Cochlear implants – wiring for sound
Box 2 | The mathematics of hearing
The cochlear implant relies on an understanding of mathematics that was discovered nearly 200 years ago by Joseph Fourier. He was actually solving equations that describe how heat flows through objects, but his technique was also found to be particularly important for understanding hearing and sound.
Frequency of sound
Sound is caused by pressure vibrations, usually in air. The rate at which the vibrations repeat is called the frequency and it determines the pitch we hear high frequencies (fast vibrations) cause high notes, low frequencies produce low notes. The variation in pressure, when plotted against time or against distance, forms a wave pattern. The simplest sort of wave, produced when there is only a single frequency present in the sound, is called a sine wave.
A single frequency wave pattern represents a pure sound like that produced by a tuning fork. Two or more sounds occurring at the same time make a new wave pattern that is simply the individual wave patterns added together. For example, these three wave patterns of different frequencies add up to give the fourth, much more complex, wave pattern. Most of the sounds we hear have complex wave patterns that are created in this way.
What Fourier discovered is that the reverse is also possible – any complex wave pattern can be analysed into a number of simple, component wave patterns. This is called Fourier analysis and is a vital tool in mathematics, physics and engineering. It is used to study the vibrations of machinery, the structure of human speech, the sounds made by dolphins and the images of distant galaxies collected by radio telescopes.
Fourier analysis in the cochlea
Fourier analysis closely resembles the process by which we hear and understand sounds. A complex sound is spread out in frequency along the basilar membrane of the cochlea, and different nerves react to different frequencies. Our ears are natural Fourier analysers and sometimes (especially with training) you can hear the different components that make up a complex sound.
The speech processor used in the bionic ear is essentially a Fourier analyser, converting sounds into 22 different components. Signals are then sent to the electrodes distributed along the basilar membrane in the cochlea to stimulate the appropriate nerves (Box 1).
Posted February 1998.