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How do we hear? - Introduction
Hearing gives us the ability to recognise the identity and content of sound. Is the troubling noise a dog barking or a running car engine? With hearing we can establish the direction of where the sound is coming from, and via echoes and reverberations we know in which environment the sound originated.
The ear has become an engineering masterpiece converting sound energy to mechanical energy and nerve impulses. The ability to discern differences in sound intensity, pitch, timbre (the tone colour or quality), and duration is essential to human communication. The ear is also a highly sensitive organ. It can recognise pitch fluctuations of just 0.3 % and can register sounds as short as 30 milliseconds. If this is not enough, the ear keeps you in the balance – literally.
It does this feat for 24 hours a day. Yes, even when your mind is at rest during
sleep, the ear keeps on working. While this was part of our survival capacity many
years ago, when noises at night could mean danger; today, in our urban environment
with its abundance of noises, the constant alertness is often the reason for sleepless
nights and stress-
Since sound is a fluctuation in air pressure, we have to assume that our hearing threshold depends on the intensity of the sound wave. The flutter of a butterfly’s wing is not intense enough to register in our ears, whereas the scream of a jet engine is so powerful to be painful. Our ability to hear also depends on the number of waves that arrive at our ear drum per time interval. In the average, we don’t register sound waves below 20 Hz and above 20,000 Hz. With age, the upper limit reduces steadily and depends also on a person’s health status and fitness level.
Our ears don’t react linear either. At low frequencies the sound has to be much louder to be heard than at higher frequencies. For example, a sound with an intensity of 80 dB and a frequency of 20 Hz will be sensed as having the same loudness as a 20 dB sound at 1,000 Hz. The lowest and highest possible sound pressure level at a particular frequency is known as the hearing threshold. When the level becomes unbearable, however, we talk of the pain threshold.
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