One of the most amazing pieces of audio hardware can be found in pairs: everybody has two ears. The most visible part of the human hearing system is the ear shell, which passes acoustic sound pressure waves though the middle ear canal to the eardrum, which sits a little deeper inside the head. The eardrum then converts the air pressure level to a smaller surface using a mechanical construction that boasts the smallest bones in the human body. The stapes is the final bone of this construction, passing the converted pressure to a liquid, housed in a miniature rolled-up tube: the inner ear, or cochlea. The small surface where the stapes bone touches the cochlea’s liquid is called the ‘oval window’; it’s the internal audio input connector of our inner ears.
This might already be an ingenious system, but the magic really begins inside the cochlea, which holds a three centimetre membrane, also rolled up, which is suspended in a liquid. (the header picture shows a rolled-out view). The membrane is covered with about 3500 rows of 4 ‘hair cells’; cell bodies each with their surface populated by 40-150 miniature hairs. The hairs ‘wave’, reacting to the pressure waves transferred from the outside world to the cochlea’s liquid by the stapes bone.
The membrane is stiff and narrow at the beginning, near the oval window, and wide and sloppy at the end. This causes the membrane to resonate at high frequencies close to the oval window and at low frequencies at the end, ‘tuning’ each hair cell to a specific frequency range. This mechanism basically constitutes a 3500-band real time spectrum analyser!
So far we have described the ‘microphone’ of our hearing system. Next step is to describe how the cochlea’s hair cells are connected to our ‘signal processor’... the human brain.
Hair cells are ‘bio-electrical’ constructions, using a chemical process to convert the movement of the waving hair cells into an electric pulse with a peak voltage of about 150 millivolts. A hair cell at rest emits about 600 pulses per second. When the hairs move, as a result of pressure waves through the liquid at the specific point of each membrane, this pulse frequency increases or decreases. This creates a coded ‘envelope’ of pulse density, representing the acoustic sound pressure at a certain frequency. These pulses are sent to the brain stem.
The brain stem is the ‘multi-connector’ of the human brain, connected to all of the human body’s sensory organs (e.g. ears, eyes, nose), and to all of the human body’s motor organs – the muscles. The carriers of information – electrical pulses – are bundles of very thin bio-electrical conductors called ‘neurons’, or nerves, and can be more than a metre long, with the main multicore passing through our spine. The ears are connected to the brain stem by a nerve string of about 40cm in length, comprising around 30.000 neurons. With every one of them transmitting, on average about, 600 pulses per second, the brain stem receives a constant stream of about 18 million pulses per second. Just from one ear!
The brain stem ‘pre-processes’ this stream of information, then passes it on to a section of the brain that specialises in handling audio: the auditory cortex. What happens there is the domain of psychoacoustics, which will be the topic of next week’s micro tutorial.
Finally, at the end of this micro tutorial, a warning. As the hair cells are very delicate constructions, extreme pressure levels can rip off the hairs. The human body has a remarkable built-in recovery function, self-repairing almost all damage. Sadly, one of the exceptions is the hair cells – they don't grow back. Once they are destroyed, they are gone. Since the pressure level is the highest at the input of the inner ear, at the oval window, the hair cells near the oval window - tuned to the highest frequencies - usually break off first. This explains why hearing loss - either caused by ageing or by over-exposure to high pressure levels - often starts with the high frequencies. There’s only one solution: keep sound pressure to a safe level and protect your ears every time you walk into a too-loud environment. And if you put an acoustic environment on your head (yes, headphones), keep the volume at a safe level!