Monthly Science: Audiograms

The audio test CD I used to measure my hearing for a Circuit Cellar project back in 2007 came to light, so I ran some tests:


I don’t have an absolute level calibration for any of those curves, so they can be shifted up or down by probably 10 dB without any loss of accuracy. The overall shape matters here, not the absolute level.

The brown curve shows my hearing as of nine years ago. I built and (of course) wrote about a rather chunky low-pass shelving filter that matched the 20-ish dB difference between my midrange and treble responses, then boosted the flattened result enough for me to hear what I was missing:

Board Top
Board Top

Surprisingly, it worked fairly well. That, however, was then and this is now.

The two red curves show my current response, under slightly different conditions: the “buds” curve uses the same earbuds as the 2007 curve and the “phones” curve uses over-the-ear headphones. Perhaps:

  • The previous (lack of) bass sensitivity came from the circuitry of the day
  • My bass has mysteriously improved
  • More likely, my midrange has gotten that much worse

The blue curve shows the response of a reference set of silver ears; the golden ears I used in 2007 were unavailable on short notice.

Given my limited bandwidth and the steep slope of that curve out toward the high end, simply fixing my (lack of) treble won’t suffice any longer: 50 dB is a lot of amplification. Compressing the bandwidth between, say, 200 Hz and 4 kHz to fit into 200 Hz to 2 kHz, then equalizing the result, might give me enough treble to get by, but it’d require re-learning how to hear.

That’s different from the straightforward frequency translation you get from a mixer. I don’t have enough audible bandwidth around 1 kHz to hear a 4 kHz slice of audio spectrum.

Back in 2007-ish, a real audiologist determined that I wasn’t “aid-able”. Maybe that’s changed.

The economics seem daunting. Michael Chorost gave a talk at Vassar lamenting the cost and terrible UX of his cochlear implants that reinforced my prejudices in that area. The discussion following my post on my Bose QC20 earphones includes useful links and rants.

The GNURadio project has enough signal-processing mojo for a nontrivial hearing aid, modulo having enough CPU power at audio frequencies. Battery power density remains the limiting factor, but I’m not nearly as fussy about appearances as most folks and some full-frontal cyborg wearables might be in order.

4 thoughts on “Monthly Science: Audiograms

  1. fixing my (lack of) treble won’t suffice any longer: 50 dB is a lot of amplification

    The way some commercial hearing aids deal with this is to move the high frequencies to lower ones where you can hear them. This helps a lot understanding speech. However it makes a piano… or, I suppose any musical instrument, sound horribly out of tune!

    1. Given that I’m basically tone-deaf, de-tuning music won’t pose much of a problem. I think I figured out to compress a range of frequencies into what I can hear, but it’s not one of the building blocks in GNU Radio’s inventory; I must learn something new: eeek!

  2. You’re not the only one with this issue. Maybe a geekish hearing aid is a viable market? Group funding for a circuit board and a shirt-pocketable case (injection-molded ?) that would hold the board and a couple of 18650 li-on cells, perhaps?

    And I’ve never encountered an audiologists that I felt I could trust. I’ve encountered a half-dozen over the years. They run the test way too long (resulting mental fatigue and skewed results). I get the impression that they are in business to sell costly hearing aids. Or the “independents” that are employed by the health plans are urged to save money and not find problems

    1. A Trusted Source aims me at the Teensy 3.2 + stereo audio board, which look like just the ticket for the hardware: 32 bit ARM with DSP hardware, nice design GUI, low power with a battery, tiny footprint, lots of I/O, no PCB layout required.

      I’m sure somebody is already working on this; the hardware & software seem cheap & readily available.

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