Waves and sound · Process
How sound is made
A guitar string, a loudspeaker cone and two folds of tissue in your throat all make sound. What do the three have in common?
Start here
Hum with your fingers on your throat.
Two fingers on the front of your throat, and hum. Now stop humming and breathe out through an open mouth, so air still pours past the same place.
If you had to guess, what makes the humming sound: air rushing past, or something shaking?
Fair guess. Your breath does keep the hum going. But breathing out with no hum moves even more air and makes no note: air rushing past is not enough on its own.
Good guess. You can feel it under your fingers. Two folds in your throat shake, and that is what makes the note.
Every sound starts with something vibrating, which means shaking quickly to and fro. Next: the four stages from source to listener, and a bench where you change what is vibrating.
Every sound starts with something vibrating — moving quickly to and fro about one place. The vibrating surface pushes on the air touching it, then pulls away from it, then pushes again. That gives the air a shove-and-release, over and over, and the disturbance travels outwards.
At the far end the same thing happens in reverse. The arriving disturbance pushes and releases whatever it meets, so a thin stretched sheet is set vibrating in time with the source. In a microphone that sheet is called a diaphragm, and its movement is turned into a changing electrical signal. In your ear it is the eardrum, and its movement is turned into signals along a nerve. A loudspeaker runs the same chain the other way: a changing electrical signal drives a cone back and forth, and the cone does to the air what your vocal folds do.
The figure
Four stages, every time
Take away stage one and there is nothing to send. Take away stage two and there is nothing to carry it. Stage four is the only stage that is not sound: by then it is electricity, or a nerve signal, and it is on its way somewhere else.
At the bench · one source, one detector, the air in between
Change what vibrates. The chain does not change.
Change a control to begin
A source at one end of the bench and a detector at the other, half a metre apart in ordinary air.
Commit first. A tuning fork is struck and rings. Someone lays a finger on the prongs and the note stops instantly. Why?
What is vibrating
What is listening
What moves at the source
—
—
How far it moves
—
typical, and greatly exaggerated in the drawing
How many times a second
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What comes out at the far end
—
Key fact
Every sound begins with something vibrating. The vibration pushes and releases the air, the disturbance travels through it, and at the far end it sets a thin sheet vibrating in time — a microphone diaphragm or your eardrum — which passes the pattern on as an electrical or a nerve signal. Stop the vibration and the sound stops at that instant.
Think again
“Sound is made by the air.”
The air is the delivery, not the maker. Hold a struck tuning fork in front of a candle flame and the flame flickers in time with the prongs: the fork is doing something to the air, not the other way round. That is why touching the prongs kills the note at once, while the air in the room is untouched and carries on exactly as before. Something has to vibrate first. The air is what is next in the chain.
“If you cannot see it moving, it cannot be vibrating.”
Almost every vibration that makes a sound is far too small and far too fast to see. A guitar string is the rare exception, and even then you only see a blur. Rest a struck tuning fork on a table top and the note jumps in volume, because the whole table is now vibrating too — and the table looks perfectly still. Sprinkle a few grains of rice on a loudspeaker cone playing a bass note and they jump about; the rice is doing the seeing for you.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Apply the rule
A wine glass is tapped and rings. Someone grips the rim and the note stops instantly. What has happened?
Rung 2 · The one that catches people
A microphone and a loudspeaker contain almost the same parts. Which statement is right?
Rung 3 · Explain
Describe the whole chain from plucking a guitar string to hearing the note, naming what happens at each stage.
Rung 4 · Take it somewhere new
You phone a friend. Trace the chain from your mouth to their ear, and say clearly which parts of it are sound and which parts are not.
Key note
Sound is produced by objects vibrating. The vibrating surface pushes and releases the air touching it, and the disturbance travels outwards. At the far end it makes a thin sheet vibrate in time with the source: a microphone diaphragm, which turns that movement into a changing electrical signal, or your eardrum, which turns it into signals along a nerve. A loudspeaker runs the same chain backwards, its cone driven by an electrical signal.
Going further
The reversibility of the chain is not a curiosity, it is engineering. A cheap intercom uses one small loudspeaker as both speaker and microphone, switching which end of the wire is driving which. Older headphones can be plugged into a microphone socket and shouted into, and they work — badly, but they work — because a coil moving near a magnet generates a voltage whether you meant it to or not.
Not everything that vibrates gets its own sound out into the room. A tuning fork held in the air is quiet, because two thin prongs push very little air; stand its base on a table and the note leaps in volume, since the whole table top is now being driven and a table top pushes a great deal of air. Nothing has been added to the fork, and it dies away faster than before. The same energy is simply being handed to the air more quickly.
Before this lesson
Connects to
At GCSE this becomes
- The range of human hearing, sound as a longitudinal wave in a solid, liquid or gas, and how the ear converts pressure changes into electrical impulses.
Where to next
Ask MrBadmus
Got a source you cannot work out the chain for?
The bench is a teaching model. Every vibration in it is drawn many times larger than it really is: a tuning fork prong moving half a millimetre would be invisible at this size. The distances the sources move and the number of times a second they move are typical values for one ordinary example of each and vary widely with size, tension and how hard the thing was struck; only the 440 times a second of a concert A tuning fork is a fixed figure, and that is a convention rather than a measurement. The loudspeaker and microphone drawings leave out the coil and magnet that do the converting. The columns of air are drawn bunched and spread to show where the air is squeezed; real air is not in rows.
Lesson content © MrBadmus.