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Human Ears Make Noise
KeelSpawn points to this article at Discover, which begins: "Until recently, scientists thought human ears were passive devices that detected and processed sounds, but new findings suggest that ears are like perpetually turned on stereo receivers that quiver spontaneously and sing along with incoming sounds."
The shape of the ears plays an important role, too. It alters the frequency response of noises heard so that you can spatially determine it's location.
In simpler words, you can tell if a sound is above or below you. The little ridges and curves of your ear accent and muffle different frequencies in different ways. It's natural for you now, because as a child, you programmed your brain by looking for the source of the sound. Later on, you stopped looking at the sound source, and could tell where the noise was coming from.
Take a cheap radio, and tune it to between stations, to generate some white-noise. Cover one ear, and move the speaker of the radio around above and below your uncovered ear. For extra fun, do it blindfolded, with a friend moving the radio around.
You'll be surprised at how well your one ear can locate sound in 3D. If it was just receiving information like an omnidirectional antenna, then it shouldn't be able to process location.
That's also why you can't determine where pure noises (like cell phones, smoke alarms, most electronic beeps, and some bird whistles) are coming from, or you have a hard time locating them. These tones do not have enough miscellaneous energy in multiple parts of the audio spectrum for the shape of your ear to fiddle with. The British are experimenting with new alarms that go "chuff", or that have white noise as part of the sound, to make it locatable.
That's also why headphones sound so different (and unlocatable, spatially), because the headphones defeat the ear shape.
No wonder I can't get Santana's Smooth out of my head. My damn ears are singling along!
Believe nothing -- Buddha
[vaguely off-topic:]
:)
I first read of something like this nearly 20 years ago. When Pink Floyd's _The Final Cut_ came out, it utilized something called "Holophonics," the creation of some guy named Zuccarelli (or somesuch). The basis of his system was that the ear produces continuous high-pitched sounds, and the sounds we hear in everyday life interfere with those sounds, and are processed by the brain to produce a 3-D soundscape.
He then went on to assert that, by combining sound effects artificially mixed with that same high-frequency pitch, they can create and record that "interference pattern," much like a visual hologram works. Then, by playing that pattern back to the user (with nicely spaced speakers or, especially, wearing headphones), they could recreate the soundscape perfectly.
On the plus side, it worked. I heard a broadcast of a Roger Waters concert, where he demonstrated walking behind the listener and striking a timpani. Sounded cool.
On the minus side, it was a load of crap. The technical issues are just way too numerous to go into here, but the bottom line is that it was probably nothing more than nicely-recorded binaural sound. No new discoveries there.
[Returning to topic]
But, still, there was that case of the sound being produced by the ear -- at about the same time as holophonics coming out, I read some articles that sounded much like the current article -- there's sound, it's unpredictable, and we're not sure why it's there. Sounds like they're starting to figure out why.
Of course, anyone who listens to Suzanne Vega would have known all this years ago -- "Blood makes noise / It's a ringing in my ear."
The best summary information on the web is the otoacoustic emissions web site. It has two lengthy reference sections, information on the use of otocacoustic emissions to test the hearing of new borns and infants, and lots more.
Other information on this is easily obtainable by browsing back issues of the Journal of the Acoustical Society of America.