
Score components:
Speech + Pink Noise : N/A
Speech + Subway Noise : N/A
SpNR
What it is: Speech to Noise Ratio is the difference in level between speech and background noise as heard by the listener
When it matters: If the microphone is going to be used in a noisy environment, it is important for it to be able to separate the speech from background noise, so the voice would be easily audible and understandable.
Good value: >24dB
Noticeable difference: 3dB
:
N/A

The noise handling has not been tested since the HD 660 S have no microphone.

0

Active Features

What it is: Headphones with active components that require a battery. This includes noise cancelling and wireless headphones that actively reduce noise or transmit audio via a wireless connection.
When it matters: How suitable the power and wireless specifications of an active headphone will be, depending on your listening habits. The range and/or discharge time of the active headphone you select will be important if you're often on the move or have long uninterrupted listening sessions.
Score components:

The HD 660 S have no active features and therefore do not require a battery. They also don’t have a dedicated app or software for added customization options.

N/A Battery
What it is: The power source of your headphones. All headphones with active features have a battery that will deplete over time.
When it matters: To continue using the active features of your headphones. Some models lose features or switch off completely when the battery is drained, which limits what you can do with them until the next charge.
Battery Type
What it is: The type of battery that the headphones use. Usually AAA or embedded, Li-ion rechargeable batteries.
When it matters: When your headphones run out of power. Rechargeable batteries usually charge via the headphones Micro-USB port whereas AAA batteries have to be replaced or charged with an external device.
:
N/A
Battery Life
What it is: The amount of time it takes for a headphones' battery to be completely drained. Battery life will vary depending on the active features used and volume level.
When it matters: For active headphones that connect wirelessly, have noise cancellation or other audio-enhancing features, that cease to work once the battery is dead.
Good value: >10hrs
Noticeable difference: 0.5hrs
:
N/A
Charge Time
What it is: The amount of time your active headphones have to be connected to a power source to charge from 0 to a 100%. However, charging time will vary depending on your power source.
When it matters: To be able to use the active features of your headphones. Especially, wireless ones that completely switch off and need to be recharged when the battery is dead.
Good value: 2h or less
Noticeable difference: 0.25h
:
N/A
Power Saving Feature
What it is: A feature that turns off the headphones, after a set time, when they're not in use.
When it matters: To prolong battery life when the headphones are not being used, or if you forget to manually turn off your headphones.
:
N/A
Audio while charging
What it is: Some active headphones remain usable while charging. They continue to stream audio and do not disable other active features.
When it matters: This makes sure that your headphones's battery are not being drained when you're relatively close to a power source. However, this means wireless headphones will need a wired connection to the power source during the charging process.
:
N/A
Passive Playback
What it is: Active headphones that still work when all their active features are turned off or out of power.
When it matters: If you run out of power and do not have spare AA/AAA batteries or access to a power source to recharge your headphones.
:
N/A

These are passive headphones with no active components and no battery.

0 App Support
What it is: The additional app provided to enhance your listening experience. They typically deliver a set of practical features that give you more control over the sound, noise cancelling and effects that the headphones produce.
When it matters: An app with a lot of features allows you to customize your listening experience to suit your taste and preferences. For example, additions like an equalizer can give you more bass or treble and room effects can simulate a bigger Soundstage in closed back headphones.
Score components: Subjectively assigned
App Name : N/A
iOS : N/A
Android : N/A
Mac OS : N/A
Windows : N/A
Equalizer
What it is: Parametric, graphic or preset sound profiles that slightly alter the frequency response.
When it matters: If you want to tailor, your listening experience. Depending on what you're listening to you may want more or less bass for some tracks or more mid-range for vocals-heavy audio.
:
N/A
ANC control
What it is: Control over the Active noise canceling technology. This could be either a simple on/off button, and adjustable slider or even adaptive self-regulating noise cancellation.
When it matters: If you're in an environment where you need to monitor your surroundings or completely isolate yourself from ambient noise.
:
N/A
Mic Control : N/A
Room effects
What it is: Room effects that enhance the audio quality to make it more immersive.
When it matters: If you want to further tweak your listening experience. Adding room effects, can simulate a more spacious Soundstage or deliver a surround sound-like feel.
:
N/A
Playback control
What it is: An in-app player that gives you access to play/stop, track skipping or volume controls directly with the app.
When it matters: It's a shortcut that allows you to control your audio without leaving the application.
:
N/A
Button Mapping : N/A
Surround Sound : N/A

The HD 660S do not have a dedicated, compatible app for added customization.

4.8

Connectivity

What it is: The inputs and outputs of wired and wireless headphones, as well as their latency performance and range.
When it matters: When you want to know whether your headphones will be compatible with your various audio sources, like your smartphone, tablet, gaming consoles, PC, smart TV, amplifiers, etc.
Score components:

The Sennheiser HD 660 S are straightforward wired critical listening headphones. Their cable doesn’t have an in-line microphone for calls or voice-chat on gaming platforms. They have a decent 10ft of cable range and their wired connection will provide a listening experience without any delay when watching video content. However, they won’t offer the same freedom and convenience of wireless headphones.

0 Bluetooth
What it is: Bluetooth support for wireless headphones.
When it matters: When you want to connect your headphones wirelessly to a Bluetooth source, like your smartphone, tablet, PC or smart TV.
Score components:
Bluetooth Version
What it is: The version of Bluetooth that the headphones support.
When it matters: Newer versions of Bluetooth, when paired with devices that support the same version, may have improved latency and wireless range performance.
:
N/A
Multi-Device Pairing
What it is: A Bluetooth profile that allows some headphones to be simultaneously connected to multiple Bluetooth sources, and have full call and media support on both/all devices they are connected to.
When it matters: To quickly switch between your Bluetooth sources. For example, switching from your phone to your home or work PC and still have call and media support on both devices.
Good value: 2 devices.
:
N/A
NFC Pairing
What it is: Near Field Communication technology that allows you to quickly, pair your headphones with your Bluetooth and NFC-enabled device.
When it matters: This makes pairing with an NFC-enabled device a lot easier than the typical and often tedious hold-to-pair procedure that most wireless headphones have.
Good value: Yes
:
N/A

These headphones are not Bluetooth compatible. For a wireless Bluetooth headset with great sound quality, take a look at the Bose QuietComfort 35 II.

7.2 Wired
What it is: The type and compatibility of audio cables for wired and wireless headphones.
When it matters: When you want to use your headphones wired with a device that has a regular audio jack (line-out), like a smartphone, PC, or gaming console controller.
OS Compatibility
What it is: Testing the headphones' cable to see which operating system it works with.
When it matters: Some wired headphones don't support all operating systems so this allows you to check if the headphones will work with your device.
:
Not OS specific
Analog Audio
What it is: When your headphones can play analog media using a standard 1/8" TRS audio jack. Includes using a 1/4" or 1/16" TRS with a 1/8" TRS adapter.
When it matters: For listening to music with devices that have a standard 1/8" TRS audio jack, like an MP3 player, tablet, smartphone or PC.
Good value: Yes
:
Yes
USB Audio
What it is: When your headphones can play digital media using a standard USB connector.
When it matters: For listening to music on a PC. A digital USB adapter can offer some advantages over a regular audio jack, like a DAC or added software support.
Good value: Yes
:
No
PS4 Compatible
What it is: PS4 compatibility with a regular 1/8" TRS or TRRS audio cable.
When it matters: When you want to use a wired headphone with your PS4 controller.
Good value: Audio + Microphone
:
Audio Only
Xbox One Compatible
What it is: Xbox One compatibility with a regular 1/8" TRS or TRRS audio cable.
When it matters: When you want to use a wired headphone with your Xbox One controller.
Good value: Audio + Microphone
:
Audio Only
PC Compatible
What it is: PC compatibility with a regular 1/8" TRS or TRRS audio cable.
When it matters: When you want to use a wired headphone with your PC.
Good value: Audio + Microphone
:
Audio Only

The HD 660S are wired headphones that come with a 1/ 4” connector, but also have a 1/8” TRS adapter for you to use on console controllers. However, the cable doesn’t have an in-line microphone so voice chat will not be supported. You can also plug in the included 0.17 inch (4.4mm) balanced cable for audio.

0 Base/Dock
What it is: The base station, dock, or dongle transmitter of wireless headphones that receive data/audio via a specific frequency range or wired headphones that have a proprietary amp.
When it matters: When you need to know which inputs and outputs the headphones support, so you can set them up with your home theatre system for gaming or watching movies.
Type
What it is: The type of base/dock the headphones use, whether a USB dongle, charging case or docking station. Wired or wireless.
When it matters: Larger docking stations tend to have more controls and sometimes even customization options while smaller USB dongles are more portable. Charging cases allow you to keep your earbuds charged on-the-go.
:
N/A
Optical Input
What it is: Optical input for audio.
When it matters: Optical can carry a bit more data at faster speeds than typical wired connection which allows for more high quality, lossless audio.
Good value: Yes
:
N/A
Line In
What it is: The regular wired input via a 1/8" TRS audio jack.
When it matters: For any device that has a line out for audio transmission.
Good value: Yes
:
N/A
Line Out
What it is: A regular 1/8TRS audio jack output.
When it matters: If you need to share the audio source with other devices. A line out lets you connect other headphones or speakers to the dock/base station.
Good value: Yes
:
N/A
USB Input
What it is: A digital USB input instead of a typical 1/8 TRS line-in.
When it matters: A USB connection can provide both an audio input and power to the Dock or Base station.
:
N/A
RCA Input
What it is: Audio input using via an RCA connectors.
When it matters: Provides better stereo audio to the dock/base that's then transmitted to the headphones.
Good value: Yes
:
N/A
PS4 Compatible
What it is: Dock/Base station compatibility with the PlayStation 4.
When it matters: To be able to use all the features of the dock/base station with out losing audio or microphone capability.
Good value: Audio + Microphone
:
N/A
Xbox One Compatible
What it is: Dock/Base station compatibility with the Xbox One.
When it matters: To be able to use all the features of the dock/base station with out losing audio or microphone capability.
:
N/A
PC Compatible
What it is: Dock/Base station compatibility with your Personal Computer.
When it matters: To be able to use all the features of the dock/base station with out losing audio or microphone capability.
Good value: Audio + Microphone
:
N/A
Power Supply
What it is: The connector type of the power source for the base/dock.
When it matters: The accessibility of the power source. For example, a power supply with USB/USB-C connects to multiple devices, PC , PS4, Xbox One or even with your regular phone charger whereas an AC adapter is less common.
Good value: USB/USB-C
:
N/A
Dock Charging
What it is: Charging the headphones via the dock/base station instead of a charging cable.
When it matters: It makes charging your headphones easier and gives you a sport to store your headphones when they are not in use.
Good value: Yes
:
N/A

These headphones do not have a dock.

0 Wireless Range
What it is: Headphones that offer a cable-free listening experience over a wireless network, typically via Bluetooth or radio frequency.
When it matters: When you don't want to be limited by the length of an audio cable. This means having the freedom to move around in your home or office with much greater range than an audio cable could provide, especially if the audio source is heavy or difficult to carry. Note that wireless range also depends on your audio source's signal strength, which may vary between devices.
Score components:
Obstructed Range
What it is: The range that the wireless headphones can reach before dropping any audio when the Bluetooth source was placed in another room. We test our obstructed range with a Moto E4 Plus. Results may vary depending on your phone model or Bluetooth source.
When it matters: If you can't or prefer not to carry your Bluetooth source on you while listening to your audio in an indoor environment. Although, the obstructed wireless range will slightly depend on your home or office layout. Note that wireless range also depends on your Bluetooth source's signal strength which may vary from device to device or depending on your phone model.
Good value: >35ft
Noticeable difference: 5ft
:
N/A
Line of Sight Range
What it is: The range that the wireless headphones can reach before dropping any audio when in direct line of sight of the Bluetooth device.
When it matters: If you can't or prefer not to carry your Bluetooth source on you while listening to your audio in a large and open environment.
Good value: 170ft or more
Noticeable difference: 10ft
:
N/A

The HD 660 S are wired-only headphones and don’t have a wireless range. You get about 10ft of range with their cable.

10 Latency
What it is: How long it takes for audio to play through your headphones once the audio signal has been sent from a source.
When it matters: When gaming or watching movies. High latency means you will hear the audio much later than the images you see on screen. Note that latency also depends on the device and applications you use.
Score components:
Default Latency
What it is: The base RF latency or the default sub-band coding (SBC) of most Bluetooth headphones.
When it matters: When watching videos wirelessly, high latency can cause sync issues between the images you see and the audio you hear.
Good value: 50 ms or less
Noticeable difference: 15 ms
:
0 ms
aptX Latency
What it is: An audio coding algorithm (codec) that improves bit rate efficiency. It reduces latency and improves sound quality over Bluetooth.
When it matters: For better sound quality if you often stream music over Bluetooth. It also slightly improves latency when watching videos with wireless headphones.
Good value: 50 ms or less
Noticeable difference: 15 ms
:
N/A
aptX(LL) Latency
What it is: Low latency variation of aptX that significantly reduces sync issues between video and sound when using Bluetooth headphones.
When it matters: Latency is a lot more noticeable when watching videos or gaming than when just listening to music.
Good value: 50 ms or less
Noticeable difference: 5 ms
:
N/A

Thanks to their wired connection, these headphones practically don’t have any latency, which is great for watching video content or when playing video games.

In the box

  • Sennheiser HD 660 S headphones
  • 1/4” audio cable
  • 0.17 inch (4.4mm) balanced audio cable
  • 1/4” to 1/8” adapter
  • Manuals

Compared to other Headphones

The Sennheiser HD 660S are good critical listening headphones that stand out by the decent value they offer. They are quite comfortable, but a bit tighter than some other models on the market and they don't sound as open as other critical listening headphones we've reviewed so far. See our recommendations for best audiophile headphones, the best wired headphones, and the best headphones for studio use.

Sennheiser HD 650

The Sennheiser HD 650 and the Sennheiser HD 660 S are very similar headphones. Their build is almost identical and their sound profile is practically the same as well. The biggest difference is how they feel once on your head. The HD 650 don’t feel as tight as the HD 660 S do, which could be more comfortable for most people. We also measure a more open-sounding soundstage on the HD 650, but since their design is the same, we expect them to sound very similarly. The HD 660 S come with a 0.17 inch (4.4mm) balanced audio cable, which the HD 650 don’t have.

HiFiMan Sundara

The planar magnetic HiFiMan Sundara are better critical listening headphones than the Sennheiser HD 660 S. they are more comfortable to wear for a long period of time and they also have a more accurate sound profile. Their bass roll-off isn’t as pronounced as the HD 660 S and they won’t sound as forward. Also, they have a noticeably more open-sounding soundstage. Overall, the Sundara will offer better value than the HD 660 S.

Focal Elear

The Focal Elear are better open critical listening headphones than the Sennheiser HD 660 S. They aren’t as tight on the head and will be more comfortable for long listening sessions. They are also one of the better-built headphones we’ve reviewed so far. Sound-wise, their treble is more uneven, but they should sound more open due to their better soundstage performance.

Sennheiser HD 800 S

The Sennheiser HD 800 S are better critical listening headphones than the Sennheiser HD 660 S. They are more comfortable, noticeably better-built, and their sound quality is also superior. There is also a big difference in soundstage, in which the HD 800 S are also superior. However, the HD 660 S are way less expensive and may offer better overall value for their performance.

Conclusion

5.7 Mixed Usage
What it is: This is the combination of the different use cases to evaluate how versatile the headphones are. Therefore an everyday headphone should be well-rounded enough to adapt to most situations and environments without significant losses in sound quality, design ergonomics or isolation.
Score components:
Sub-par for mixed usage. These open-back over ears are designed for critical listening and won’t be suitable for any other use case. Their open design will let noise seep into your audio and will also leak a lot, which isn’t suitable for commuting or the office. Their bulky design isn’t designed for sports and they leak too much for watching TV without disturbing your household. They can be decent for games if you have a stand-alone mic and are playing in a very quiet room, but overall, a gaming headset will be a better option for most.
7.6 Critical Listening
What it is: The level of audio fidelity a headphone can reproduce. Therefore a balanced and true representation of bass, mids, treble, soundstage and imaging, as well as a comfortable listening experience, is essential for critical listening.
Score components:
Good for critical listening. Their bass is very good, consistent, and punchy, the mid-range is well-balanced and clear, and their treble is very good as well. However, their bass lacks a bit of thump and is a bit muddy; they also sound a bit mid-rangy and forward. Also, their treble lacks a bit of detail and might sound slightly veiled. Overall, they are great versatile headphones for a wide variety of music genres but their sub-bass might be lacking a bit for bass-heavy music.
4.5 Commute/Travel
What it is: How well the headphones handle the loud environments involved in commuting or traveling. Therefore your listening experience should be comfortable, hassle-free and as isolated from noise as possible.
Bad for commuting. These headphones are open-back and won’t be suitable for this use case. All the noise of public transit will seep into your audio and people surrounding you will be able to hear what you’re listening to.
5.6 Sports/Fitness
What it is: How well-adapted the headphones are, to use while doing sports or strenuous exercise. Therefore the headphones should not be too cumbersome and deliver a stable and comfortable listening experience.
Sub-par for sports. While they are stable on the head due to their tight fit, these headphones shouldn’t be used for sports. They aren’t portable and you will also sweat more than usual since they create a tight seal around your ears. Also, their wired design means you’ll have a cable in your way during exercising.
5.0 Office
What it is: How well the headphones can deliver a comfortable and isolated listening experience in an office-like environment. They should not leak much and should block the noise of a busy office.
Poor for the office. The open design of the HD 660S means that they won’t block work environment noises and will also leak quite a lot, which will be disturbing to people surrounding you. These headphones can be used if you’re working alone but won’t be suitable for a common office.
Sub-par for TV. While they are quite comfortable and have good audio reproduction, these headphones don’t offer the range of a wireless design. Also, your household will be able to hear what you’re listening to and they also don’t block ambient noise well.
5.7 Gaming
Sub-par for gaming. These headphones don’t have a microphone for online games, can’t be customized like most gaming headsets with a software, and can’t be used in loud environments. However, if you have a stand-alone mic and play in a quiet room, these are quite comfortable for long gaming sessions and they have a good sound quality.

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