Harman Kardon AVP-1A Owners Manual - Page 26

SOLUTIONS TO ROOM REFLECTIONS: DIFFUSION AND ABSORPTION Loudspeaker Loudspeaker iE I Amami. slide the mirror 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. A variation of this method is especially helpful in rooms which are already fairly "dead" acoustically. Rather than using absorptive material in homes 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 randomized way which 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 installations, particularly when the home theatre 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 else 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 the pink noise found in Chapter 6 of the WOW! laserdisc. If you can move the cloth in front of the speaker without hearing a difference, you are all set. Large expanses of glass can be challenging. They reflect mid and high audio frequencies but often pass bass energy through almost as though they didn't exist. The result is a characteristically bright, rough sound which can be difficult to correct electronically. The best treatment is generally the heaviest insulated drapes which can be found. (Incidentally, these serve double duty, controlling light which might otherwise fall on the screen.) The materials just discussed here 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 People are sometimes tempted to go overboard with absorptive material once they discover how powerful its use can be. While the ideal home theatre should be considerably "deader" acoustically than a typical living room, it still needs some reflectivity and diffusion. In particular, the surround speakers depend on non-absorptive surfaces for their operation, since they radiate virtually no sound directly at the listeners. The best arrangement of the absorptive and non-absorptive surfaces in the room can be seen in the diagram below. Most of the room surfaces are relatively absorptive, with the notable exception of the rear wall and the highest portions of the other walls, which should be diffusive. "SLAP" ECHOES "Slap" echoes are common in rooms which have parallel walls with little or no absorption or diffusion. Sounds tend to bounce back and forth between the parallel wall many times before they die out, causing a characteristic bright, "zingy" sound and interfering with the intended tonal balance and acoustic nature of the soundtrack. Walk slowly through the room, clapping your hands. No clear reflections should be heard at any point in the room especially not near the primary seating area. Listen for a "flutter echo" of the hand clap (a rapidly-repeating percussive sound, indicative of the sound bouncing between two parallel walls) Again, the best home theatres are fairly "dead' acoustically. This allows the program material and the playback system to create the environment, rather than having the room's native acoustic signature color everything. You can also use the hand claps in chapters 17 and 18 of WOW!