Preferred headphones store
Updated , Sam Vafaei

Our Imaging Score and Tests
Heaphones

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.
Score distribution

Imaging determines where, how far, and how wide each object should be in the stereo image. That is, it controls the location, transparency, and stereo balance of objects in the mix, as intended by the audio source. In other words, imaging is the localization and spatial cues inherent to the audio content that loudspeakers/headphones have to "reproduce" rather than "create". This differs from soundstage, which is the localization and spatial cues not inherent to the audio content.

Our imaging test evaluates Weighted Group Delay, Weighted Amplitude Mismatch, Weighted Frequency Mismatch, and Weighted Phase Mismatch, for the tested headphones below.

Test Results

When it matters

Imaging is mostly important for critical listeners and gamers since accurate placement and localization of objects (voice, instruments or video game effects like footsteps) in the stereo field is quite crucial. Some aspects of imaging are very hard to notice for the average listener and therefore, a great imaging performance may not be as critical for everyone. However, a decent imaging performance is one of the essential qualities that a headphone should have, and a headphone with very poor imaging would be rather unpleasant to listen to, even for use cases other than gaming and critical listening. For example, a headphone with a poor stereo balance (with a left-heavy image for example) would have a noticeably negative effect on the listening experience even for listening to a podcast during a commute.

How it works

The slight differences in time and amplitude/intensity between the two channels of stereo audio are responsible for creating the stereo image (also known as ITDs and IIDs). As an example, for the voice of an individual to be perceived to be coming from a specific location to the left side, the left channel's signal should arrive a specific amount sooner and louder compared to the right channel's signal. This way, the brain will localize the speech to be coming from the left side.

A headphone with good imaging doesn't tamper with these inter-channel time and amplitude differences inherent to the audio content, and reproduces them faithfully (driver-matching), resulting in an accurate placement and localization of objects in the stereo image. It also doesn't tamper with the intra-channel time differences inherent to the audio content, since they affect the tightness of bass and the transparency of treble reproduction (Group Delay).

Our Tests

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
Score distribution

Group delay is the time it takes for the amplitude of each frequency to reach their maximum. Group delay looks for single channel frequency-dependent time distortions created by the headphones, that is, it checks to see if a portion of the frequencies in the response of the headphone arrives later than the rest. Group delay differs from phase mismatch, by being a monaural quality, that is, it can be perceived even with one ear. But phase mismatch, by definition, is a stereo quality.

For our group delay calculation, we have implemented a weighting filter based on a number of studies1,2,3 done on the audibility threshold of group delay on headphones at different frequencies, in order to make our results more perceptually relevant. Since the research was limited to the 500Hz-8KHz range, we extrapolated the results for the bass and high-treble regions. Below is our current audibility threshold for headphone group delay, in milliseconds.

Headphone Group Delay Audibility Threshold (milliseconds)

Usually, headphones have higher group delays in the bass range compared to the higher frequencies. For example, it may take 25ms for a headphone to produce a 40Hz tone at 90dB SPL, while it can produce the same amplitude at 1KHz in less than 2ms. A high amount of group delay in the bass range results in a slow and loose bass. A high amount of group delay in the treble range negatively affects the transparency of the sound. Also, some Bluetooth headphones have a spike in group delay around 20KHz, but that won't be audible to most people.

Excellent group delay

Barely audible group delay at 60Hz

Sub-par bass group delay

Barely audible group delay at 20KHz

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
Score distribution

The Left/Right balance of our test unit, that is, the amount of amplitude difference between the left and right drivers. A headphone with good amplitude matching reproduces objects in their intended location in the mix. Conversely, a headphone that has, say, a 3dB amplitude mismatch favoring the L channel, will have its center offset to the left, and in accordance to that, all the objects in the stereo image shift to the left, resulting in a "left-heavy" image.

It should be noted that this is not a design test, but since in essence, this is a driver matching test, it could also be considered a marker for manufacturing tolerance and ergonomics.

Also, since the graphs on the website are level-matched, they don't show the actual measured amplitude mismatch between the L/R drivers of the headphones. However, they are reported in the imaging box, under amplitude mismatch.

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
Score distribution

The amount of difference (standard deviation) between the frequency response of the left and right drivers of our test unit. A headphone with poor frequency response matching will have an uneven stereo image, with certain frequencies offset to the left or right, instead of being centered. In other words, frequency response mismatch is similar to amplitude response mismatch, as in they look for the same kind of error, but frequency mismatch does it per frequency, as opposed to amplitude mismatch which does it per channel.
 
It should be noted that this is not a design test, but since in essence, this is a driver matching test, it could also be considered a marker for manufacturing tolerance and ergonomics.

Excellent frequency response matching

Sub-par frequency response matching

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: <90°
Noticeable difference: 30°
Score distribution

The amount of difference between the phase response of the left and right drivers of our test unit. A headphone with poor phase matching will have an incoherent stereo image, which in extreme cases, could push the objects to the sides of and leave a hole in the middle of the stereo image.

For our phase mismatch calculation, we calculate the standard error of one channel phase response against the other, but we have implemented a weighting filter based on the research done on the audibility threshold of phase mismatch on headphones at different frequencies, in order to make our results more perceptually relevant. Since the research was limited to the 63Hz-8KHz range, we extrapolated the results for the sub-bass and high-treble regions. Below is our current audibility threshold for headphone phase mismatch, in degrees.

headphones phase mismatch audbility threshold

Headphone Phase Mismatch Audibility Threshold (degrees)

It should be noted that this is not a design test, but since in essence, this is a driver matching test, it could also be considered a marker for manufacturing tolerance and ergonomics.

Excellent phase matching

Barely audible phase mismatch

Sub-par bass phase matching

Sub-par treble phase matching

Additional Information

What's not included

  • Factoring frequency (treble) response in the imaging calculations
  • CSD: Cumulitive spectral decay could be considered part of imaging, may be considered in the future
  • Head tracking
  • Surround capabilities

If you feel there is an item missing that should be included, please let us know in the Q&A section or via email.

Discussions

LOG IN

JOIN RTINGS.com

Be part of the most informed community and take advantage of our advanced tools to find the best product for your needs.
Join our mailing list:

Create Discussion

Preview Back to editor Format guide

The editor uses special characters (aka. markdown).

To post formatted content follow these rules:

What you typeWhat it will look like
*italic text*italic text
**bold text**bold text
[link](http://rtings.com)link
> quoted text
quoted text
# header

header

- item 1
- item 2
- item 3
  • item 1
  • item 2
  • item 3

* Quotes and lists must be followed by a blank line