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Microphones are nothing more than sound sampling devices; they tell us about the sound waves as they pass by their tiny diaphragms, and from that signal, replayed over loudspeakers we deduce the larger recording environment in our mind. Sound waves are in fact nothing more than localised modulation of atmospheric pressure, and you can imagine how weak and diffuse their energy is (compared to atmospheric pressure).
The very earliest recording of a sound field (above) was made in 1881, when a series of between ten and eighty telephone microphones were arranged cross the stage and connected by individual wires to telephone receivers in a nearby hotel. Visitors could select a receiver for each ear and listen to the live sound from the hall. The width of the image would have depended upon which receivers they selected: No.1 (left ear) used with No.10 (right ear) would have given a wide and diffuse field with a hole in the middle; No.5 and No.6 would have painted an intense binocular-like perspective of the centre of the orchestra.
By the 1930's both loudspeakers and the means of making recordings were established thanks to the pioneering work of the USA's Bell Labs. and the UK's Alan Blumlein inventor of the so called 'Blumlein pair' or 'crossed figure-of-eight' ribbon microphone recording technique. |
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The above picture shows a typical minimalist modern recording set-up. Just two microphones of cardiod pick-up pattern have been arranged at 90 degrees to each other, some distance back from the performers so as to include all of them in the angle of acceptance.
Accurate stereo reproduction places a performer exclusively 'in' the left loudspeaker if he/she is actually on the extreme left edge of the performing stage picked up solely by the left facing microphone. Conversely, a performer hard right would emanate from the right loudspeaker alone.
Much more challenging is reproducing a performer dead central, picked up equally by both microphones because performers anywhere than at the extremities of the sound stage are spaced on the loudspeaker soundstage entirely in the mind of the listener. The central image is psychoacoustically extremely critical and is called the 'phantom mono image', and should appear ethereally hanging in the air and not at all 'projected' by either loudspeaker: a true curtain of sound.
As we will see on the next page, the all-important phantom image keys the entire left-right sound stage's depth and width perspective. The correct perceived energy in and around the phantom image is is a design characteristic of Harbeth's life-like, natural sounding stereo.
Our interpretation of a virtual sound stage across the plane of the loudspeakers is due to the directionality of our outer ear and the mental map we individually build of the world we live in. Our auditory system primarily exists to give us accurate localisation of threatening sounds around us: if we hear a twig snapping behind us we instinctively turn and examine the threat. The stereo illusion persuades our auditory system's twenty million years of evolutionary fine tuning into believing that two loudspeakers (or even five or seven) are an real solid soundscape. Few loudspeakers can execute this illusion properly - as one would actually hear at the recording; most 'spice-up' the sound which to our trained ears destroys the subtle illusion.
For Harbeth users, the thrill is in the clarity of sound; that makes for a better deception and rewarding listening experience. But there are many alternative design solutions as we will see . . . |
Pictures: (Top) The new stereo soundbook Everest/Streicher ISBN 0-8306-3903-9
(Bottom) Music, Physics and Engineering Harry F. Olson ISBN 0-486-21769-8 combined with Broadcast Sound technology Michael Talbot-Smith ISBN 0-240-51355-X (all recommended reading) |
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