Perception of sound: Beatings

When we have two sounds whose frequencies differ little between each other, we perceive an extra sound, similar to the other two and with a sound that is similar to a beat whose rhythm is dictated by the the two main frequencies. If these two frequencies are too different to each other, our brain no longer perceives the difference in sound. This is because for the two frequencies to be perceived as a beating they have to stimulate the lashes that correspond to the same critical band. The beatings' frequency is equal to the number of times the two sinusoids are in phase and out of phase in the space of one second. Let's use a practical example.

Let's consider two pure sinusoids with frequencies 400Hz and 405Hz respectively. When the two sinusoids are summed together a new wave-form results which is perceived as a beating and whose rhythmic flow is illustrated in the following figure:

Pure sinusoidal wave (f=400 Hz)  [Track 11]

Let's now consider another pure sinusoid with a 405Hz frequency:

Pure sinusoidal wave (f=405Hz)  [Track 12]

The sum of the two sounds can be heard in the following sound:

Sum of two sinusoids with frequencies 400Hz and 405Hz  [Track 13]

As we can hear, a new ossilation takes place. If the two frequencies were to have been more distant from one another this phenomenon wouldn't have taken place (see the example in the sound theory section [Combination of pure sinusoids ] ).

The figure shows the wave form obtained by combining the two previous ones:

Sum of two sinusoids with frequencies 400Hz and 405Hz