See Figure, However, since it is not possible to

FINE-TUNING YOUR AUDIO SYSTEM

However, since it is not possible to

SOLUTIONS TO ROOM REFLECTIONS:

“toe-in” flush-mounted speakers, the

DIFFUSION AND ABSORPTION

next step is the strategic placement of

absorptive materials on the side walls.

Loudspeaker

These range from commercially avail-

able fiberglass and dense foam to heavy

ect

ath

fle

Pat

efl

ath

fle

draperies and even large, overstuffed

Path

furniture. The optimal position for these

Direct

Absorptive Material

materials can be found with a small

Absorptive Material

hand mirror and the help of an assistant.

Sit at the primary listening position and

have the assistant slowly slide the mir-

Listener

ror along the wall. When you can see

any of the front speakers reflected in the

mirror, mark the wall at the mirror for

later placement of absorptive material

Figure 23.

(see Figure 23).

A variation of this method is especially helpful in rooms that are already fairly “dead” acoustically. Rather than using absorptive material in rooms like these, try using diffusion instead. Commercially built diffusers are available, but large bookcases and irregularly shaped furniture will also serve the same purpose. They reflect sounds in a highly random way that effectively “scatters” the sound in all directions. Place the diffuser where you would otherwise place the absorptive material (using the “mirror trick”), to break up the first early reflections and scatter them randomly throughout the room.

Commercially available fiberglass, foam and diffusion panels may not be aesthetically acceptable in many installa- tions, particularly when the home theater room serves multiple purposes. All of these materials can be covered with acoustically transparent cloth for design considerations. It is important that the cloth be acoustically transparent, however, or the effectiveness of the absorptive material will be greatly reduced. The simplest test for this is to hold a large sample of the cloth in front of a speaker playing pink noise. If you can move the cloth in front of the speaker without hearing a difference, the cloth is good for use in this application.

Large expanses of glass can be challenging. Glass reflects mids and highs, but often lets bass pass through, almost as if the glass were not there. The result is a characteristically bright, rough sound that can be difficult to correct electronically. The best treatment is generally the heaviest insulated drapes that can be found. (Incidentally, these serve double duty, controlling light that might otherwise fall on the screen.)

The materials just discussed are ineffective at lower frequencies. See the discussion on standing waves for more information about treating environments with low-frequency response problems.

EXCESSIVE USE OF ABSORPTIVE MATERIALS

Absorptive materials should be used judiciously, as overuse can acoustically deaden the room and create additional difficulties. While the ideal home theater should be considerably “deader” acoustically than a typical living room, it still needs some reflectivity and diffusion. In particular, the surround speakers depend on nonabsorptive surfaces for their operation, since they radiate virtually no sound directly at the listeners.