Frequency Response Consistency describes the variations in a headphones' frequency response due to their fit on your head. Headphones will sound slightly different depending on the size and shape of your head, how they interact with your ears and whether you wear glasses.
Frequency response is the most important part of good audio reproduction. So although most listeners naturally compensate for variations in frequency response caused by their unique features, such as the shape and size of their heads, other factors like their preferred headphone position, and how the ear cups' acoustics interact with their ears may cause additional variations in the response. An inconsistent headphone may have a drastically different sound from listener to listener. This means a headphone that will sound bass-heavy for most could lack a lot of bass for listeners who wear glasses. The ideal headphone should consistently reproduce the same response whether open or closed-back and regardless of positioning or head/ear shape.
We evaluate the consistency of the frequency response of our over/on-ear headphones by measuring them 5 times on the HMS (Head Measurement System) and 5 times on 5 human subjects. For in-ears, we measure them 5 times only on the HMS. We then calculate the deviation of each response compared to the average frequency response.
Frequency Response Consistency is important if you're a critical listener who wants the most accurate listening experience from your headphones. Bass and treble are often dependent on how the headphones interact with your unique features. Therefore, some headphones will sound drastically different, especially in the bass range, from listener to listener. Typically, inconsistency is more an issue for closed-back models due to the ear cups' acoustics being strongly affected by the quality of the seal they create around your ears.
The ideal headphones should have the same frequency response regardless of head size or ear shape, but depending on the use case, it may not be as significant. For example, sports headphones may not need to be as consistent as critical listening headphones.
Since variations in positioning preference and head/ear shape have a significant impact on the measured frequency response of headphones, and also because of the consistency and reliability issues with the HMS, we decided to measure the low-end of our over/on-ear headphones on 5 human subjects, 5 times each. And the result of those 25 measurements will be averaged-out to get the final bass response. Since for in-ears, we don't have the ability to measure them on humans, we only do 5 re-seats on our HMS instead.
The human measurements are performed using a pair of blocked ear canal (BEM) microphones. Since these microphones don't have the same frequency response as the ear simulator inside the HMS, we have calibrated our mics against the ear simulator of our HMS so that they perform the same.
For over/on-ear headphones, we merge the results of human measurements and HMS measurements by crossfading between them from 350Hz-450Hz. Below 350Hz, we only use human subject results and above 450Hz, only the HMS results. This is done in two steps: level matching and crossfading. During level-matching, the level of the human measurements is matched to the HMS measurement, and over the crossfading range, the human measurements are slowly faded out as being replaced by the HMS measurement.
Average Standard Deviation is calculated by comparing the difference between each frequency response pass, against the average of all passes.
A number of factors influence the measured frequency response of a headphone on a human subject. The main 3 that we have encountered so far are:
Note that the influence of these factors decreases on open-back headphones since compared to closed-back variants, they don't rely on fit/seal as much for creating their bass.
We noticed that humans with bigger/wider heads tend to get more bass:
Hair length and hairstyle also affects the measured bass response of headphones. We have noticed that women tend to measure less bass, especially on closed-back headphones, probably because they usually have longer hair.
We also noticed that different glasses affect the frequency response differently. Glasses with arms that were not flush to the temple (probably on skinnier humans), tend to break the seal and cause a big drop in bass. Again, this effect is much smaller with open-back headphones.
For more information, check out our video on measurement inconsistencies and target curves: