Sound & Volume

Sound is pressure moving through a medium - air, water, wood, metal, even your body. No single particle travels from source to ear. The push is passed along, particle to particle, like a wave through a crowd.

Press the button below to start a pure tone and watch what happens.

Machine 1 - Pure Tone

Volume -5.8 dB

What just happened

Your speaker pushed and pulled the air beside it. That pressure change passed from particle to particle at about 343 metres per second until it reached your ear.

Sound is a pressure wave. It travels through liquids, solids, and through your body - not just air.

Look at the oscilloscope in the navbar while it plays. The line is the electrical signal going to your speaker. Up means the speaker cone pushes outward. Down means it pulls inward. The middle line is the resting position.

The sine wave

This signal is a sine wave - the simplest possible vibration. One smooth hill, one smooth valley, repeating forever. A tuning fork does approximately this. Nothing in nature does it exactly, because nature doesn't do "perfectly boring."

The tone vibrates at 261.63 times per second. Musicians call this middle C, or C4. We cover what those numbers mean in the next lesson. For now: that one repeating pattern is all it takes to make your brain perceive a sound.

The oscilloscope line glows and shifts colour with the signal. This is intentional - we like instruments to look the way they sound.

Amplitude = volume

Drag the volume slider while the tone plays and watch the oscilloscope. The wave gets taller when you turn it up and shorter when you turn it down. Same shape - just bigger or smaller swings.

That height is called amplitude: the distance from the centre line to the peak. It is what your brain perceives as volume.

Loud

Quiet

Amplitude = distance from centre to peak

Try this: play the tone and slowly drag volume all the way down, then back up. Watch how the oscilloscope wave shrinks to a flat line and grows back. The shape never changes - only the size.

Decibels

The number next to the slider says something like -7.5 dB. That unit - the decibel - is one of the most beautifully deranged units in all of science.

Decibels are logarithmic. Every increase of 10 dB means the sound energy has multiplied by 10. Every 20 dB means the energy has multiplied by 100. This seems like a terrible way to measure something until you learn why: your ears are logarithmic too.

The quietest sound a healthy human ear can detect is about 0 dB SPL - roughly 20 micropascals of pressure. A jet engine at close range is about 140 dB SPL. The difference in actual energy between these two is a factor of 100,000,000,000,000 (one hundred trillion). Your ears handle this entire range and make it feel like a smooth gradient from "quiet" to "loud."

The volume slider uses a cubic curve: gain = slider position cubed (after normalising to 0--1). The bottom of the slider gives fine control over quiet volumes; the top covers the loud end more coarsely. This matches how your ears work. A linear slider would feel wrong: the first 10% would seem to do nothing, then it would get loud too fast.

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