Beats — Definition
Definition
Imagine you have two sound sources, like two tuning forks, that are producing sounds at almost, but not exactly, the same pitch. For instance, one might be vibrating at 440 Hz (Hertz, cycles per second) and the other at 442 Hz. When these two sound waves travel through the air and reach your ear simultaneously, they combine or 'superimpose'.
What happens when they combine? Sometimes, their peaks (compressions) and troughs (rarefactions) align perfectly, causing them to add up and create a louder sound. This is called constructive interference. At other times, the peak of one wave might align with the trough of the other, causing them to cancel each other out, resulting in a quieter sound or even silence. This is destructive interference.
Because the two frequencies are slightly different, this alignment and misalignment don't happen constantly. Instead, they occur periodically. For a moment, the waves are in phase, making the sound loud. Then, as one wave slowly 'lags behind' or 'gets ahead' of the other due to its slightly different frequency, they gradually move out of phase, and the sound becomes soft. This cycle of loud-soft-loud-soft repeats itself.
This periodic variation in the loudness of the sound is what we call 'beats'. You literally hear a 'wa-wa-wa' or 'throb-throb-throb' sound. The rate at which this loudness fluctuates – how many times per second you hear the sound get loud and then soft again – is called the 'beat frequency'.
Remarkably, the beat frequency is simply the absolute difference between the frequencies of the two original sound waves. So, for our 440 Hz and 442 Hz tuning forks, the beat frequency would be Hz.
This means you would hear the sound get loud and soft two times every second. This phenomenon is a direct consequence of the principle of superposition of waves and is a beautiful demonstration of wave interference.