Excuse my English plz
In: Biology
You have (presumably) two ears and your brain can do stuff like compare volume and timing to figure out where the sound is coming from.
Technically you need three sensors to triangulate a point in 3d space and if you did this with two microphones, a computer and some math you would end up with a circle of possible origins, that intersected the ground in two points.
But mother nature is able to make do with just two ears by making them in clever shapes and sligthly asymmetrical and taking advantage of the fact that experience tells you that most sound come from near ground level.
The 2 ear drums vibrate at a different rate and angle so it makes it possible to hear different points around you and at many distances around you.
Because we have two ears. The sound won’t be exactlu the same in both ears, and thus the brain can determine the origin of the sound.
Because you have two ears and they are on different sides of your head. Sound arrives at slightly different times at each ear based on the sound’s origin and you brain interprets those timing differences as location data. Not everyone can do this, like me. I have single sided deafness so I have no ability to localise sound.
Your brain can recognize in which ear the sound arrived first. If the sound wave fjrst crashed into your right eardrum, the sound came from your right side, the same for your left. Regarding in front or behind, that is different because your ear is shaped like this. So sounds from in front of you sound crisper than from behind. So a crisp sound first arriving in your left ear comes from in front of you, left hand side.
Literally my PhD topic here. So it’s gonna be a bit of a long one, but hopefully interesting. I’ll maybe add a comment with some other related stuff (like VR) at some point 🙂
There are roughly 4 ways we work out sound direction.
1) Interaural time difference (ITD). We have two ears. A sound on the right side of the head will reach the right ear before the left ear. This time difference can let us work out the angle of approach. But it works best for lower frequency (deeper) sounds. This is because at higher frequencies, it’s difficult to compare waves and work out if you’re looking at the same part of the same wave in both ears, or one part of one wave in one ear and one part on the next or previous wave in the other.
2) Interaural level difference (ILD). Same kind of idea. Our head creates a shadow, so sounds on the right are louder in the right ear. But ILDs work better for higher pitched sounds. This is to do with how sounds bend around a sphere. Lower sounds are better at bending, so there isn’t as much of a head shadow effect for low sounds.
These two approaches are pretty good, but are ambiguous. For example, a sound directly in front of you will reach both ears identically. But so will a sound directly behind you. And one right overhead. And one below you. In fact, for any location (except directly in like with your ears), there is what’s called the *cone of confusion*, a cone of location stretching out from your ear that would all have the same difference cues. So, that’s why we need to use…
3) Spectral cues. Those flappy things you call ‘ears’ are just part of your ears, specifically your *pinnae*. They act like dishes, helping funnel sound in. But they’ve got a distinct shape. In fact, every pinna is about as unique as a fingerprint. Anyway, these unique twists help filter sounds depending on the direction of arrival. If a sound is in front of you, you’ll hear more of the high pitched information. If behind you, you’ll get more low end. The problem here is that the sounds need to have a bit of complexity to them. Ideally, we’d also be familiar with the sound to know what it’s ‘meant’ to sound like. But it’s okay, because we also use…
4) Dynamic cues. Go back to methods 1&2. We only talked about how these work when you’re stationary. But suppose you move… Imagine that situation where there is a sound directly in front or behind you, and it’s arriving at both ears the exact same. Now turn your head to the left. If the sound was in front of you, it would now be closer to the right ear. If it was behind you, it would be closer to the left. By seeing how the possible locations change as we move our head, we can work out exactly where the sound is.
I should add that this is just direction. Sound distance is difficult. It mainly relies on knowing how loud something should be, and what the echoes in the room are like.
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