About the inaccuracy of the measurements of the frequency response of headphones / Pult.ru / SurprizingFacts

Today, many people buy the headphones blindly, without listening, relying on the technical characteristics and the frequency response curves, carefully laid out by users of numerous audio forums. Especially such purchases are typical for online stores. It is well known that the frequency characteristics of sound determines the frequency response. At the same time, if you try, you can find several completely different graphs on the same model, with significant differences, which is alarming.

Not infrequently the result of the purchase is disappointment and discrepancy between the pair bought and the expectations that arose during the review of the schedule – they wanted, they say, bass players, and they more often distinguish the middle, etc. In addition, subjective comparison of different models of headphones with the same or very similar frequency response curves, there is a significant difference in the subjective evaluation of sound.

Under the cut description of one of the common reasons, ways to solve it and several recommendations for self-improvement of the stand.

I believe that many have already realized that the key problem lies in the measurement technique. It is about her that will be discussed. At measurements the "bare" microphone is most often used, which is not in real conditions, since headphones are pressed to the head with one or another force. In addition, sound waves before the moment of impact on the eardrum are caught by the auricle and pass through the external auditory canal. It is natural that the results of tests without taking these parameters into account do not demonstrate what is actually.

The reason for the creation of this post was the master's thesis of Marco Hiiipakka "Measuring Apparatus and Technical Modeling of Ear Acoustics" written at the Department of Signal Processing Acoustics of the Faculty of Communication and Automation of Helsinki University of Technology, as well as the experiments of videloglogers stalker29218 and Alexander TheHitty, who drew attention to this problem.

Problems and solutions

According to Marco Hiiipakka, when measuring AFC, acoustic characteristics of the ear itself are not taken into account. It is well known that the properties of hearing are extremely individual and are directly dependent on our anatomical features. As a rule, measurements of acoustic parameters of headphones (as, indeed, of other audio equipment) are conducted without taking into account anatomy.

Comparison of the frequency response of headphones (especially closed type) when using a "bare" microphone reveals a significant difference with the results when using a stand where the ear pads are pressed To the plane. Even greater differences are demonstrated by the stand with an imitation of the external auditory canal and auricle.

Red – an open stand
Yellow – pressing the ear cups to the foam rubber;
Green – silicone skin imitation;
Blue – silicone + auricle + external auditory meatus.

Open Stand Alexander TheHitty

Closed Stand Alexander TheHitty

Alexander TheHitty stand with imitation of the skin

Alexander TheHitty stand with imitation Skin, external auditory canal and auricle

As can be seen from the graphs published by Alexander using a more reliable technique, there are significant changes. The most significantly affects the sound of the enclosed space, as well as the length and width of the external auditory canal. As the length of the pass is increased, the resonance peak arising between 2 and 4 kHz shifts towards low frequencies. When the width is reduced, on the contrary, towards the RF, which is clearly visible on the graphs.

Graph of results Alexander TheHitty

Among other things, changes in the frequency response caused by the deposition of earwax in the external auditory canal are possible. As the graph below shows, the presence of sulfur (imitator-wax) has a certain effect on the curve.

Marco Hiipakka and Alexandra TheHitty's research data coincide according to the results of the test measurements The main points.

One of the Marco Hiipakka stands in the section

Everything that has been described mainly concerns full-size models, and is especially useful for measurements with closed-type and semi-closed headphones. In his work, Marco Hiipakka also proposed original ideas for in-channel headphones. Here are some examples that allow you to make measurements without using a large scale stand.

From what to do it

Before modeling the auricle and external ear canal, it makes sense to provide proper sound insulation and emit the skin. As an insulator, you can use any sound-absorbing material, for example, foam, as Alexander TheHitty did.

Silicone with a density of 29 to 30 units is usually used as a skin simulator. According to Shore.
The same silicone can be used to create an imitator of the auricle and an external auditory canal.

It should be taken into account that it is quite difficult to remove the impression from the ear canal, it is easier to make a hollow silicone tube with 2 bends, a narrowing, 28 mm long and 5-7 mm wide (average parameters for the human ear).

Marco Hiipakka used plastic tubes, and in a number of studies limited himself to a standard syringe (in cases, it was required to measure the frequency response of the intra-channel headphones).

As fasteners for this whole case, Plywood, drywall, chipboard or something else. The main condition is the isolation of the imitator of the external auditory canal and the hermetic fixation of the microphone.

The result

Given the low accuracy of measurements, it is naive to believe that it is possible to understand how headphones sound using the results of the frequency response tests found on the network. It is also foolish to believe that any measurements do not reflect the real picture, and sound should be evaluated only subjectively. When receiving the test results, it is important to know how this test was conducted and how much the human anatomy was taken into account, how "pure" is the study (this concerns not only the frequency response but the CWI, IMD and other characteristics). Before buying headphones, they should still be listened to, for which, in fact, many showrooms have been opened, where it can be done with comfort.