Sound in Motion: Anatomy of a Simple Wave
The easiest answer to the question “What is sound?” is that sound is energy that transmits through matter, called the medium, in the form of wave. Sound energy takes the form of acoustic vibrations and the medium that transmits most of the sounds we hear is air molecules (although sound can also move quite well through denser matter, like water). Because we can’t see sound moving in air, let’s compare the process to a process that we can see, and that is familiar to most people: waves moving across water. As we compare both processes for similarities and differences, we can use the process that is easy to visualize as a framework to help us “see” the physical attributes of the sounds we hear.
When you drop a pebble into a smooth pond, what happens? You get ripples that move out in all directions from the location where the pebble hit the water. These ripples are created by the kinetic energy released as the pebble strikes the water’s surface. Some ripples will travel smoothly through the water in one direction until their energy becomes diffuse and fades away. Others will react with objects in their way (bounce back, recombine with other ripples), transferring some of their energy each time they react with those objects, until eventually they expend enough energy that they too die out. Over time, all of the kinetic energy radiating from the initial source and carried along with the ripples is dissipated and the water’s surface returns to its original state, a smooth pond.
Sound interacts with our environment in approximately the same way. From where a sound is initiated by a source (handclap, gunshot, etc.), the ripples of energy (in this case acoustic energy) move out in all directions. Like the ripples on a pond, the sound waves created travel outward from the sound source in all directions. If unimpeded they will travel smoothly, if not they will interact with objects in the way first. (One example of sound interacting with objects in its path is when sound waves bouncing back from a large object produce an echo.) Either way the sound will eventually transmit all its energy until it cannot be heard anymore.
Back at the pond, when the pebble releases the energy of its movement, the water itself does not travel. Instead, the water molecules become excited by the expanding energy wave and quickly cycle in place before passing the energy to neighboring water molecules as the wave propagates. The up and down oscillations of the water molecules create the wave shape we are all familiar with. At the crest of each ripple, all the energy of the wave has pushed the water together, making it rise up. When the energy that created the crest has moved on, the molecules suddenly have extra space between them, creating a trough or dip in the water. From this dip the water molecules return to rest at their original positions as the wave energy transfers to the water further out. Depending on how much energy was released by the pebble, for a short while additional waves of energy may follow the first, before all the energy is spent and the pond returns to its original, calm state.
Sound does something quite similar to air molecules as it travels through them as sound waves. From where the sound starts, the acoustic energy spreads out through the air causing the molecules to cycle in place as they pass the energy along to the molecules further out, repeating this cycle until all the energy waves created by the sound source have passed. As in the water wave, where the energy of the traveling wave pushes the water molecules into the crest of the wave, the air molecules excited by sound energy become compacted together into an area of high pressure. In audio we call this the compression part of the wave cycle.
Then, much like the water drops into a trough as the crest of the wave energy has passed on, as a sound wave’s energy moves past, the compressed air molecules no longer have the acoustic energy pushing them together, and snap back out the same relative distance of the compression. In other words the air molecules refract back the same energy that is no longer present, resulting in a brief pulse of low pressure roughly equivalent in value to the high-pressure pulse of the compression that preceded it. This low-pressure fluctuation is called the rarefaction part of the wave cycle.
Finally, just as the water springs to its original position after dropping into a trough, the air returns from rarefaction to rest once again at its original state, the ambient pressure of the atmosphere (also called atmospheric pressure). In audio, the level at which the air molecules rested originally, before being compressed or refracted, is called the zero crossing point. At this point in the process, after the wave has passed beyond that position, the medium has no more of the compression or rarefaction energy that created the pulse of high- and low-pressure fluctuations, and will remain at rest unless another wave moves through with the energy to repeat the cycle.
The last thing we need to look at before we leave the pond behind are two key qualities that make sound waves different from water waves. The two waveforms are so similar that virtually all visual examples of sound waves show the iconic upward swells and downward dips, or the radiating nested circles, of waves on water. This is because water waves are easier to represent in two dimensions. This practice makes it easier to represent and label the components of sound waves in print, but in doing so it disguises a big difference between the two. While their components and the processes that move them work the same, the way they move is not exactly the same. The first key difference between sound and water waves is the direction of their oscillations. Water waves have oscillations that are perpendicular to the outward direction the wave energy is traveling and so are called transverse waves. Sound waves are longitudinal waves because their fluctuating air pressure oscillates in the same direction as the wave energy expanding outwards from the source.
The next key difference between sound and water waves is the direction they move as they expand outward. Both types of waves expand outward from their source, but water waves are confined to the two dimensional surface plane of the water by the force of the water’s surface tension, radiating in a 360 degree circle, but only along a single plane. Sound energy is not confined to a single plane, so the energy traveling in all directions from the source creates an expanding three-dimensional sphere, similar to a balloon inflating (note that this sphere is distorted when the sound meets objects and barriers). Keeping these two differences in mind will allow you to visualize sound moving outward from its source the way it really behaves, not as undulating up and down waves traveling out from the source 360 degrees on a flat plain, but as pulsating bands of high and low pressure moving out from the central sound source like an expanding bubble.
Excerpt from Basic Live Sound Reinforcement: A Practical Guide for Starting Live Audio by Raven Biederman and Penny Pattison © 2013 Taylor & Francis Group. All Rights Reserved.