3. How do people / animals / machines "hear" sound?
4. Why do people have two ears?
Having two ears allows humans to distinguish from where a sound originated. If you close your eyes & someone calls your name from your left hand side, you can tell that the person is standing to your left without having to see them. How does this work?
Firstly, sound travels at a fixed speed (or velocity) in air which is approximately 340 metres per second. If a person calls your name from your left hand side, the sound will reach the left ear sooner than the right ear because the sound has the width of your head extra to travel before it reaches your right ear. It takes time for the sound to travel that extra distance. Even though the time difference in the arrival of the sound at your left & right ears is minute, your brain is able to interpret the time difference & deduces that the sound has come from the left hand side. These are called INTERAURAL ARRIVAL-TIME DIFFERENCES.
Secondly, the quality & intensity of the sound reaching your left & right ears may be different. Again, imagine your eyes are closed & someone calls your name from your left hand side. The sound waves reaching your left ear are unimpeded & mostly direct sound reaches your left ear. However, your head creates an acoustic shadow that impedes the sound reaching your right ear. Some sound will find its way around your head & into your right ear & some sound will be reflected off any hard surfaces & back into your right ear. What is important is that the quality of the sound reaching your left & right ear is different. Again your brain will recognise these differences & deduce that the sound is coming from your left hand side. These are called INTERAURAL INTENSITY DIFFERENCES.
If a sound originates from directly in front of you, both ears will receive virtually identical sound with very few interaural arrival-time & intensity differences. We can simulate this when we listen back to sound or music over two speakers. If we sit between two speakers in an equilateral triangle & play an identical sound through both speakers, we can trick our brains into thinking that the sound is coming from directly between the two speakers. Identical sound is reaching both ears so your brain assumes the sound is coming from directly in front of it. If we now send the signal through the left speaker more loudly than the right speaker, our brain will interpret this as the sound originating from the left hand side. This forms the basis of stereo sound. A stereo signal whether it comes from a record, cassette, CD, mini disc, MP3 or WAV file has a left & a right signal, each sent to its respective speaker. By making signals louder or quieter in each speaker we can simulate the position of sounds in the stereo field. When you listen to a well recorded piece of music, each instrument & singer seems to occupy their own space. The guitarist may sound like they are on the left hand side, the bass player may sound like they are on the right hand side while the vocalist may sound as if they are singing directly in front of you. If you listen to the music in mono (or through only one speaker) this sense of where a sound is coming from is lost. In the same way that you need two ears to determine where a sound originates from, you need two (or more in surround sound) speakers to represent the direction a sound has come from in a recording.