Definition · Plain-language
Frequency and amplitude
Frequency is how many complete cycles of a wave occur each second; amplitude is the size of the wave’s oscillation from its rest position.
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Frequency: how often the wave repeats
Frequency counts how many complete cycles of a wave occur in one second. Its unit is the hertz (Hz), where one hertz means one cycle per second. A wave that completes 50 cycles each second has a frequency of 50 Hz. Frequency is closely tied to the period — the time for one cycle — as the two are reciprocals: a higher frequency means a shorter period. For sound, frequency determines pitch; for light, it determines colour. Frequency is a property of the source and does not change when a wave passes between materials.
Amplitude: how big the oscillation is
Amplitude measures the size of a wave: the greatest distance it moves from its undisturbed, rest position. A tall wave has a large amplitude; a small ripple has a small one. Amplitude reflects how much energy the wave carries — a louder sound or a brighter light corresponds to a larger amplitude. Crucially, amplitude is independent of frequency: you can have a loud low note or a quiet high note. Amplitude tends to decrease as a wave spreads out or loses energy, which is why distant sounds are fainter.
How they shape sound and other waves
The two quantities map neatly onto what we perceive in sound. Frequency sets the pitch: a high frequency is a high-pitched note, a low frequency a low one, with the human ear hearing roughly 20 Hz to 20,000 Hz. Amplitude sets the loudness: a larger amplitude is a louder sound. The same pairing applies elsewhere — for light, frequency relates to colour and amplitude to brightness. Because the two are independent, a single wave is fully described only when both its frequency and its amplitude are known.
Key facts
At a glance
- Frequency: the number of complete cycles per second
- Frequency unit: hertz (Hz), one cycle per second
- Amplitude: the maximum displacement from the rest position
- Amplitude relates to: the energy the wave carries
- For sound: frequency sets pitch; amplitude sets loudness
- Independence: frequency and amplitude can vary separately
Common misconceptions
What people often get wrong
Often heard: A higher-frequency sound is automatically louder.
Actually: Frequency controls pitch, not loudness. Loudness depends on amplitude. A high-pitched note can be quiet and a low-pitched note can be loud — the two properties are independent.
Often heard: Amplitude and frequency are just two names for the same thing.
Actually: They measure different features of a wave. Frequency is how often it cycles each second; amplitude is how large each oscillation is. One can change while the other stays fixed.
Often heard: A wave slows down its frequency when it gets weaker.
Actually: A weakening wave loses amplitude, not frequency. As a sound fades with distance its amplitude shrinks, but its frequency — and therefore its pitch — stays the same.
Going deeper
