Harman Kardon AVP-1A Owners Manual - Page 28

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, lowlevel signals which are frequently important in films, and music as well. 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 narrowbandwidth 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. BACKGROUND NOISE SOLUTIONS Many sources of noise in a home environment can be addressed simply. Locating the home theater in the basement often removes it from many household noises as well as isolating it from the other family members. Taking care to completely seal windows and doors can also make a significant difference in reducing outside noise. Heating and cooling systems are more challenging. Sometimes, the answer may be as simple as using a "whistle-free" diffusion grille rather than one which creates undue noise from turbulence. In cases of new construction, using larger-diameter air ducts for lower air velocity is very beneficial. You can go further by using ductwork which is lined with acoustically-absorptive material. Where possible, longer ducts which have several turns further reduce the sound of the airflow, by eliminating the straight path from the heating/cooling system to the room. Some of the construction techniques used to minimize the transmission of external sounds into the listening environment include: •Double or triple layers of sheet rock (gypsum board). • Double wall construction, meaning two complete sets of studs (preferably stuffed with fiberglass insulation). • Double wall construction with staggered studs (minimizes transmission of vibrations from one set of studs to the next). • Floating floor construction (again, preferably stuffed with fiberglass; this also can enhance the perceived bass, since the subwoofers may cause structural vibrations through the false floor which then get transmitted up through furniture). • Seal all windows, doors, vents. •Seal and caulk all apertures in the wall (electrical outlets, through-wall plumbing, etc.). Finally, transient noises (traffic on the street, dripping faucets, etc.) distract your attention away from the program material, and remind you that you are in your home theatre/living room rather than a participant in the action of a movie. STANDING WAVES A "standing wave" is what causes a pipe of a particular length in a large pipe organ to have its characteristic pitch. The pipe literally amplifies certain frequencies. based on its length and the wavelength of the frequency. A typical rectangular room has three characteristic "lengths," and thus three fundamental standing wave frequencies. In addition, multiples of these frequencies are also amplified. 2,