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An Optical Analogy
Let us use a visual analogy to aid our understanding of acoustics. Imagine that you are in a
room that is lit only by a candle in its center. There is (approximately) a uniform amount of
light cast in all directions. If a large mirror is held closely to candle, one half of the room
becomes darkened, while the other half receives twice as much light. This is because there are
effectively two candles now illuminating that half of the room, the real candle, and the virtual
(or reflected) candle. The energy that had been sent to both sides of the room has now been
concentrated in one side only.
If we repeat the same experiment using a large piece of black cloth instead of a mirror, the
results will be somewhat different. The side of the room behind the cloth is darkened, just as
before, but the level of light on the side of the candle remains unchanged. This is because the
light is absorbed by the cloth, rather than being reflected back into the room.
Thus we can see that the energy can either be absorbed or reflected. A similar situation occurs
with sound waves, although we must account for the much greater wavelengths of audible
frequencies. Of course no material is a perfect absorber or an absolute reflector. Furthermore,
the sonic absorption coefficient of a given material usually varies with frequency.
Basic Room Acoustics
The great majority of all listening rooms are rectangular, with parallel surfaces. The walls and
ceiling are typically hard surfaces, which are acoustically reflective. These large areas are the
predominating factors in the overall room acoustics, although the other items in the room
(furnishings, carpeting, wall hangings, doorways, etc.) will also play a role. Without going
into excessive detail, there are four primary areas of potential concern:
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1. Standing waves.
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2. Flutter echo.
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3. Early reflections.
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4. Bass reinforcement.
The first three items are problems which should be reduced or eliminated. The last item, bass
reinforcement, needs to be matched to the entire system for proper tonal balance.