Sciences
Amplitude: Measured from the Middle, Not Top to Bottom
Amplitude is the distance from the rest position to a crest — half the crest-to-trough height. Bigger amplitude means more energy: louder sound, brighter light.
Amplitude
Amplitude is the maximum distance a point on a wave moves away from its undisturbed rest position.
- Où les élèves le rencontrent
- Grade 7 science, on the first labelled wave diagram, and again in Grade 9 physics when a sound wave is displayed on an oscilloscope.
La réponse en bref
The amplitude of a wave is the greatest distance a point moves away from its rest position. It is measured from the centre line to a crest, not from crest to trough, which is twice as large. A bigger amplitude means the wave carries more energy: a louder sound, or a brighter light.
Un exemple
Crest to trough measures 6 cm, so the amplitude is 3 cm
This is the single most commonly lost mark in the topic. The wave rises 3 cm above the rest line and falls 3 cm below it, and the amplitude is one of those, not the pair added together. If a diagram gives you the full height from the lowest point to the highest, halve it. If it gives you the distance from the undisturbed line to a peak, that number is already the amplitude.
Where you measure from
Every wave has a rest position: where the water sits when nothing disturbs it, where the rope hangs before you shake it, where the air pressure sits in a silent room. On a drawing that is the horizontal centre line, and the amplitude is measured from there to the top of a crest.
Because a wave swings symmetrically about that line, the trough is the same distance below as the crest is above. The amplitude is stated once and describes both. Measuring all the way from trough to crest gives double the amplitude — sometimes called the peak-to-peak value, a real quantity but not the one the question asked for.
What amplitude tells you about the wave
Amplitude is the energy measurement. A wave with a larger amplitude is carrying more energy past each point every second: a bigger sea wave does more damage, a louder sound moves your eardrum further, a brighter light delivers more energy to a surface.
Watch what happens as a wave loses energy and the point becomes clear. Pluck a guitar string and the note fades in volume but not in pitch, because the amplitude falls while the frequency stays exactly where it was. Shout across a field and the sound is quieter at a distance, not deeper. Amplitude and frequency are independent, and changing one leaves the other alone.
Amplitude on an oscilloscope
Connect a microphone to an oscilloscope and a sound becomes a picture you can measure. The height of the trace above the centre line is the amplitude, and it tracks loudness. The horizontal width of one complete wave is the period, and that tracks pitch. Turn the volume up and the trace grows taller; play a higher note and the waves bunch closer together without getting taller.
One detail worth being precise about: the vertical axis on an oscilloscope is voltage, not distance. The amplitude read off that screen is measured in volts and stands in for how far the air particles are moving. For a wave on a rope or on water, where displacement really is a distance, the amplitude is in metres.
Questions fréquentes
Is amplitude the distance from crest to trough?
No, that is twice the amplitude. Amplitude runs from the undisturbed rest position to a crest, so it is half the full crest-to-trough height. The larger measurement is a real quantity, sometimes called the peak-to-peak value, but questions often supply it deliberately, because halving it is the step being tested.
What units is amplitude measured in?
Whatever unit the displacement is in. For water waves or a wave on a rope it is a distance, so metres or centimetres. On an oscilloscope trace the vertical axis is voltage, so the amplitude is read in volts. The idea is the same in both cases: how far from rest.
Does amplitude affect the pitch of a sound?
No. Pitch is set by frequency alone. Amplitude sets loudness. This is why a guitar string plucked hard and plucked gently sounds like the same note at two volumes, and why turning up a radio does not make the singer sound higher.
Why does a wave's amplitude get smaller as it travels?
Because it loses energy or spreads that energy over a larger area. Ripples from a stone spread out into ever-bigger circles, so the same energy is shared along a longer wavefront and each part of it moves less. Friction and absorption remove energy too. The frequency, meanwhile, does not change.
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