Frequency response of a hearing aids

[AudioSim]

I’m a Computer Science student currently working on a research project involving hearing aid signal processing. To move forward, I need to measure the frequency response of several hearing aids.

In the professional world, I see two standard methods: Ear Measurement and 2cc Coupler measurements. Unfortunately, professional equipment for these is way outside my student budget.

I’ve been looking into affordable alternatives and found the IEC-711 (clone) artificial ear/couplers often used by the IEM community.

My questions for the experts here:

Can an clone of IEC-711 coupler provide reliable enough data for a comparative study of frequency responses to data provided by manufacturer

Since I couldn't find a cheap 2cc coupler clone, is there any?

As my background is in Software Development, I’m a bit of a novice in acoustic measurement physics. Are there specific pitfalls (sealing, calibration, vent leakage) I should be aware of when using these clones with hearing aids?

Any advice, papers, or alternative low-cost setups would be greatly appreciated!

 

[RandomEar]

To my knowledge, the main disadvantage of 711 couplers in general is that their nozzle is not a good representation of the average ear canal. That means any frequency repsonse measurement and the typical resonances you see would not represent what you would actually hear as a human when using them. However, that would not be a major problem for a comparative study. It's still not ideal for that use case, but it's probably good enough.

Fitment and seal are very important for all headphone measurements, including IEMs - and by that logic, also for hearing aides. I have seen different approaches to these challenges. One was to adjust the IEMs until they all produced a resonance at the same frequency (typically around 8 kHz for that example). From my understanding, this would guarantee that the "effective" insertion depth is the same for all tested devices. Seems reasonable to me, but there might be better methods.

That being said, I don't have any personal experience with headphone measurements. There are forum member who do, so maybe they will chime in.

 

[Soandso]

Human ear canals can be of different lengths. Adult women have been assessed as having +/- 13% shorter ear canal lengths than adult men. Which means their ear canals are relatively more rigid than adult males. The implication is that as women age they've greater potential to hear higher frequencies yet not low frequencies than similarly aged men.

Ear canal width also exhibits human variation; which can extend to different widths for the left and right ear of the same person. A wide ear canal chamber promotes relatively less center portion air (sound) flow. While a narrow chamber of the ear canal elicits faster air (sound) flow in the center portion than the main air column.

Factors such as length, rigidity and width interact with ear canal resonance, which is modulated by aperture seal. The relevant context of seal is that when a canal chamber air's sound (audio frequency) corresponds to any resonant frequency in the canal chamber (a factor of the degree of seal) that creates a synchronicity which adds amplitude to that particular sound (audio frequency) signal.