This is a post about a possible misconception about sound that can be gained from a common method of drawing sound waves.
Sound is caused by vibrating objects. Those vibrations cause disturbances to the particles of the surrounding medium (air, for example). Those particles disturb their neighbours, and the disturbance propagates outwards as a wave of regions of compression (higher particle density) and rarefaction (lower particle density).
Sound as a longitudinal wave
The following animation of that process comes from Dan Russell at Penn State University, and is reproduced under the CC BY-NC-ND 4.0 licence.
Imagine an object on the left vibrating in the left-right direction and imagine your ear at the right hand side of the picture. Look at how the wave propagates from left to right, even though each particle vibrates backwards and forwards about its mean position and doesn’t go anywhere overall (a few are coloured red to highlight that fact). This picture describes a ‘longitudinal wave’, which means that the vibrations are in the same direction as the wave motion.
The compressions have a higher pressure than the rarefactions, due to the higher particle density. As the wave travels from left to right and hits your ear drum, your ear drum experiences those fluctuations in pressure, and it vibrates accordingly. Sound is often described as a pressure wave.
Transverse waves
‘Transverse waves’ behave differently: the disturbance of each particle is perpendicular to the wave motion. This excellent animation is once again due to Dan Russell.
Whereas the imaginary object vibrated left and right in the first diagram, here you can imagine a similar object being moved up and down. Provided there is some ‘coupling’ of the black dots from left to right (that is, they are sort of attached to each other), then the disturbance will propagate as shown. Sound does not routinely travel as transverse waves (because the particles/molecules in air are not ‘coupled’ in that way). And yet…
What happens when we connect a microphone to an oscilloscope?
The microphone contains a ‘diaphragm’, whose role is similar to the role of the ear drum in hearing – it senses the pressure waves of sound, and oscillates in response. This oscillation of the diaphragm is then converted into a voltage. Don’t worry how – different types of microphone do that in different ways; the point for us is the generation of an oscillating voltage due to the oscillating air pressure.
Then we get a ‘wave trace’ or ‘waveform’ produced. If the sound is a pure tone, like that from a tuning fork, the waveform will be a sine wave as in the picture (most real sounds are not sine waves, but they are easy for me to draw, so you’ll have to put up with it).
And then we can investigate the properties of sounds waves, and their relationships to our human perception of the sound, by changing the volume of the sound (hit the tuning fork harder) or its pitch (get a tuning fork of a different size, and therefore a different note). We might get waveforms like those below. The height of the trace is a measure of its amplitude (so the green and blue ones are louder) and the number of waves on the screen is a measure of frequency (so the blue and yellow ones are higher pitched).
Now these pictures are graphs, created by the oscilloscope. The y-axis of these graphs is voltage (I know UK exam specifications use the term potential difference, but all the electrical scientists I know call it voltage…). The height of the wave doesn’t tell you the pressure of the wave – it tells you the voltage generated in the microphone by the sound. You would need a cleverly calibrated microphone to be able to relate the voltage to the pressure causing it.
So what’s the problem?
Well, maybe there isn’t one. But don’t these lovely oscilloscope traces look just like transverse waves? Like the ripples on the surface of water (when viewed from the side, obviously, not from above!). And sound is a wave, after all. So how easy it is to fall into the trap of starting to picture sound as a transverse wave, even when we know it is actually longitudinal. Remember, these aren’t pictures of sound, they are graphs of sound (that happen to look like transverse waves!)…
There’s a related misconception that’s weirder than this one. It concerns electromagnetic waves, which are transverse, but which can still be misinterpreted from their graphical picture. We wrote about that in “Why don’t the microwaves go through the holes?”