I have a right and left ear, so I get how I can tell if a sound comes from my right or left. How can my 2 ears tell me if a sound is coming from in front of me, behind me, above me or below me?
In: 2
Mostly context. But also, your ears can pick up subtle echos and reverberations from your surroundings helping your brain perceive the environment as a whole and pinpoint the source of the sound. *Very* slight delays can be registered subconsciously.
First of all, having two ears doesn’t just let you tell if a sound comes from right or left, your ears can [locate the sound](https://en.wikipedia.org/wiki/Sound_localization) source anywhere around you horizontally. To figure out how, you can draw lines from the source of the sound to both your ears, and realize that a longer line to one ear means sound will arrive at that ear slightly later. And your brain can calculate the minute difference in “lateness” and figure out WHERE the sound source must be.
For detecting up and down and forward vs. behind you, the [ear cartilages](http://www.earwellcenters.com/wp-content/uploads/2018/09/ear-anatomy-infant-.jpg) play a role of bouncing the sound into your ear canal but dampening some of the frequencies. So your brain detects the slight absence of high pitch or low pitch sounds that marks the difference between sound coming from in front of you (and reflecting off the cartilages) vs. sound coming from behind you (and having to pass through the cartilages).
> How can my 2 ears tell me if a sound is coming from in front of me, behind me, above me or below me?
By the shape of our ears. The top 1/2 of our ears don’t look the same as the bottom 1/2.
SmarterEveryDay has a video on this, including a fun little experiment showing that if you put playdoh on your outer ears, you no longer can discern these things (4:30 mark):
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It also isn’t just a delay difference between the ears, it also is an volume difference and a frequency difference caused by the sound wrapping around our skull.
Head Related Transfer Functions (HRTFs) are very important in getting headphones to sound right (especially in-ears) and super important to get spatial audio to sound right.
When Sony announced spatial audio for the PlayStation, they showed the insane rig they used to measure an individual’s HRTF:
Sound localisation comes down to a few factors
1) Sound travels at about 330m/s in air at normal atmospheric conditions*. This means that if a sound is coming from one side or another, there will be a small difference. .6ms or something, it’s tiny, but your brain uses this information. This gives horizontal plane information, A sound directly in front, above, behind… will reach each ear simultaneously though.
2) Your head casts a ‘shadow’. About 40dB in the high frequencies is blocked by your big old noggin. Your brain also uses this difference to calculate location. Again, this is horizontal information though. We know which side it is coming from, but not its height.
3) The shape of your outer ear (the pinna) and ear canal have a load of turns and bends, this actually affects the way sound travels, affecting sounds coming in from different directions in different ways. Your brain uses these subtle changes to calculate vertical direction.
*so to count how far lightning is and listening for thunder, every 3secs is 1km
The other answers are correct but incomplete.
For sounds with wavelengths smaller than your head, the left/right position is located simply by paying attention to the relative loudness. Your head blocks some of the sound so that it will be *slightly* quieter on the side away from the source.
For a bit of context, your ears can hear sounds that move your eardrum less than the width of an atom. Your ears are *very* sensitive to small differences in volume.
Sounds with wavelengths larger than your head can wrap or bend around your head. Lower-pitched sounds do not *attenuate* as fast as high sounds – that is, they don’t lose volume over distance as quickly – and your head can’t block some of the sound because it goes around.
To locate these sounds, your brain looks at the relative *phase* of the sound. That is, sound is made of waves of high pressure and low pressure, and if you graph the waves it creates a sine wave with peaks and troughs. Going around your head changes the distance ever so slightly, which changes where the peaks and troughs line up on either side of your head. Your brain is sensitive to the timing of these differences in phase, and uses that to determine which side it came from.
For forward/backwards, the shape of your ear changes the sound. Partly it’s that sounds in front tend to be louder because your pinna (the fleshy outer part of your ear) funnels sound from the front and partially blocks sound from the back. Your pinna also resonates with different frequencies depending on where they come from, so that they sound *barely* different, which your brain can recognize.
Also, don’t discount your eyes. If there is ambiguity about the source of the sound, your brain will look for potential sources in what you can see. If something in your field of vision looks like it should be the source, your brain will try to use it as the source. Even if you can’t see something, if you know that there is something around you that should be the source, you will use that as the source if there is enough ambiguity.
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