The Human Ear

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The human ear is a truly remarkable organ.

From Whispers to Jet Engines

The softest audible sound produces an intensity equal to the light and heat given off by a fifty-watt light bulb at a distance of 3000 miles. The movement involved here is sub-microscopic - it displaces the eardrum by a distance of one-tenth the diameter of a hydrogen atom1 This is one four-millionth part of the diameter of a fine silk thread. It is a response to a sound pressure change of one billionth of an Atmospheric Pressure

It can also respond meaningfully to sounds up to 10 trillion (1013) times greater in intensity, the threshold of pain. This range of response is so vast that we tend to measure it using logarithms (decibels are a logarithmic measure). Because of this we sometimes forget the vast differences of scale in the sounds we listen to. A rock band playing at 90 dB is actually producing sounds of a billion times more intensity than the softest whisper.

As if this were not demanding enough, the ear must also respond to waves recurring with frequencies between 15Hz (cycles/sec) and about 25 kHz. With age (and misuse), deterioration will occur, particularly at high frequencies.


The ear consists of three main sections: Inner, Middle and Outer.
The Outer Ear consists of the lobe which is essentially a sound-gathering device. It allows us to hear more easily those sounds that come from in front of us. The fact that we have two ears gives a 3-dimensional perspective: we have directional hearing.

The Outer Ear focuses the sound waves into the auditory canal, which ends in the "Tympanic Membrane" or eardrum. This membrane vibrates in accord with the received sounds, and thereby passes sound into the Middle Ear.

The Membrane is connected to the first of 3 small bones in the Middle Ear: Mallus, Incus and Stapes. These are alternatively known in English as the Hammer, Anvil and Stirrup. These perform the vital function of "attenuation". It is these bones, which enable us to cope with such extremes of sound pressure. When loud sounds occur, they alter their arrangement (varying their elasticity) to attenuate (reduce) the effect. For this reason, it tends to be sudden loud sounds that can damage our hearing; the bones do not react quickly enough and too much power is transmitted through to the Inner Ear.

Another vital function of the 3 bones is to 'match the impedance' between the air in the outside world and the fluid-filled Inner Ear. When waves in air hit the surface of another medium such as water, the majority of the power is reflected because of the different Impedance (elasticity and density) of the fluid. Matching the Impedance allows more power to transfer into the other medium.

The Inner Ear is fluid filled and contains an organ called the cochlea. It is this that converts the detected vibrations into what we perceive as sound. It is only recently that scientists have begun to understand its mechanism, and is certainly beyond the scope of this article.

To prevent echoing and ringing within the cochlea, there is a so-called "round window", that allows the waves to escape instead of reflecting back along the length of the organ. Any blockage here may result in a hearing dysfunction called "Tinnitus" where noises and distortion continuously disturb the sufferer.

The "Cocktail Party Effect"

This is the strange and wonderful ability we have to focus on a single conversation in the midst of a party full of noise and other conversations. When watching an orchestra, we can decide to listen especially to the oboe, or a flute, and within reason we are able to do so. (The ability to do this depends somewhat on our musical ability and familiarity with the characteristics of instruments).

Listening is therefore an active process, that involves the brain. The more we learn about sight and hearing, the more we appreciate the role of the brain in shaping what we actually perceive, and "filling in the gaps".

No Ear - No Sound

There is an old argument which goes something like this: "If a tree were to fall in a forest and no-one was there to hear it... would it make a sound?"

Of course the falling tree would produce vibrations in the air and ground. But, for these vibrations to be perceived as sound, an organ of hearing is required. Just as I can walk past a radio mast and detect nothing at all without a radio receiver, so the movements of layers of air are meaningless to me without the Ear.


Douglas Adams in "Hitchhiker's Guide to the Galaxy" used the Babel Fish as a translation device that, once inserted into the ear, allowed the listener to understand any language. He cited the fish as so remarkable a creature that it was used as proof-positive of the existence, and then the non-existence of God. It is easy to see why he was drawn to such thoughts, when one considers the ear itself. How could such an intricate device have evolved? Like the eye, it is made up of several different constituent parts, each useless without the others; and like the eye, the ear gave Darwin grave doubts about his own theories.

The point of this article is not to answer such questions, but simply to point out the marvel of something we usually take for granted.


1 "The Fundamentals of Acoustics" by Kinsler and Frey.

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