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Direction Finding

Bio-Acoustics 108

Sound intensity declines with distance from its source (inverse square law) due to scattering as well.

If the two ears on a head get the same intensity, then the direction must be from dead ahead or behind. If the sound comes from the right or the left, then the right or left ear gets a higher intensity respectively.

The difference in intensity from ear to ear just isn't much. 10,000 Hz sound attenuates 70 dB per kilometer. A 1 cm span between the ears would have a gradient of 0.0007 dB and a 100 cm skull 0.07 db.

The head may cast a sound shadow on one of the ears. For example, the left ear is shaded when the sound is from the right and the intensity between the ears is greater. As a direction finding method it works less well at lower frequencies and less well in water than in air.

The difficulty of distinguishing right in front from right in back or ahead 45 degrees right or behind 45 degrees left is reduced by head moving, cocking or cupping your ear and by having ears that are directional "antennae" like LTER PIs (see Shaw 1974, in Handbook of Sensory Physiology). We humanoids angle our ears 30 degrees on the average.

Now if there isn't much of a sound shadow, then it is much more difficult to assess direction. This becomes a problem when the head circumference exceeds 1/4 of the wavelength of the sound (thank Lord Rayleigh for that one).

The human head has a radius of 8.75 cm and a circumference of 0.55m. [See earlier CEDs for differences in head size among LTER scientists and NSF bureaucrats.] We have a harder time direction-finding with long wavelength (low frequency) sound than high frequency sound.

If you are in a tight game of hide and seek and the seeker wants to "make a sound" use the lowest voice possible and the seeker will remain confused. With a squeaky high voice at, say, 10,000 Hz, the sound shadow of the human head is strong and directions finding is possible with some precision.

It should be noted that fatheads have an advantage over pinheads as they can cast a greater sound shadow and do better at direction finding.

Our friend the elephant with its 4.5m circumference head can use direction finding with 25 Hz sounds. We would be lost trying to find our mates with such low frequency wooing.

Another way of direction finding by sound takes advantage of the difference of arrival time for sound waves from one ear to the other. Sound from the right gets to the right ear before it gets to the left ear.

Bats, for example, with 5 cm from ear-to-ear need to be able to sense an arrival time difference of only 0.15 microseconds. They do it.

LTER scientists run about 20 cm ear-to-ear and need only detect a difference in arrival time of 0.6 micro seconds and our nervous system is easily up to this task.

Fatheads here, too, have the advantage over pinheads.

However, underwater with a faster velocity of sound means that the arrival time differences are much less and direction finding by scuba divers is difficult.

Direction can also be determined based on the phase-shift (like Doppler shift for light) of the sound wave.

Head size is important and high frequency sounds (as long as they are not an exact multiple of the distance between the ears) work better than low frequency sounds.

In Hide and Seek it is better to fool the Seeker with low frequency "I'm over here!" then a contralto "Hey!" Choose up sides now for the next All Scientists Meeting.



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