The Physics of Sound
A sound is a complex interaction between vibrating molecules in matter and the specialized sensory structures of the brain and ear. Much of what makes sound meaningful has more to do with our brain than with any of the physical properties of oscillating molecules of air, but we can get at how sound is propagated. We can describe the physical properties and learn to see sounds and from this learn to appreciate sounds in ways that may go beyond the ability of the human ear.
Sound begins with vibrations, periodic oscillation. If we stretch these out over time while tracking position, they trace out a wave form. Frequency defines the number of vibrations per unit time. The more vibrations/time the greater the frequency. Figure 1 shows two waves tracked over the same time period. The blue wave has twice the frequency because it make twice as many oscillations in the same period of time. The height of the wave is called the amplitude. It is a function of loudness in sound. It also tells us something about the energy of the wave.
A pure tone would produce a uniform standing wave with a definite frequency, but most natural sounds are not this clean. When more than one frequency of wave interact, this is called interference. With waves interference may act constructively combining amplitudes or destructively cancelling out amplitudes. If the waves in Figure 1 were to interfere with each other the result would be a compound wave form like that of Figure 2. Most natural sounds have complex wave forms that combine many individual frequencies and amplitudes like those in Figure 3. This is what give natural sound its richness and depth.
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