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Treebeard's Stumper Answer
4 May 2001

The Mind's Ear

Ilana and Maggie are making audio recordings as part of their project for the Dunn Middle School Science Fair next Friday, May 11, at 5:00 P.M. (Don't miss it!) We noticed that everyone thinks their own recorded voice sounds wrong. It's thinner and less powerful. It's not the voice I'm used to. What's odd is that everyone else sounds normal. How can we all think that, even when we're listening to the same tape? Is it an illusion that my own voice sounds different, or is it a real effect? Which is my real voice? Are there other examples of sounds based on the same principle?


A recording of my own voice sounds thinner and less powerful than I'm used to because I usually hear my voice conducted through my skull bones as well as through the air. Solid bone carries the low frequency bass sounds better than air, and adds some resonance or echo. It's not an illusion. You can easily hear the difference if you open and close your ears with your fingers while you hum or talk. Small Walkman-style headphones use the same principle to provide a full sound. They sound great to the listener, but they sound tinny and thin to other people nearby.

Notes:

If other people can hear the music from your headphones, it's too loud! Hearing loss and tinnitus can result. Unfortunately this is a positive feedback situation. As your hearing is damaged, you will automatically turn the volume even higher and cause even more damage.

I picked this easy stumper because the Dunn Middle School Science Fair was looming, and we were all busy with our own stumpers. Graybear explains:

The sound we hear when we speak is partly transmitted through our body as well as through the air. This would seem to make the sound richer and more powerful. Other similar effects are underwater or in a small, enclosed space. Sometimes, when flying, noises sound different to me shortly before my ears 'pop'.

Air and bone are very different media for transmitting sound. I normally sense my own voice both ways, but when I listen to a recording I only hear the air-transmitted sounds. Sounds transmitted by bone conduction go directly to the inner ear with no volume loss. That's why you can still hear your own voice when you're listening to music through headphones.

Low frequency sounds travel better through solid bone than air, and all sounds travel faster through solid materials. This creates the resonant bass and the reverb effect since I hear my own voice with it's own slightly-delayed echoes. I figure my voice sounds even louder when I plug my ears because it creates an additional delayed echo back through my eardrum and middle ear. I remember playing my guitar late at night in the stairwell in Ellison Hall at UCSB when I went to college. The echos made it sound much fuller. Sometimes I would have late night jams with other unseen guitar players on different floors.

Here's a Fletcher-Munson equal loudness curve that shows how we hear different sound frequences (in air) at different intensities.


The sound pressure of a source is a physical quanitiy that can be measured in decibels. But perceived loudness is a psychological quantity. These F-M curves represent equal loudness as perceived at different pitches. They show that the ear is less sensitive at low frequencies, and the insensitivity of low sounds becomes even more so at low volumes. That's why my stereo amp has a loudness setting to boost the bass at low volume settings. We hear best around 3-4 khz which is great for speech and telephones and lo-fi 28K streaming audio, but you don't hear that resonant bass that makes you sound so cool, except in your own inner ear as you speak.

I think the more interesting question is to find other examples of sounds that depend on the difference between air-conduction and bone conduction. Here are a few:

I've already given lots of good links for your own research, but here are a few more general links on sound and acoustics:

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Copyright © 2001 by Marc Kummel / mkummel@rain.org