Why does the auditory system analyse sound by frequency ?
Some animals do not analyse sound by frequency, but simply transmit the pressure waveform at the ear directly. We could do this by having hair cells on the eardrum. But instead we have an elaborate system to analyse sound into its frequency components. We do this because, since almost all sounds are structured in frequency, we can detect them, especially in the presence of other sounds, more easily by “looking” at the spectrum than at the waveform. In the six panels below, the left-hand column shows plots of the waveform of a sound – the way that pressure changes over time. The right-hand column shows the spectrum of the sound – how much of each sine-wave you have to add together in order to make that particular waveform. The upper panel is a sine wave tone with a frequency of 1000 Hz. A sine wave has energy at just one frequency, so the spectrum is just one point.
