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."

Comments

Well, duh....

[Anonymous Coward]

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.

Re:Well, duh....

[Zinho]

Help me out here, because I don't see your experiment being conclusive. I've been led to believe that the determination of the direction of a sound had much more to do with the timing with which it reaches the ears than with the volume level. My hypothesis is that the subject in your experiment is still using the covered ear to determine the origin of the sound.

An experiment I saw on Mr Wizard's world supports the timing theory. It involves the subject putting a tube up to one of their ears and being really confused about the direction of sounds originating near their head - the increased path length to the ear with the tube changed the timing enough that the subject judged the sound to be coming from a location much nearer the unblocked ear.

A similar experiment (that I haven't seen performed) involved a pair of speakers used to simultaneously play a short tone (as close to a single wavelength as possible). When asked which direction the sound appeared to come from, the subject pointed in the direction of the closer speaker, even when the futher one was set to a higher volume.

Perhaps another experiment is in order - have the subject put an earplug in one ear and place a tube up to the same ear, and repeat your experiment. If the subject is still correct about the direction, then your hypotesis that one ear can determine direction will be supported; otherwise I'd assert that the subject in your experiment was still using the covered ear to determine the direction of the sound source.

ps-I'd also attribute the trouble identifying the direction of a siren in the city to the good reflectivity of the building surfaces - it's easy to catch spurious reflections, correctly interpret the direction of the reflected sound, and incorrectly assume that the sound originated along that path. I'm not convinced that adding white noise to the siren will help with that.

pps-before I get flamed about my experiment being centered around left-right awareness and the parent post discussing up-down, I'm aware of that. Give the tube a right-angle turn and point it up or down as you see fit; it's easy to do with either pvc or paper towel tubes and duct tape. Join me in ignoring the acoustical properties of either of those materials... ;^) If you're lucky, after work I'll run this experiment myself and post the results, but don't hold your breath - family life can wreak havoc on an amateur experimenter's lab schedule.

Holophonics

[dschuetz]

[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." :)

Re:Holophonics

[Eigenray]

There are some nice samples here and some others here.
Listen to them with headphones to get the full effect.

It says,

Holophonics is based on the hypotheses that we do not perceive sound in a passive manner, with sound waves impacting our eardrums and our brain computing characteristics and location from the frequency, intensity, and arrival time at each ear. Instead, the technology is based on the premise that the human ear generates its own Rreference tone which interferes with incoming sounds to create the necessary spatial information for analysis by the brain. It is the interference between external sounds and the reference tone that provides the brain with spatial information. If external sounds are tape-recorded with synthesized reference sounds, the brain provides its own second reference beam. Like a laser beam shining through a hologram, it decodes, reproducing the original ambient conditions.

Hearing loss with age primarily due to noise

[obtuse]

"Most hearing loss in old age is caused by the death of the hair cells, which are not regenerated."

Hearing loss with aging is unnatural. Hearing loss in the elderly is not common in primitive societies. We assume it is a normal part of aging because we are exposed to so much noise, but in fact it is not. In many non-industrial societies, older people hear quite well.

Hearing loss is caused by death of the hair cells, but that death in turn is caused by overdriving them with excess energy, that is, loud high-amplitude sounds.

70 decibels is a normal conversation, 80 decibels is damaging over time, and city noise is commonly louder than 80 dB. You probably listen to your music louder than that, too. Concerts? Wear earplugs, or you'll probably have demonstrably worse hearing afterwards. Any sound that makes your ears ring is probably damaging.

Ontopic: The idea that the ears are active, and produce sound is not at all new, although the demonstration that those sounds are externally audible is surprising. The very statement that doctors are listening to those sounds to check the health of infant hearing, tells you that this isn't brand new research.