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:

• You can still hear your own voice with earplugs or blasting headphones by direct bone conduction, but you can't hear anything else. Be careful, or that will be the norm! I expect a big increase in hearing problems due to Walkman-style headphones.

• Tie a string to the corner of a metal oven rack. Wrap the free end of the string around your index finger, let the rack hang free, and press your finger with the string against your ear. When you strike the rack with a stick or screwdriver, it sounds like temple gongs. (hah!) But only you can hear it because the sound is traveling directly from the rack, up through the string and your finger to your skull, and directly on your inner ear. Ckbarlow has an MP3 music piece The Troll Dance that captures the sound, but it's more fun to do it yourself! How can you record this inner sound?

• Snap your middle finger and your thumb with your hand open (if you can!), and then try it again with your hand closed. It's much louder with your hand closed. This shows the amplifying effect of cavity resonance. It's a similar effect when you "listen to the ocean" with a conch shell or just by cupping a hand over your ear. You're actually hearing the blood rushing in your ear and the ambient noises around you. Plug your ears completely, and you'll hear a rumbling sound that comes from your heartbeat and arm muscles. Try flexing your arm muscles to change it.

• Take a light-weight archery bow (without the arrow!) and hold one end against your jaw while you twang the string with a pick. Change the sound by flexing the bow and opening and closing your mouth. That's a mouth bow. The sound goes right from the bow through your jaw bone to your inner ear, just like whales hear. Sounds great to you, but your audience will miss most of the sound. This is an ancient instrument. Folk singer Buffy Sainte-Marie has some fine recordings with mouth bow, including Cripple Creek.

• Walkman-style headphones use bone conduction to provide a full sound. They sound great to the listener, but they sound tinny and thin to other people nearby. If you press the headphones to your head, they sound even fuller. Small computer speakers also sound tinny if you're not sitting in the "sweet spot".

• Some hearing aids amplify outside sounds into your ear drum, but others pass the sounds directly to the solid mastoid bone behind your ear which allows you to hear directly by bone conduction. There are microphones that work the same way and record your voice like you expect it to sound. Radio DJs enhance their voice with electronic reverb effects to get that "inner voice" sound.

• Sub-woofers and movie theater surround-sound systems are usually pointed at the floor rather than the air. We feel these sounds as much as we hear them.

• The moving Clasical Kids' CD Beethoven Lives Upstairs tells the life of Beethoven from the point of view of a child. It relates the story of Beethoven's pianos. Beethoven's hearing loss made it difficult for him to hear the notes he played. So he would cut the legs off the piano, lay on the floor, and hold a stick on the piano frame with his teeth in order to feel the sound vibrations. I have some hearing loss myself, and this segment brings me to tears every time.

• Sound travels better through water than air. Whales actually "hear" through their lower jaw bones directly to their inner ear, and their ear canal is reduced or blocked completely. Whale songs can cross the oceans! There is special hardware to allow scuba divers to "talk" the same way.

• I'm impressed how I can listen to good music on poor quality equipment, and it doesn't matter! I have a vintage hand-crank Victrola and my Mom's old record collection. Audio quality is not an issue when listening to original Billie Holliday and Duke Ellington 78s! Our ears are an amazing sensory organ. Take care of them!

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

• Encyclopędia Britannica: the human ear
• HyperPhysics: Sound and Hearing
• The Fridge, Environmental Architecture at UWA.
• Spectrogram - a freeware spectrogram program for PCs so you can do your own acoustic analysis!

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