Harman Kardon AVP-1 Owners Manual - Page 8

■ 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 theater room from scratch, consider using non-parallel surfaces in the construction of the room. A difference of as little as 6 degrees will break up the slap echoes very effectively. For example, "flaring" the side walls out from the front by approximately 6 degrees and having the ceiling rise toward the rear of the room at a comparable rate will do wonders for 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 identifying 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. 10 dB 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 its 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 rattle 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 rattle 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 subject 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. BACKGROUND NOISE The effect of background noise on system performance is dramatic, yet often overlooked. Most people might think of it as merely a minor inconvenience, yet it has a profound effect on the way we perceive sound. The presence of more-or-less constant background noise alters the way we perceive volume, since subjective loudness is a relative measure. In a quiet room, even a 70 dB SPL sound can seem fairly loud. In a noisy convention center the same volume would be barely audible. Since there is a practical upper limit to both the volume to which we should expose ourselves and to the volume a given system can reproduce, having a relatively noisy environment effectively limits the perceived dynamic range of the program material. This, in turn, limits the dramatic effect which might have been intended by the director (or the performer, if listening to music). Constant background noise also obscures, or masks, low-level signals which are frequently important in films. Many scenes use subtle ambient noises to set the mood prior to an important event - without the full perception of the whispered secret or the barely-heard creaking of a door, the impact of the following scene is diminished. It has been demonstrated that even a relatively narrow-bandwidth noise can effectively reduce our hearing acuity over a broad range of frequencies, far greater than the noise itself. When you add up all the various sources of noise from electric motors, noisy heating/cooling systems, outside noises, plus noises that even audio and video components can introduce such as noisy transformers, motors in laser players, or projector fan noise, our ability to discern the low-level information in the soundtrack is greatly compromised - and the director's intention along with it. 7