The Propagation of sound

Sound is a sequence of waves of pressure which propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes of propagation). During their propagation, waves can be reflected, refracted, or attentuated by the medium. The purpose of this experiment is to examine what effect the characteristics of the medium have on sound.

All media have three properties which affect the behavior 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 attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.

What happens when sound is propagating through a medium which does not have constant properties? For example, when sound speed increases with height? Sound waves are refracted. They can be focused or dispersed, thus increasing or decreasing sound levels, precisely as an optical lens increases or decreases light intensity.

One way that the propagation of sound can be represented is by the motion of wavefronts-- lines of constant pressure that move with time. Another way is to hypothetically mark a point on a wavefront and follow the trajectory of that point over time. This latter approach is called ray-tracing and shows most clearly how sound is refracted.

In the simulation which follows, the effects of the medium on sound propagation can be visualized. The user can generate a variety of sound-speed profiles and wind-speed profiles by clicking on the profile choices and dragging the red dots to establish amplitudes. Two sound sources are available: a spherical source, in which initial sound waves emanate uniformly in all directions; and a planar source , in which initial sound waves emanate in a single direction. The location of the source and it orientation can be changed by dragging the red dots. Sound propagation in this simulation is in two dimensions; and media profiles depend on height only. Pressing 'Start' will begin the simulation. Propagation is represented both by rays (black) and wavefronts (red). Note that the sound speed C0 is artificially low to accentuate the effects of the medium. (Sound speed in air is nominally 340m/s; in water, 1500m/s.) Data, including sound speed, wind speed, and derivatives, may be obtained by clicking anywhere within the orange propagation field.

Begin the simulation