Building the “Diplacusis Piano”, Part 1/3: Background

Introduction

In a previous post I described my struggles with diplacusis and my intention to build a “piano” that could reproduce the sounds that I actually hear for the benefit (?) of others. This series of posts will document the progress I have made so far and the exciting compositional possibilities that are opening up as a result.

Diplacusis is a disturbing phenomenon in which the two ears hear a given musical note at two different pitches. It is yet one more from the smorgasbord of symptoms associated with Ménière’s Disease (see this post for a detailed account of my Ménière’s experiences), alongside vertigo, hearing loss, tinnitus and aural fullness.

I decided to try to build a musical instrument that would convey to others what this sounds like. I wanted this to offer me a creative opportunity to make some beautiful music. What it is in fact providing is not just that, but a whole new direction for my composition.

This post is a detailed account of the first steps in building this instrument. It is necessarily a digital instrument: there is no way this could be done using traditional technologies. I have been greatly helped by my GNResound Linx Quattro hearing aids and by my friend, the composer and Professor Craig Vear, who provided not just technical fluency in the studio and an otologically “normal” pair of ears, but also the ability to describe each sound to me as it emerged from this new instrument.

Starting Points

I decided to start with a piano simply because that is the instrument I used to play back in the days when I regularly made music. Piano sounds also have a pleasing decay which I instinctively felt would work well with this phenomenon. Nobody wants to listen to sustained diplacusis!

In my previous scientific study of my own diplacusis, I mapped the differences in pitch across my own singing range by laboriously stopping the good ear and singing the pitch I heard in Hertz, then comparing it with the correct pitch. This gave me a verified chart from F#2 (~92Hz) to C4 (~261Hz). To understand what comes next, you need to see my audiogram:

Andrew Hugill’s audiogram (July 2017)

This one is a little bit out of date, but my hearing has not changed much since then. Observe that (as is usual in audiology) the right and left ears are reversed in the image. You will also notice that audiology takes no interest in frequencies below 125Hz or above 8kHz. This is because audiology is mainly interested in speech and, frustratingly, takes little account of music.

Anyway, you will see quite clearly that my right ear (in red) is way below my left ear. This is what severe hearing loss looks like. My left ear has normal hearing (above 10dB) in the region between 1500 Hz and 4000 Hz. This is my salvation in speech situations. But there is quite a lot of hearing loss around that. Nevertheless, my pitch perception in that ear is tolerable.

One other thing to notice is that the lower frequencies show a marked decline in both ears. This is typical of Ménière’s Disease, where the bass disappears first. By contrast, in age-related hearing loss (presbycusis) the high frequencies deteriorate first, which is why so many hearing aids concentrate on the high end.

First efforts

Now you can see why the next step in preparing for the instrument was so daunting and has taken me many months of struggle to figure out. I could no longer rely on either my audiogram or my singing voice to help me understand my own pitch perception, because the rest of the piano keyboard is simply out of range. To make matters worse, every time I tried it was like working in a hall of endlessly reflecting mirrors. I would listen to my diplacusis with my diplacusis… it was very uncomfortable and very tiring.

So with considerable effort, I worked on trying to understand my own hearing by feeling my way with trial and error. Gradually a number of key features emerged:

  1. There is an octave between F#5 (~698Hz) and F#6 (~1397Hz) where there is no diplacusis at all. In other words, I hear a piano just like a normal piano, as anyone else would, albeit with greatly reduced hearing in one ear.
  2. In the range above that, the diplacusis gradually reappears, getting worse the higher up you go. However, since the piano sounds pretty metallic in that register anyway the effect is not as disturbing as you might expect.
  3. The range from C4 (~261Hz) down to F2 (~87Hz) is affected by random amounts of diplacusis as per the chart from the earlier study.
  4. Below E2 (~82Hz) this random diplacusis effect continues, but now a new phenomenon enters, presumably resulting from the general loss in low frequency hearing. The fundamental frequencies of each note and then the first and second partials, gradually disappear, leaving a thudding sound and a collection of higher overtone frequencies. This complex spectrum is then subject to precisely the same diplacusis that affects the higher register, resulting in a perceptible shift in spectrum but no discernible change in pitch.
  5. And this is, I think, a novel finding: every diplacusis induced detuning is flat! This seems to contradict the received wisdom that diplacusis notes are sharp. I need to do more research into this.

Given the difficulties of translating the above into any kind of instrument, I eventually had to admit defeat and seek help. This is where Craig Vear enters the picture and the account of our building session yesterday will be the subject of my next post.

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