What characteristic of sound is defined by its number of cycles per second?

The characteristic of sound defined by its number of cycles per second is frequency. Frequency refers to how many times the sound wave completes a full cycle—oscillating from a peak to a trough and back to a peak—within one second. This is measured in hertz (Hz). Higher frequency sounds, such as a whistle, have faster cycles per second, while lower frequency sounds, like a rumble, have slower cycles.

Understanding frequency is fundamental in the field of audiology and hearing aids. It affects how we perceive pitch; higher frequencies correspond to higher pitches, and lower frequencies correspond to lower pitches.

In contrast, amplitude reflects the strength or intensity of the sound wave, which is perceived as loudness but does not influence the pitch. Intensity is a measure of the power of the sound wave over area, also related to loudness, but not to its pitch. Quality, on the other hand, refers to the timbre of sound, which helps to differentiate between different types of sound sources, but again is not concerned with the rate of oscillation defined by frequency. Thus, frequency is the accurate characterization of sound regarding its cycles per second.