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Our Imaging Score And Tests
Headphones

Updated
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.

Having good matching between the left and right driver of the headphones in amplitude, phase, and frequency response helps accurately reproduce the positioning of objects/instruments in the stereo image that was designed or recorded without imbalances or holes. A low group delay can help you hear each layer of the audio content clearly and faithfully.

When it matters: When accurately positioning the objects in the stereo image, and clear and transparent imaging is desired.
Score distribution

While you may not immediately know it by name, imaging is a very important component of the headphones-listening experience. When a pair of headphones have good imaging, you can place an enemy's footsteps at the right side of your screen when you're gaming or hear where each of the band's instruments are on stage. Conversely, if a pair of headphones have bad imaging, it can be harder to accurately distinguish where sounds are in space. Audio cues may be skewed to one side and you may even feel 'listening fatigue' after listening for some time. 

This is what we call imaging. It's the localization of spatial cues such as instruments, sound effects, or voices inherent to your audio content. It also controls the transparency, and stereo balance of objects in your mixes. Unlike soundstage, which determines the space and environment of sound as it's 'created' by the headphones, imaging is the 'reproduction' of audio cues. 

Our imaging test evaluates Weighted Group Delay, Weighted Phase Mismatch, Weighted Amplitude Mismatch, and Weighted Frequency Mismatch.

Test results

When It Matters

Imaging is mostly important for studio work and gamers since the accurate placement and localization of objects (voice, instruments, or video game effects like footsteps) in the stereo field is crucial. Some aspects of imaging are hard to notice for the average listener, meaning a great imaging performance may not be as critical for everyone. However, a decent imaging performance is one of the essential qualities that headphones should have. Headphones with very poor imaging would be rather unpleasant to listen to, even for use cases other than gaming and neutral sound. For example, over-ears with a poor stereo balance (with a left-heavy image, for example) can have a noticeably negative effect on your listening experience, even if you're listening to a podcast.

Imaging can also be a measure for quality control and ergonomics. For example, it's harder to manufacture headphones with planar magnetic transducers than more common dynamic driver headphones. This complex driver design can make it harder to match the L/R drivers, and as a result, there may be slight errors in phase response or small shifts in the stereo image. Another factor is how the diaphragms are positioned concerning your auditory canal. Some manufacturers place their diaphragms so that they're more forward-facing to replicate a speaker-like listening experience rather than playing directly into your auditory canal.

It should be noted that we don't test mono headsets for imaging. Mono headsets, by design, can't produce stereo sound.

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). If an individual's voice is intended to be perceived as 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.

Headphones with good imaging don't tamper with these inter-channel time and amplitude differences inherent to the audio content since they affect the tightness of bass and the transparency of treble reproduction (Group Delay). Instead, headphones reproduce these differences faithfully (driver-matching), resulting in an accurate placement and localization of objects in the stereo image.

Our tests

Weighted Group Delay

What it is:

The average amount of group delay calculated based on the group delay graph and using a perceptual weighting filter.

Group delay indicates how long it takes for each frequency to reach its maximum amplitude in the time domain and plays a role in the reproduction and distinction of each layer in the audio content improving imaging fidelity. 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 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 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 its maximum. Group delay looks for single-channel frequency-dependent time distortions created by the headphones. In other words, 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; it can be perceived even with one ear. But phase mismatch, by definition, is a stereo quality.

For our group delay calculation, we've implemented a weighting filter based on several studies done on the audibility threshold of group delay on headphones at different frequencies 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. Some Bluetooth headphones have a spike in group delay around 20kHz, which can be caused by different things, including signal interference. However, this won't be audible to most people.

Excellent group delay

Barely audible group delay at 30Hz and 80Hz

Mediocre bass group delay

Barely audible group delay at 20KHz

Weighted Phase Mismatch

What it is:

The amount of difference between the phase response of the left and right drivers of our test unit, based on the data in the phase mismatch graph.

A bad phase mismatch at a specific frequency region can alter its amplitude, having the effect of altering the original image the audio is meant to deliver, making it less accurate or clear. This test is a marker for quality control.

When it matters: When an even and stable stereo image, as well as good quality control, is desired. A poor score indicates there may be inaccuracies in the stereo image reproduction at certain frequencies.
Good value: <16
Noticeable difference: 3
Score distribution

This test measures the amount of difference between the left and right driver's phase response. If you're not familiar with phase, we'll need to go back to the basics of sound. One way of representing sound is by using sinusoidal waves. The height of a wave tells us its amplitude (or loudness), while the length of the wave indicates time. Since soundwaves have a repetitive form, we want to focus on a wave's cycle or wavelength.

Phase is the location of a point within a repetitive sound's wave cycle, which we can express in angles, starting from 0° until 360°. For example, if we're looking at max amplitude, we want to look at phase at 90°, as this is the highest point within the sound wave's cycle. However, this on its own doesn't tell us much. Phase is most useful when we can compare the differences in phase between more than one signal.

 

 

Several soundwave cycles.1
 Two sinusoidal waves with the same frequency but have phase difference or phase mismatch2.

Stereo headphones use a left and right driver to produce coherent sound as our brains process both signals simultaneously. Assuming you're using a consistent tone, each time the left driver's sound wave reaches its peak amplitude at 90°, the right driver's sound wave should also reach its peak amplitude at 90°. When both waves are matched in phase, their amplitude increases. In headphones, a well-matched phase response indicates a stable stereo image. However, a bad mismatch in phase response can point to inaccuracies in the stereo image reproduction at certain frequencies. At worst, it can push objects to the sides and leave a hole in the middle of the stereo image. 

Our graphs reflect when you'll hear phase mismatch in different frequencies. When the response crosses above the audibility threshold, the mismatch may be audible. 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 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)

 

That said, the amount of deviation from 0° and the frequency range of the peak can impact whether or not you hear the mismatch. For example, the Focal Celestee have a peak at just under 16KHz, or in the high-treble range. However, this can be very hard to hear, especially with real-life content. This is because the higher the frequency, the harder it is to sync each wave cycle as the degree between them is becoming smaller and smaller. It's also harder for us to detect these mismatches with our ears, and we lose sensitivity to this range as we age.

In another example, the JBL Tune 510BT Wireless have a more prolonged peak between 200Hz and 900Hz, or between the high-bass and mid-mid. This peak indicates that the amplitude of these sounds won't be representative of the amplitude desired by the audio you're listening to, and they sound quieter than intended. You can hear it in real-life content, but it may be more obvious to some users than others. 

A lot of phase mismatch can look like the Anker Soundcore Life Q30 Wireless' graph. There are a lot of peaks, which indicates that objects like voices skew to one side of the stereo image. This also means that the phase mismatch is noticeable in real-life content. When listening to your favorite tracks, audio is skewed to either the left or right driver at different frequencies, making for a harsh and fatiguing listening experience.

Jaybird Vista 2 Truly WIreless

Excellent phase matching

Focal Celestee

Barely audible phase mismatch

JBL Tune 510BT Wireless

Poor phase matching

Anker Soundcore Life Q30 Wireless

Terrible phase matching

This isn't a design test, but since it's essentially a driver matching test, it could also be considered a marker for manufacturing tolerance and ergonomics.

Weighted Amplitude Mismatch

What it is:

The Left/Right balance of our test unit, based on our sound profile measurements; that is, the amount of amplitude difference between the left and right drivers.

Higher amplitude from either the left or right side can create unwanted shifts in the image the audio content is meant to create. This test is a marker for quality control.

When it matters: When you want a properly balanced stereo image and good quality control. 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

Weighted amplitude mismatch assesses a model's left and right driver's balance with amplitude or volume. When headphone drivers are matched in amplitude, they can reproduce objects in the intended location within your mix. Conversely, high amplitude mismatch can create unwanted shifts in the stereo image and potentially lose or exaggerate details. For example, if a pair of in-ears have 3dB of amplitude mismatch favoring the left channel, the stereo image's center (and the objects within your mix) will shift to the left.

This isn't a design test, but since it's essentially a driver matching test, it could also be considered a marker for manufacturing tolerance and ergonomics.

Our sound graphs are level-matched at 90dB SPL and 100dB SPL so that test results are consistent and comparable across reviews. However, these graphs don't show the actual measured amplitude mismatch between the L/R drivers of the headphones. Instead, we report this measurement in the Imaging box under 'Weighted Amplitude Mismatch'.

Weighted Frequency Mismatch

What it is:

The amount of difference between the frequency response of the left and right drivers of our test unit, based on our left and right sound profile curves.

Mismatch in frequency response can create imbalances or holes in the image generated by the audio content. This test is a marker for quality control.

When it matters: When an even and stable stereo image, as well as good quality control, is desired. A poor score indicates there may be holes or imbalances in the stereo image reproduction at certain frequencies.
Good value: <2
Noticeable difference: 0.1
Score distribution

This test assesses the amount of difference or standard deviation between the frequency response produced separately by a unit's left and right drivers. This mismatch results in an uneven stereo image where some frequencies are skewed to the left or right instead of being centered. 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.

That this isn't a design test. Since this is essentially a driver matching test, however, it could be considered a marker for manufacturing tolerance and ergonomics.

Outstanding frequency response matching

Mediocre frequency response matching

For example, the Samsung Galaxy Buds Live Truly Wireless display minor frequency mismatch. Their left driver's bass range is more emphasized and boomy than their right driver, which could offset sounds like thump and rumble in EDM tracks and skew them to the left. Conversely, the left driver's mid-range is less emphasized than the right driver, offsetting voices and lead instruments to the right.

What's Not Included

  • Factoring frequency (treble) response in the imaging calculations
  • CSD: Cumulative spectral decay could be considered part of imaging, may be considered in the future
  • Head tracking
  • Surround capabilities (whether virtual or true surround can accurately place objects in the image)

 

If you feel something is missing that should be included, please let us know in the discussions or via email.

Conclusion

Whether you're trying to place an enemy's footsteps in gameplay, or if you want to distinguish the location of instruments in your mixes, a good imaging performance can make your audio sound accurate and enjoyable. The four different elements of this test—group delay, phase, amplitude, and frequency response—reflect different elements of the stereo image as it's recreated by your headphones' left and right drivers. While some driver mismatch can be hard to hear with real-life content, its presence can also point to larger issues in the headphones, like ergonomics and quality control.

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