In many ways, small rooms pose greater acoustical challenges than large rooms. This is because in small rooms, the wavelength of sound can be similar to, or longer than, the room’s dimensions. This promotes a modal response, standing waves, and a lack of diffusion. These effects create anomalies in the room’s low-frequency response (for example, below 300 Hz) and other problems. In contrast, large rooms have more diffuse sound and the low-frequency response is not dominated by modes; therefore, the frequency response can be flatter. Small rooms also have relatively less absorption and thus a shorter reverberation time than may be desirable. This book deals primarily with the acoustics of small rooms. The special problems of small dimensions will appear in many of the discussions of these room designs.
Let’s try another experiment. While indoors, for a few quiet minutes, just listen. Can you hear traffic on the road outside, or aircraft passing overhead? Do you hear interior conversations, footstep or plumbing noise, air conditioners, or fans? Clearly, even if a room has “good acoustics,” it isn’t worth much if other noises intrude into the space. In other words, good isolation is an important consideration. Further, if you intend to play your home theater loudly, you’ll have to consider other people in your house or your next-door neighbors. Isolation is also important for them. Isolation also depends on the frequency of the sound. Generally low frequencies are harder to isolate than high frequencies. For example, you might strongly hear the bass content from a stereo playing down the hall, but less of the song’s midrange and treble content.
Adequate isolation is critical in any acoustically sensitive space. For example, recording studios, concert halls, libraries, and home bedrooms require isolation. Conversely, isolation is usually less critical in spaces such as retail stores, restaurants, gymnasiums, and home kitchens. The task of achieving isolation begins with the building’s blueprints. Good isolation across the full audio frequency range usually demands heavy walls, decoupled noise sources, and other specialty architectural features. Other features might include floating floors and a specially designed heating, ventilating, air-conditioning (HVAC) system. In addition, good isolation demands attention to detail; for example, sound leaks between rooms must be eliminated and any accidental couplings in a decoupled element must be prevented. In short, good isolation is hard to obtain. Furthermore, as noted below, good isolation should begin in the blueprints; adding considerable isolation to an existing structure is very difficult and sometimes impossible and is usually quite costly.
To the average person, if they notice it at all, a room is simply an assemblage of building materials. They see a tile floor, rough stucco walls, arched ceiling, heavy wooden doors, and large glass windows. Unless the person is an architect, the room’s paint scheme and furnishings might make a greater impression than the room itself.
To an acoustician, a room is a matrix of sound processing devices. To greater or lesser degrees, as shown in Fig. , every partition or barrier in a room will reflect, absorb, and transmit sound that strikes it. Reflected sound will continue to play a role, while absorbed sound will disappear. In addition, some elements will diffuse sound that strikes it; instead of a simple bounced reflection, sound is returned over a range of angles. The balancing of these phenomena (reflection, absorption, and diffusion) is key to a room’s acoustical treatment. Some sound that strikes a barrier such as a wall may be transmitted through that barrier to the adjoining room. The sound that is transmitted always has less energy than the original sound because of attenuation provided by the barrier. Because of this attenuation, a barrier can provide sound isolation.
To an acoustician, the tile floor and stucco walls would be sound reflectors, and the arched ceiling might create troublesome sound focusing. The heavy doors (and the stucco walls) might provide useful sound isolation, while the large windows might allow noise intrusion. The paint scheme is relatively unimportant (although paint can affect factors such as absorption). Because their surface areas are relatively small, furnishings are usually less important than the building materials but, for example, a stuffed sofa and chairs would add absorption to a room.
An acoustician may also view a room as an opportunity for improvement. Through room treatment, its acoustical characteristics can be adjusted to provide more suitable performance. For example, a room with tile floor and stucco walls would have a long reverberation time. If it was a media room used for teleconferencing, the long reverberation times might reduce the intelligibility of the speech that is conveyed. To improve this, absorption in the speech frequency range could be added; for example, heavy tapestries could be hung from the walls. Similarly, heavy fabric banners could be hung from the arched ceiling to lessen focusing effects.
When treating a room, it is naturally easier to add treatment rather than take it away. For example, some acousticians prefer to design rooms that are slightly too live, and then add absorption as needed. This kind of room tuning is an important part of room treatment. It is also important to note that room treatment must closely observe the frequency response of any particular problem, and use this to design the most appropriate treatment solution. For example, if a room has an unwanted low-frequency resonance, a panel absorber could be designed with peak absorption in that particular low-frequency band.
Finally, another aspect of room treatment is the spatiality of the sound. For example, a room might be designed with absorption on one end, and diffuse reflectors on the opposite end. Or, for example, a large wood panel might be hung from the ceiling and angled so that it reflects sound to another part of the room. As with other acoustical treatment, spatiality depends on the purpose of the room. For example, a recording studio would require good diffusion throughout, whereas a home theater might require much less. In fact, in the latter, too much diffusion might degrade playback imaging, that is, the ability to localize where sounds are coming from.