Acoustic consideration: It isn't just for concert halls anymore.



In the last week, I've listened to the following lines: The first is the logic of an architect who is trying to cut costs on his project: "I don't see any reason to acoustically treat these two rooms. I mean they're just small hospitality rooms."

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.

Low frequency "swells" cause the sound pressure throughout the room to rise and fall in unison.

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.

Mid frequency "rollers and chop" cause the sound pressure to shake, rattle and roll.
Strictly speaking, this "specular reflection region" is the only one in which the classical large room acoustic equations and techniques are valid. The other three regions define rooms that are acoustically small. Notice that by using the size of the sound waves to judge smallness, there is a critical sound wavelength for any room that divides the long, lower frequency sound waves for which the room is considered small, from the short, higher frequency sound waves for which the room is considered large. This critical wavelength is equal to about 1/9 the acoustic length of the room. The acoustic length of the room is an average room dimension found by multiplying the total surface area of its floor, walls and ceiling by the average sound absorption coefficient of its surfaces, and then finding the square root of that number.

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.

High frequency "ripples" disperse as an average sound pressure throughout the room.
The sound of the same whip in a small room appears to be instantaneous with maybe the perception of a reflection from a wall. This difference is heard because all of the sound in a small room has less distance to travel before it hits something and is partially absorbed. In the small room cited above, sound can only travel an average distance of 61/2 feet before it hits something. Each time it hits something, 20 percent of it is absorbed so that after about 62 hits, only one millionth of the original sound remains.

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 rooms

Low-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 the diaphragm absorber, a wall surface is made to vibrate like a drumhead at the desired frequency. Soundwaves at this frequency tend to pass right through the wall with little resistance. Once the sound energy is outside the wall, it is kept out by absorbing it with massive layers of sound-absorbing material. Helmholtz absorbers trap soundwaves in tuned cavities. These can be recognized by their perforated or slotted faces.

Mid- to high-frequency soundwaves in the "diffusion region" can be dispersed with the use of diffusers. The most common diffusers are shaped like pyramids and attach to walls and ceilings. These diffuse soundwaves by scattering them in four different directions. Other diffusers look like flat sound panels, but are really an array of reflectors and absorbers, arranged in a mathematical pattern, which scatters soundwaves in many directions.

High-frequency soundwaves in the "specular reflection region" can be handled with sound absorbing panels and sound reflectors. Since, in small rooms, soundwaves only last a short time, it is critically important to place diffusers, absorbers and reflectors only where they are sure to intercept the soundwave that you want to control.

Classroom acoustics in the United States are so poor that students are not able to hear 70 percent of what their teachers say. One third of office workers say they are very disturbed by noise with a resulting loss in productivity.

It's interesting to read the complaints of Vitruvius, an architect from the first century B.C., that "improvements in architecture and the arts are always held back by baseless approval of inferior methods under the influence of social connections."