Sound Wave
Light, Sound and Waves

How are we able to hear?

Physics Narrative for 11-14 Supporting Physics Teaching

Vibrations from source to detector – that's how we hear

So, how is it that you are able to hear the radio playing in all parts of the house or the bacon sizzling or the notes from the guitarist at a concert? The guitarist is up there on the stage and you are in the audience. What happens to allow you to hear the sounds from the guitar, which is at least 20 metre away?

The first step in answering this question is to treat the act of hearing as involving a chain from source (which is vibrating) to medium (which enables the vibrations to pass) to detector (which in this case is you!) For vibrations to travel from source to detector there must be particles of matter in the gap, and these form the medium. If there are no particles then there is nothing to carry the sound from source to detector.

The medium transmits information from the source to the detector.

Source and detector

All sources of sound have the same kind of to and fro motion. All sources of sound vibrate. It is an interesting exercise to think of a sound (yes, any sound!) and trace it back to the vibration that is producing it.

Here are some examples: voice, vibration of vocal cords; notes from a sitar, vibration of a string; toot of a trumpet, vibration of lips at mouthpiece.

The vibrating source acts on the medium around it, setting the medium moving to and fro, in a way that matches the motion of the source.

The detector also vibrates as it is set in motion by the medium adjacent to it. The motion of the detector matches the to and fro motion of the medium, so the sound is carried from source to detector – transmitted by one block of particles acting on the next.

Density varies as the vibration travels

What happens in the medium to allow the sound to pass? If the medium is air, the air particles are initially in a state of random motion.

Here we show what happens as the cone of a loudspeaker acts on the air in front of it. As you can see, the to and fro motion of the loudspeaker cone produces changes of density in the air.

As the cone moves forwards, the air particles are squashed together to make a region of high density. As the cone moves backwards, the air particles in front of it spread out to form a region of low density.

The animation helps to visualise the patterns of high and low air density. These are first created directly in front of the loudspeaker cone and then farther away, as the disturbance in one block of air affects the next block of air. In this way the sound travels out from the loudspeaker cone through the surrounding air.

What sound is

As the disturbance of high and low-density regions travels out through the air, each block of air moves backwards and forwards following the motion of the loudspeaker cone. If you're not sure about this, step back to the previous screen and review the animation there. Each block of air moves backwards and forwards. It is not the case that the block of air that starts directly in front of the loudspeaker cone ends up at your ear. The sound travels through the air but the air itself simply moves backwards and forwards.

You might ask the question:

Teacher: But what exactly is the sound?

Well, as we've emphasised, the cone moving to-and-fro is not the sound. Rather it is the source of the sound.

Equally, the detector is not the sound.

The sound is actually the disturbance travelling through the air, the to-and-fro motion of the air that creates the pattern of high and low-density regions. That is what we detect with our ears.

Making connections to energy descriptions

Loudspeakers enable you to hear by turning the to-and-fro movements of electrical currents, which encode the sound, into the to-and-fro movements of air. Later, and remotely, your ear is affected by these to-and-fro movements. The loudspeaker is working remotely on your ear. While it is working, you can hear a sound. Once it stops working, the sound stops. The sound must happen over time: you cannot freeze it. So sound, or, more precisely, hearing a sound, is a process. So it cannot be sensibly thought of as an energy in a store, which is as a result something that has happened (there is much more on this in the SPT: Energy topic).

In everyday life this makes perfect sense: You buy a sound system rated by watts, not joules.

That is a clue that making connections between hearing and energy descriptions is best done through power. The sounding is a pathway that empties or fills stores of energy.

Our ears are very sensitive, so the power is tiny (150 W of illumination in a room is bright, but 150 W of sound system running at full power is just painful). So as a means of filling or emptying stores sound is very ineffective. But for those who like a neat systematic view sound is a pretty good match for the mechanical working pathway.

Teacher Tip: Hearing sounds is linked to the energy description via power in pathways—in fact it's best thought of as the mechanical working pathway, because of the mechanisms at either end.

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