The softest audible sound has an energy equivalent to that given off by a 50-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 atmospheric pressure (10^-9 atm).
It can also respond meaningfully to sounds up to 10 trillion (1013) times greater in intensity, at the threshold of pain. This range of response is so vast that we tend to measure it using logarithms (the decibel scale is logarithmic). An intensity increase of 1dB is roughly the smallest audible level change, a factor of 10 increase in intensity (a 10dB increase) sounds to us as something like a doubling of volume. Because of this we sometimes forget the vast differences of scale in the sounds we listen to. A rock band playing at 90dB is actually producing sounds a billion times more intense than the softest whisper.
As if this were not demanding enough, the ear must also respond to waves recurring within frequencies between 15Hz (cycles/sec) and about 25kHz. 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, (or pinna), is partly a sound-gathering device. It helps us to hear more easily those sounds that come from in front of us, but also subtly alters sound depending on the direction it comes from. Adding to that the fact that we have two ears gives the brain the ability to extract good spatial information from sound; ie we have directional hearing. Using just a single ear, we can often locate sounds surprisingly well, especially with the assistance of slight head movements.
The outer ear focuses the sound waves into the auditory canal, which ends in the tympanic membrane, or eardrum. This membrane vibrates with the received sounds, and thereby passes sound into the middle ear.
The membrane is connected to the first of three small bones in the middle ear - the 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 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 three 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. Combinations of hair cells, which are resting in the fluid, pick up fluid waves. The hair cells are in different lengths and in different positions so they perceive sound differently. The hair cells trigger nerves that rest in a particular configuration to detect sound.
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 balance-sensing organs are located near the cochlea, and inner ear problems or infections can also affect balance.
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 (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 philosophical question that 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! Sound, radio transmissions, and even light are all merely manifestations of energy. If the energy is there, so is the manifestation; irrespective of someone being present to perceive it.