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