Beats from nearby frequencies
Slow modulation comes from superposing close frequencies.
Key knowledge
At a fixed observation point, equal-amplitude coherent sinusoidal components with nearby frequencies f₁ and f₂ have amplitude factor 2Acos[π(f₁−f₂)t] multiplying a carrier at their average frequency. The rate of successive loudness/intensity maxima is |f₁−f₂|; the envelope’s signed cosine has half that cycle frequency.
Values and equations
y₁+y₂=2Acos[π(f₁−f₂)t]cos[2π((f₁+f₂)/2)t]; f_beat=|f₁−f₂|
- y₁,y₂,A
- Wave variable values and each component amplitude; units of the chosen wave variable
- f₁,f₂,f_beat
- Component and beat frequencies Hz
- t
- Time s
Conditions and scope
- Linear superposition, a fixed relative phase chosen as zero in the displayed example and sufficiently close frequencies for a resolved slow envelope.
- Unequal amplitudes leave nonzero minima. The identity remains algebraically true for separated frequencies, but interpreting them as slow audible beats requires additional perceptual/bandwidth conditions.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read §17.6 superposition identity and beat frequency17.17. Distinguish signed-envelope period from intensity maxima and exclude the worked example’s unqualified audible-beat interpretation for widely separated tones.
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Entry details
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- foundation-physics-beat-frequency
- Revision
- foundations-refresh.1
- Structured reference record
- Foundational reference additions