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An Optical Analogy
Let us use an optical 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 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:
1. Standing waves.
2. Flutter echo.
3. Early reflections.
4. Bass reinforcement
The first three items are problems that should be reduced or eliminated. The last item, bass reinforcement,
needs to be matched to the entire system for proper tonal balance.