The second are the pleas of a client whose architect didn't expect a major acoustic problem in such a small room: "We just remodeled our clubhouse and the dining room is noisy even with only two people in it! Can you do something?"
It is a common misconception that the only rooms that require acoustic design are concert halls, churches, theaters and other large acoustic performance spaces. The acoustics of small rooms are very different than those of large rooms and this difference in auditory perception may add to the illusion that small rooms don't have acoustics. As my two friends above demonstrate, this conclusion is wishful thinking.
The truth is that the only rooms that really need acoustic design are those, large and small, that contain sound. Once convinced that small rooms do require acoustic design, the inexperienced quickly run into trouble when they use the equations and techniques that were developed for large rooms. The result is often a room that sounds worse than it would if the acoustics had been neglected.
What is a small room?There are a couple of ways to judge the acoustic size of a room, but one of the easiest is to compare it to the size of the sound waves. When comparing the size of a room to the sound waves in it, it is useful to think of the room as a boat riding on the waves of sound. Low-frequency swells that are much larger than the room cause the sound pressure throughout the room to rise and fall in unison. Acousticians call this the "pressure region." Low- to mid-frequency rollers that are approximately the same size as the room cause the sound pressure to rock violently back and forth and roll from side to side. Acousticians call this the "normal mode region." Mid- to high-frequency chop, while not big enough to make the room rock and roll, do cause the sound pressure to shake and rattle (apologies to Charles Calhoun, Big Joe Turner, Bill Haley and the Comets). This is the "diffusion region." High-frequency ripples that are much smaller than the room are easily dispersed by the hull 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