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Topic: ear distortion (Read 6081 times) previous topic - next topic
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ear distortion

Looking for some help on something. I've been told that the ear can suffer distortion at a high SPL level.

This article :

http://www.silcom.com/~aludwig/Nonlinear.htm

Says

Quote
Non-linearity in a sound system produces unwanted harmonic and intermodulation distortion, and perfect linearity is the ideal. But for the ear, non-linear behavior is far from being a flaw; in fact it is a critical feature that enables the large dynamic range of human hearing. The inner hair cells (IHC) of the cochlea, which convert sound to nerve impulses, have a dynamic range of less than 50 dB. But we can hear over a 120 dB dynamic range! How is this possible? It turns out that the ears have a built-in sound level compression system, created by the outer hair cells (OHC) of the cochlea. In the most active region of the cochlea basilar membrane, a 4 dB increase in sound pressure at the eardrum increases the membrane motion as little as 1 dB, due to mechanical action of the OHC.

The non-linearity of the ear has been known over a century, but it was relatively recent that the OHC of the cochlea were identified as the primary cause. The middle ear is quite linear over sound pressures of 40 to 110 dB SPL, and does not result in noticeable distortion at normal listening levels (Hartmann page 512). The inner ear non-linearity does produce distortion, which can be heard, and measured in the ear canal. In fact, the measurement of distortion products in the ear canal is used as a hearing test for newborn infants, since the distortion products are absent for certain forms of hearing impairment!


Now it also says that the OHC is the primary cause of nonlinearity. It also says the inner ear non-linearity does produce distortion in the ear canal. But does this mean that distortion in the ear only exists at 110 dB, since the middle ear is so very linear over the range of sound pressure? Or can it occur below this?

The reason for this question is that one may mistake compression from loudspeakers at very high volumes for their own ears compressing. Just want to understand the article better as I'm not sure about the "40-110 dB SPL" thing and how the inner ear/OHC relate.

ear distortion

Reply #1 –
I'm not a hearing expert, but I see two unrelated things being discussed together. Compression is not the same as distortion, and you can have compression without distortion. But ears definitely distort at very high volumes. One of my favorite examples derives from when I played percussion in a local symphony. A glockenspiel is similar to a vibraphone but smaller, and the metal bars are higher pitched and have no resonating tubes underneath. A glockenspiel can also play very loud! I noticed while playing a chord of two high notes that I heard difference frequencies as overtones when those notes aliased down into the audible range. That is, I'd hear low tones even though both of the notes were very high pitched. But the distortion was created entirely within my ears. You have to be very close to the glockenspiel in order for the volume to be loud enough to distort your ears. I've never noticed this from out in the audience.

--Ethan
I believe in Truth, Justice, and the Scientific Method

ear distortion

Reply #2 –
I think I've heard my ears distorting before, but nothing as definite as what Ethan is describing.    I don't even remember the occasion, but maybe it was like I felt something vibrating inside my ears.

Certain kinds of speaker noise/distortion can be hard to hear unless you are close to the speaker.  Especially things like a loose rattling voice coil, or some hardware or rattling, or a cabinet vibrating/buzzing.

I'm not sure if that's the case with normal-everyday amplifier clipping or speaker-overload distortion.




You can get a kind of compression (more like automatic volume control) when you get a temporary threshold shift (temporary hearing loss) from listening to loud sounds.  Well, hopefully it's temporary.  But, that's not going to sound like compression or distortion.  It's just going to sound like someone turned-down the volume a bit, and you might hear a change in tonal-balance.  Then of course, when the music (or noise) stops, it's seems extra-quiet, you feel a little deaf, and you might hear ringing in your ears.