Sound involves the compression and rarefaction of a medium like air.

Sound can radiate in all directions, but the shape of the source has an effect.

Musical tones are usually periodic. Different instruments have different waveforms. Noise is not periodic.

A plucked string has vibrational modes at integral multiples of the fundamental frequency. These are called overtones.

A sound can be described in the time domain or the frequency domain. Both descriptions can contain the equivalent information.

The anatomy of the ear.

Schematic anatomy. Cochlea is unrolled into a straight tube for simplicity

The cochlea is a tube that is tapered and rolled up like a snail.

A tone produces a traveling wave that has maximal displacement at some point along the basilar membrane.

Tonotopy: different frequencies lead to strong vibrations at different locations along the basilar membrane.

Cross section of the cochlea.

Close-up showing the details

Cells in auditory nerve are tuned to particular bands of frequencies.

The development of speakers. First there was one simple speaker. But it couldn’t handle the huge range of frequencies, so it was split into two speakers: a tweeter for high frequencies and a woofer for low.

Adding stereophonic cues. Next, two speakers were used, to give a left channel and right channel. This gave sound localization. Total: 2 woofers and 2 tweeters. Today it is common to have two midrange speakers and one sub-woofer. The midrange speakers are small and easy to arrange for a good stereophonic effect. The subwoofer reproduces low bass; it can be placed almost anywhere because humans can’t localize low bass tones.

How do the pinnae affect what we hear? The way things sound depends on the shapes of your ears. It’s slightly different for everyone. The subtle effects of how the sounds bounce around can be used for localization in the vertical direction. If you go into a sound-proofed room and listen to the world through microphones it sounds funny because you are bypassing the pinnae.

Replicating the sound of the world. Make a life-sized model of your head and ears, and put the microphones in the ears. Listen to the sound through earphones. It sounds natural again. (This won’t work if you use someone else’s head/ear model because the sounds bounce around differently from what you are used to).