Psychoacoustics

Overview

Some understanding of the way in which we hear is essential to follow the discussion of different microphone techniques, the way they work, and the advantages and drawbacks of each one.

Hearing

The human ear can detect sound waves with frequencies from about 20Hz to 20kHz.  At the ends of this range, the sensitivity is much reduced.  The high frequency limit reduces with age, even when the hearing is otherwise perfect.  Considerable exposure to very loud noise can cause permanent damage to the hearing.

Directional Hearing

We are able to detect the direction of sounds.  A number of mechanisms are used for this, but different mechanisms may be effective for different sounds or situations.

Our ears are on opposite sides of the head, baffled by the pinnae.  This gives the possibility of several ways of detecting the direction of a sound, each of which may be predominant under different circumstances.

• Volume difference - the head shades each ear from sound coming from the opposite side.  At high frequencies this gives a difference in volume which is used to detect which side a sound is coming from.
• Time difference - sound from different directions arrives at the two ears with a time difference that depends on the direction.  For steady tones this is ambiguous at high frequencies, but at lower frequencies the ear uses this mechanism. When considering steady tones, this is often described as a phase difference rather than a timing.
• Tonal difference - sound from different directions will have different tonal qualities, both because of differences in the diffraction of different frequencies round the head, and because of comb filtering caused by reflections from the complex surfaces of the pinnae and the shoulders.  Of course, there is no control signal to compare with, so this is not an absolute effect; rather, we have each learned the individual effect of our bodies on the sound in circumstances where the direction is known by other means (e.g. sight), and so for recognisable sounds we can conjecture the direction - including vertically.

Movement of the head while listening can reinforce all of these effects, increasing the amount of information available for the ear and brain to analyse.  Vision also reinforces the perception of direction.

Distance

The ear has no direct way of determining the distance of a sound source.  However, there are clues that can be used effectively. 

• Tonal changes - high frequencies are absorbed quite strongly in air (more so in dry air), so tonal changes in a familiar sound give an indication of distance.
• Reflections and reverberation - the timing, intensity and buildup of reflections from walls and objects around the sound source and listener give clues which can help the listener get some idea both of the space they are in and the position of the sound source within that space.

Again, vision can help resolve ambiguity or uncertainty in another sense.