The two sound waves should have a rhythm in order to create beats.
Option d). slightly different frequencies
The frequency of an event is its repetitions per unit of time. In contrast to spatial frequency, it is also sometimes referred to as temporal frequency, and in a contrast to angular frequency, it is also referred to as ordinary frequency. Hertz (Hz), which equals one (event) per second, is the unit of frequency. The reciprocal of frequencies is period, which is defined as the length of time for one cycle of a recurring occurrence. For instance, if a heart beats 120 times per minute (2 hertz), its period (T), or the space between beats, is half a second (60 seconds divided by 120 beats). In science and engineering, frequency is a crucial specification parameter.
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