What are soundwaves? Definition and examples

From a human perspective, sound is received via the ears and then interpreted by the brain. Sound reaches the ears because it is a vibration which can be transmitted through gases, liquids, or solids: “a mechanical disturbance […] that propagates through an elastic material medium”. All sounds, then, are made by something shaking (whether this is perceptible or otherwise). These vibrations – a “pattern of disturbance caused by the movement of energy traveling” – are soundwaves. (Alternatively, they may be known as ‘acoustic waves’, particularly in relation to sounds outside of human hearing.)

Soundwaves don’t transmit particles of air (or other media) from their source to the ear of the listener – rather, the vibrations making up the wave cause a knock-on effect from one particle to the next; the vibrations are passed on by particles moving back and forth, but the particles don’t themselves travel. Because these particles vibrate in parallel to the direction in which the sound travels, this makes soundwaves an example of longitudinal waves. (In contrast, transverse waves cause particles to vibrate latitudinally or perpendicularly to the direction of the wave; think of ripples on water, or even a Mexican wave.) 

The nature of soundwaves means that they can bounce off of obstacles (particularly hard, smooth ones); this is the cause of echoes and reverberations. Rough or soft surfaces, on the other hand, will absorb more soundwaves, rather than reflecting them.

The loudness of a sound depends upon how much the particles transmitting its soundwave move; a loud sound (one with a large amplitude) is caused by large movements and a quiet one by smaller movements. Similarly, the pitch of a sound is caused by the speed of the particles’ vibrations: the quicker the vibrations, the higher the pitch; the slower the vibrations, the lower the pitch. 

Vibrations per second – the frequency of a sound – is measured in hertz (Hz), with the highest pitch audible to humans being 20,000 Hz and the lowest around 20. Owls and bats – which, as nocturnal hunters, rely much more on their hearing – are able to perceive far higher frequencies, while greater wax moths (Galleria mellonella), which have “the best hearing in the animal kingdom”, can hear frequencies of up to 300 kilohertz (kHz; one kilohertz is equivalent to 1,000 hertz). At the other end of the spectrum, the animal with the best capacity for hearing infrasounds (sounds below the range of human hearing, AKA low frequency or subsonic sounds) is the humble pigeon, which can perceive frequencies as low as 0.5 Hz, meaning that it can detect events like volcanic eruptions and earthquakes before they become obvious to humans.

Soundwaves can be visualized through the use of an oscilloscope; recording a sound transduces it from one form to another, in this case into an electrical signal. An oscilloscope can display such signals as a wave trace: a graph which appears as a wiggly line. 

While it’s helpful to see sounds visualized as a wave, this is also misleading, since wave traces present soundwaves as transverse waves (up-and-down movements) rather than the longitudinal ones (side-to-side) that they are in reality. Nevertheless, wave traces show the size of particles’ vibration on the vertical axis, against duration on the horizontal axis. As loud sounds cause large movements of particles, wave peaks on the graph will be large in this case, but smaller for quieter sounds. Because high-pitch sounds cause fast vibrations, peaks will then be close together, while, for lower pitches, the peaks will be more spread out. Without any sound, a wave trace simply displays as a straight line.

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Featured photo by Pawel Czerwinski on Unsplash