IGCSE Physics / Sound
Pitch and loudness revision notes
Edexcel International GCSE Physics (4PH1), chapter 13: Sound, specification point 3.28P, 3.29P. Free lesson.
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Pitch depends on the frequency of vibration, while loudness depends on its amplitude. These are separate features, so a sound can change in pitch without changing in loudness, or vice versa.
What controls pitch and loudness
The pitch of a sound is how high or low it seems. It depends on frequency, which is how many times something vibrates each second. Loudness depends on amplitude, which is how far the source moves as it vibrates. Because they have different causes, pitch and loudness can change independently. Higher frequency means higher pitch. Bigger amplitude means a louder sound. On an oscilloscope trace, closer waves show a higher pitch. Taller waves show a louder sound.
How to compare oscilloscope traces
Compare the width and height of the waves separately. The width shows the period, the time taken for one complete wave. A shorter period means a higher frequency. The height shows amplitude, so a taller trace means a louder sound. With a time-base of 1 ms per square, a wave covering 4 squares has a period of 4 ms, or 0.004 s. Frequency = 1 ÷ period, so the frequency is 1 ÷ 0.004 s = 250 Hz. If waves are twice as close together, the frequency is twice as high. If the heights are the same, the sounds are equally loud. If waves have the same width but one trace is twice as tall, the taller trace…
How strings change the note
A string produces a higher note when it vibrates faster, because faster vibration means a higher frequency. Changing the string or how you pluck it can affect different sound features. A shorter string vibrates faster and produces a higher note. A tighter string also vibrates faster and produces a higher note. Plucking the same string harder gives a bigger amplitude. The sound is louder, but the pitch does not change.
How to compare two sounds
For a comparison question, use the trace width to describe pitch and the trace height to describe loudness. Give the direction of the change and link it to frequency or amplitude. If B's waves are twice as far apart as A's, B has a lower frequency and a lower pitch. If B's waves are half the height of A's, B has a smaller amplitude and is quieter.