l and are felt as an averaged sound pressure throughout the room.
For example, a room with dimensions 10-feet-by-12-feet-by-8-feet, having floors, walls and ceilings with an average sound absorption coefficient of 0.2 has a total surface area of 592 square feet and an acoustic length of 11 feet. The critical wavelength is 1/9 of that or 1.2 feet. The frequency of this wavelength is found by dividing 1,130 by the 1.2 feet to get 942 Hz. So, this room is small for all sound up to about 1,000 Hz.
Doing the same calculation for a much larger room of dimensions 30-feet-by-50-feet-by-20-feet with a 0.05 average absorption coefficient, it is seen that even this room must be considered small for sound frequencies less than 600 Hz.
Audible differences between small rooms and large rooms
As useful as the above method of judging room size by comparison to sound wavelengths is, unless the room has an acoustic problem, people don't perceive these differences as an indication of room size. What we do perceive is that sound tends to stick around longer in large rooms than it does in smaller rooms. When we listen to the crack of a whip in a large room, we may notice a delay between the time we see the whip crack and when we first hear it; the time it takes the sound wave to travel from the whip to our ears. There is then another silent delay before we start to hear reflections from the various surfaces of the room and these blend into a dying hiss, lasting a second or two.
At this point, the sound has only traveled a distance of 6.5 feet times 62 hits equals 403 feet at a speed of 1,130 feet per second so the sound is effectively gone from the room within 403/1,130 equals 0.36 second.
For the large room in the example above, sound can travel an average distance of 19.4 feet before it hits something. Each collision with the large room's surfaces only absorbs 5 percent of the sound energy so it takes a total of 270 hits to absorb all but one millionth of the original sound energy. This occurs after the sound has traveled 19.4 feet times 270 hits equals 5,238 feet or nearly a mile! This takes 5,238/1,130 equals 4.6 seconds to do.
What denotes good acoustics in a small room?As in any room, the goals for the acoustics of a small room depend on what the room's function is to be. If the room is to be used for listening to recordings, then the acoustics of the room are made to be subordinate to those on the recordings and reproduced by the sound system. If the room is for lectures, then the acoustics of the room are made to be comfortable with an optimum signal-to-noise ratio for the listeners.
The acoustics of individual music practice rooms are adjusted to mimic the acoustics of the large performance halls where the polished performance will be made. Studios are designed to add pleasant and adjustable room acoustics to sound recordings. Control rooms are made to optimize their stereo monitoring function. Teleconference rooms are built to optimize the acoustic presence of conference participants.
For small rooms, these acoustic goals are met by controlling the timing, strength and location of sound waves within the room. This requires the exact placement of various types of acoustic treatment. The type of treatment to be used depends on the region of soundwaves to be controlled.
Controlling soundwaves in small roomsLow-frequency soundwaves in the "pressure region" can be controlled using active noise control (see "Silence, Please," Sound Advice, Walls & Ceilings, September 2000). This technique uses a microphone to pick up the low frequency soundwave. It then uses a loudspeaker in the room to create a mating soundwave having the same amplitude but opposite phase (i.e., the mating soundwave pressure goes down when the original soundwave pressure goes up and vice versa). The result is that the two soundwaves tend to cancel each other out.
Low- to mid-frequency soundwaves in the "normal mode region" can be controlled using diaphragm and helmholtz-type sound absorbers (see "Helmholtz Resonator Sound Absorbers," Sound Advice, W&C, April 2000). In t