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CSEC Physics: Sound Wave Production and Propagation

Sound is propagated as a longitudinal wave of pressure produced by vibrating systems. Since sound is a wave (a disturbance which travels through a medium transporting energy), the medium is simply the material through which the disturbance moves. A medium has several particles that interact with one another- so when a wave moves through it, one particle is able to transfer the energy of the wave to an adjacent particle. All media have three properties which affect the behaviour of sound propagation:

  1. A relationship between density and pressure. This relationship, affected by temperature, determines the speed of sound within the medium.

  2. The motion of the medium itself, e.g., wind. Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported.

  3. The viscosity of the medium. This determines the rate at which sound is diminished. For many media, such as air or water, weakening of sound due to viscosity is negligible.

When considering the production and propagation of sound waves, a tuning fork can be used as an example. A tuning fork consists of two tines capable of vibrating if struck by a hammer. As the tines of the tuning forks vibrate back and forth, they begin to disturb surrounding air molecules. These disturbances are passed on to adjacent air molecules by the mechanism of particle interaction. The motion of the disturbance, originating at the tines of the tuning fork and travelling through the medium (in this case, air) is what is referred to as a sound wave. The generation and propagation of a sound wave is demonstrated in the animation below.

Similarly, when you pluck a rubber band, it produces compressions and rarefactions in the air as a sound wave.

However, not all sound is the same (of course). Why is this? Well, every longitudinal sound wave is different based on its wave parameters. As discussed in a previous post on wave motion, a wave can be described based on its amplitude, frequency, wavelength, period and wave speed. However, the amplitude and frequency affect sound the most.

The amplitude of a sound wave is its loudness/volume. The amplitude of any wave describes the maximum displacement of vibrating particles of the medium from their equilibrium positions. So, a sound wave with a larger amplitude will displace the particles of a medium farther from their equilibrium positions, and therefore is a larger disturbance.

The frequency of a sound wave is its pitch. The human ear can only hear between 20 Hz and 20000 Hz while dolphins (obviously superior to humans) can hear between 0.25 Hz and 200000 Hz. Below 20 Hz is known as infrasound and above 20000 Hz is ultrasound. Nonetheless, the pitch of any sound we hear is determined by its frequency, or the number of waves per second. Higher frequencies are higher pitched while lower frequencies are lower pitched. For example, if you play the piano or a similar instrument, middle C (C4) is 262 Hz.

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