3.28P-3.29P - Pitch, loudness and amplitude
Sound begins with a source that vibrates. Two features of that vibration matter in this lesson: how often it vibrates each second, and how far it moves from its rest position. Edexcel expects you to connect those two source properties to the sound heard: frequency controls pitch, and amplitude controls loudness.
Sound from vibrations
A sound source is an object or system that produces vibrations. A tuning fork, a guitar string, a loudspeaker cone and a human vocal cord can all act as sound sources.
When the source vibrates, it disturbs the air around it. Those disturbances travel away as a sound wave. You do not need the practical details of microphones or oscilloscopes in this lesson; the important starting point is that the sound wave copies two useful features of the source vibration:
| Source vibration feature | Sound feature heard |
|---|---|
| frequency of vibration | pitch |
| amplitude of vibration | loudness |
Keep the phrase of the source in mind. This lesson is about the source's vibration: how often it vibrates and how far it vibrates.
The speed of the sound is not one of the features the source controls. Speed is set by the material the sound travels through, so in the same air, high and low, loud and quiet sounds all travel at the same speed.
Pitch and frequency
Pitch describes how high or low a sound seems. A whistle has a higher pitch than a deep drum note. A bass guitar note has a lower pitch than a violin note.
The pitch of a sound is related to the frequency of vibration of the source. Frequency means the number of complete vibrations each second, measured in hertz (Hz): 1 Hz is one complete vibration each second. A source vibrating more times each second produces a higher-frequency sound, so the sound has a higher pitch. A source vibrating fewer times each second produces a lower-frequency sound, so the sound has a lower pitch.
For example, if two tuning forks vibrate with the same amplitude but one vibrates at 880 Hz and the other at 440 Hz, the 880 Hz fork produces the higher pitch. The important comparison is frequency. The amplitude has been kept the same, so loudness is not the reason for the pitch difference.
Do not write that a sound is higher-pitched because it is louder. Loudness and pitch are different observations, linked to different features of the source vibration.
Loudness and amplitude
Loudness describes how loud or quiet a sound seems. A shout is louder than a whisper. A strongly struck drum is louder than the same drum tapped gently.
The loudness of a sound is related to the amplitude of vibration of the source. Amplitude is the maximum displacement from the rest position. For a sound source, that means how far the vibrating object moves from its middle position during the vibration.
Continue this lesson.
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