Harman Kardon AVP-1A Owners Manual - Page 27

The solution for slap echoes is usually a combination of absorption and diffusion. Specifically, placing absorptive material behind the front speakers (heavy drapes, fiberglass, dense foam) and diffusion in the rear of the room (bookcases, irregularly-shaped furniture, etc.) will deliver the greatest benefits. This will effectively suppress the slap echoes while at the same time providing a diffusive surface in the rear for the surround speakers. This enhances the enveloping characteristic of the surrounds even further. In those relatively rare cases where you have the luxury of building the home theatre room from scratch, consider using non-parallel surfaces in the construction of the room. A difference of as little as 6° will break up the slap echoes very effectively. For example, "flaring" the side walls out from the front by approximately 6° and having the ceiling rise toward the rear of the room at a comparable rate will dramatically improve the room's acoustics, if the wall design is solid and the angles are clearly intentional from the outset. RATTLES Rattles in the room are structural resonances (as opposed to standing waves, which are airborne resonances) which the system may stimulate due to its broad frequency response and wide dynamic range. They are particularly prominent for sounds in the lower frequencies, and can sound like distortion. Sources of rattles include: furniture, loose window frames, walls, lighting, fixtures, ventilation systems, and even knick-knacks on various shelves around the room. The simplest way of identifing these rattles is by using the Rattle Test found on WOW! (Chapter 16). This is an extremely slow low frequency sweep from 20 Hz to 500 Hz, recorded at reference level. 10dB of output level increase over standard level might be necessary to allow hearing all the room rattles. Be careful with this test, as it is also a severe test of associated amplifiers and speakers. As the sweep makes it way up the frequency range, you will probably find a surprising number of rattles in your room. All of these rattles will occur at one time or another during music or movies, but are usually perceived as background noise or distortion in the system. Once identified, eliminating the rattles is usually straightforward. As an example, small pieces of felt can be affixed to the back of a painting (in the bottom corners) to prevent audible rattles against the wall. Likewise, strips of felt can be wedged into a loose window rattling in its frame. Recessed lighting fixtures can be tightened up. A piece of cloth can be placed under offending knick-knacks. Every Home THX Audio System should be subjected to the rattle test at least once-the difference in low level resolution and in freedom from pseudo-distortion is sometimes large, and the effort involved is quite small. ROOM ABSORPTION FOR HOME THEATRE SYSTEMS Surround speaker Screen speaker • "Dead" zone absorbs front speaker reflection. • "Live" zone provides surround propagation Reflective •live' zone Absorptive dead' zone 24