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4845 Steeds Crossing Circle
Park City KS 67219
316-265-0366 (voice)
tech@apluselectronics.com
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ACOUSTICAL ROOM TREATMENT

You've spent thousands of dollars on the perfect viewing screen. Thousands more on that top-of-the-line surround sound system. Words like "subwoofer", "DVD", and "multi-channel sound" now hold court in your vocabulary.

FACE IT . . . You're hooked. You love sittin' in the sweet spot!

So with all the time and energy you've put into your home theatre, why stop short of the perfect acoustical treatment?

A Plus Electronics can install a complete home theatre acoustical kit . . for less than you'd think.

This is how it works.

We sell and install acoustical treatment products that are specifically for home theatres. Some are engineered to absorb low frequency sound that tends to gather in corners, these products eliminate excess low frequency noise for improved listening conditions. Correctly positioned midwall panels controls unwanted sound reflections for optimum sound quality.

It's simple really - most rooms require four PressureZone corner absorber panels, two front wall panels, two midwall panels and one back wall panel. Wrapped in fabric to coordinate with your room, PressureZone acoustical panels are easy to install and easy to clean - the perfect acoustical and aesthetic enhancement for your home theatre.

And that's not all! Every component of the PressureZone acoustical home theatre system comes with a one year warranty - our promise of your satisfaction.



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A Treatise on Acoustical Room Treatment

(long & windy, but good information!)

The home theater room is the last link in the chain that brings audio signals to the listeners' ears. The size, shape, and construction of a listening room reacts in concert with the reflective/absorptive properties of the room's interior and unfailingly couples with the speaker system output. The listener never hears the sonic output of a speaker alone; aural perception is always a combination of the room and the speakers.

Many A/V installers are beginning to use acoustical treatments within listening rooms to tailor audio quality. Customized room treatments range from simple and inexpensive to complex and costly. Typically, surface-mounted materials like carpets, fabrics, foam/fiber panels, etc., are the easiest and least expensive items that can be implemented. Because wall, ceiling, and floor coverings are usually no more than a few inches thick, their use and effectiveness is limited to acoustic functions in the shorter wavelengths of the mid and upper frequency ranges, such as direct reflections, echo, and reverberation. Design and application of specific coverings and treatments can dictate whether they absorb or diffuse sound waves.

Low-frequency sounds are much more difficult to isolate and control because of their long wavelengths. The longer the wavelength the more easily sound will pass through walls, floors, and ceilings. In residences, the cost of containing low frequency sound can be high because special building techniques and materials are required. Double-wall construction (dead air space between two non-touching walls), floating floors and ceilings, minimum air leakage, and extra mass in all wall, floor, and ceiling materials will be needed.

As if that weren't enough, certain bass frequencies will resonate within a room, causing boomy bass in some areas and cancellation in others. These resonant frequencies and their interaction within the room are called "modes." Room shape and dimensions play a large role in the number and intensity of modes, and to a certain degree, rooms can be designed so that their proportionate internal dimensions will smooth out the sonic mischief modes create.


Absorbing lessons

In most installations, you will not have the luxury of deciding room dimensions As a result, your options are more or less confined to arranging the furniture and subwoofer locations, or applying acoustical wall treatments or self-standing cylindrical absorbers and fixed-mounted corner absorbers. Be advised, though, that especially offensive bass problems (if they exist), will require time-consuming experimentation with several cylindrical and/or fixed absorbers.

Corner Traps will do little to attenuate the really low frequencies from sub/room interaction, but the problems of mid-bass bloom (centered around 100150Hz) should be smoothed out with its application. (Polar plots of box-mounted woofers show an almost uniform amplitude 360 degrees around a speaker in both the vertical and horizontal planes at 100Hz and below.) Used in conjunction with other absorbers in the room corners, you could expect a "tightening" of bass reproduction and an increased sense of deeper bass.

Many corner traps position in a trihedral corner (where two walls and the ceiling or floor converge). Typically, this style of absorber is made of foam cut to form multiple wedges to increase surface area.

The mounting method may require adhesives, fasteners, or hangers. Two companies well known for having a wide selection of these absorbers are Acoustics First (Richmond, VA) and Auralex Acoustics (Indianapolis, IN).

Foam corner absorbers are considered to be broadband because they absorb all frequencies, albeit the high frequencies are effected far more than the bass registers. Nevertheless, foam more than three inches thick has an absorption coefficient above 1.0 for 500 Hz and higher, i.e., the thicker the better for trapping bass frequencies. Even then, like free-standing absorbers, only mid-to-high bass absorption is practical. Net effect of fixed-mounted corner absorbers is to reduce midrange and high-frequency reflections generated in corners, and to smooth out modes for a deeper sounding subwoofer.


Stereo vs. multichannel

Rooms that sound good for audiophile stereo recordings may or may not work well for multichannel reproduction. The evolution of "music" reproduction dates from the early decades of this century and is based on two-channel (stereo) playback through two speakers within an "average" living room. Binaural, three-dimensional realism is theoretically possible from this setup, but there are too many variables that must be exactly perfect. Also, because the recording industry has no standards in place and because no two living rooms or speakers exhibit uniform acoustic characteristics, attainment of realism within this system is mainly accidental. Even when audio perfection is attained in a two-channel system, it would occur on only a few songs (recordings) among thousands, and the listener's head (only one listener) would have to be positioned straightforward in the exact same spot each time.

Additionally, the two-channel-music recording industry has always relied heavily on the listener's room acoustics to enhance the sense of spaciousness within each recording. It was long ago determined that an "average" residential listening room displayed a reverberant time of approximately 0.30.6 seconds (in the voice range), and most stereo music is recorded with this characteristic in mind. Stereo music is typically improved in rooms with a discernible (but not extreme) amount of reflective sound.

Movie soundtracks, on the other hand, are recorded under specific standards and assume that a listening environment's acoustics are somewhat dead. In multichannel mixes (future music and all cinema) the dynamics of all channels should be left to the discretion of the recording engineers. For a listener to be aurally transported from a mountainside to a train station to a car, a home theater's reverberation must not overwhelm the spatial intentions of the script.


Diffusion dos and don'ts

Some room furnishings and acoustical treatments cause sound to scatter or reflect in a random way (bookcases and irregularly shaped furniture are good examples). Commercially available diffusors are quite effective in scattering sound so that it eventually reaches absorbing surfaces in a room, and they come in a wide variety of colors and coverings.

Diffusors should be used to redirect troublesome reflected sound when additional absorptive devices would be overkill or make the room acoustically dead. Essentially, it is absorbers and diffusors working together that will produce the optimum reverberation and mode-smoothing conditions under which the speaker/room relationship will sound best.

According to Jeff Szymanski, chief engineer for Auralex, obtaining the right balance between absorption and diffusion usually requires a 30 to 40 percent wall coverage, respectively, for the average residential listening room. However, he cautions to point out that no two rooms behave exactly the same. Basement rooms with concrete walls, for instance, represent an extreme that would not be indicative of the "typical" residential construction normally encountered.


Bigger is better

Large rooms generally sound better than small rooms because there is a wider range of room resonances and therefore each is less apparent. Front-channel speaker placement away from room boundaries (walls, floor, and ceiling) is easier in larger rooms, and these spans improve clarity and imaging. As room size increases, the problems associated with modes will drop to lower-frequency registers. Small listening rooms are mostly plagued by modes in the upper bass regions and can play havoc with setting subwoofer crossover/phase adjustments. Larger home theater rooms have mode problems in the mid-bass areas, but can still effect subwoofer crossover at 100 Hz and below.

No matter the size, it would be wonderful to be able to create a one-time acoustically perfect room and use its design over and over. Unfortunately, things are not that simple. Speaker systems display a wide variety of response characteristics that react in different ways to their surroundings. Using common sense, it is obvious that bipolar, dipolar, and front-radiating speaker designs project their sound in unlike manners.

Sonic reflections within a listening room can be classified as early or late. Early reflections arrive at the listener's ear at approximately the same time as the direct sound from the speaker itself. The delay time for this reflection is so short that the brain does not process the sound as distinct reverb or echo, yet the clarity and transparency of the perceived direct sound will be blurred. The primary culprits of early reflections are floors, ceilings, and walls located near the LCRs. If these boundaries are hard-surfaced, carpets, drapes, or acoustic panels can tame the offending waves.

Should a speaker be near a corner or in a cabinet cavity, loading in the lower voice ranges will augment the distortions to further degrade audio quality. Selecting a box-type (not open backed) satellite speaker designed for inwall use and filling cabinet cavities with insulation or acoustic foam will help solve this type of problem.

Late reflections are those sounds reflected from surfaces that are further from the speaker than the listening position. A late reflection commonly found to be a problem comes from the back wall behind the listener. Audiophiles ordinarily treat rear walls with diffusors to nullify the problem, but this approach is not really appropriate. Diffusors in too great a concentration or located too close to the listener will smear the real audio image; it is probably best to use absorber panels positioned to block the offending reflections.

Echo is another variable. Echo is a specific path of reflected sound that is repeated between two or more surfaces. To hear echo (and for it to be a serious problem), the listeners' ears must be in the pathway.

No doubt you have seen "experts" wander around in a room clapping their hands together while listening for the "slap echo" problems. This procedure has become ritual for many installers, but in real acoustic investigations it is of little merit. A better experiment would use two people, one seated at the listening area and the other clapping at the speaker locations. Depending on the deadness of the room, echo pathways can be reduced with diffusors, absorbers, or better yet, both.

Still another factor, reverberation, is made up of many low-intensity sound reflections of a diffused (scattered) nature. A small amount of reverberation within the listening room is desirable for a couple of reasons. First, rooms that are too dead can create an illusion of "pressure" on the ears, which is distracting and not at all natural. And, the sense of spaciousness from surround speakers is always better when their subtle ambient sounds are not sucked up by too much wall absorption.

If the room is perceived as too reverberant, just add more sound-absorbing items. If you elect to use commercially available absorption devices, try to cover large areas instead of the pillow here, pillow there approach.


Shattering a myth

Over the years, audio installers have come to accept the rumor that parallel walls represent some horrific acoustical enigma. Mostly, this anxiety has little basis in fact. At low frequencies, where room modes are a problem, nonparallel walls demonstrate very small differences to parallel boundaries. The lower the frequency the less effect nonparallel surfaces will have. In fact, for the upper bass registers, wall surfaces would have to be more than 15 degrees from parallel before even measurable changes occurred.

Probably the worst problems that can come from parallel walls are early reflections and flutter echo. And, these difficulties are hardly mysterious, as we have already discovered, and can easily be rendered harmless with acoustic wall treatments.

Likewise, I am amazed at the number of A/V professionals who overlook the obvious. For example, many installers go to extreme lengths and expense to install tube traps and other acoustical treatments, but overlook the elimination of rattles and background noises. They just do not see environmental noises as a real problem-wrong!

Sometimes it is a fine line that divides a good audio system from a great one. That line is usually defined in terms of dynamic range and clarity, both of which are negatively affected by environmental sounds such as rattles and background noises. There are few other things an installer can do for so little time and money that are guaranteed to deliver positive results.

Rattle sources can be located by using a low frequency sweep tone through the audio system. A small inexpensive knob-controlled tone generator is ideal (cost is around $150), but you can use the low frequency sweep from the Wow! Disc, too. Just turn the audio system up to a moderately loud level and start the sweep. Rattles from windows, wall hangings, light fixtures, etc., will appear one by one. Silicone chalk and adhesive felt pads will come in handy.

The two worst offenders of background noise are HVAC air ducts/registers and poorly sealed doors and windows. If possible, have the HVAC contractor use larger than normal ducts and registers. Reducing air velocity through the registers will decrease air noise notably. Noises from outside the listening room can be reduced through isolation techniques mentioned earlier. If the noise infiltration is extreme, so too will be the cost of the solution. But, most home theaters will benefit from less expensive formulas such as extra insulation in the walls, weather sealed doors and windows, and exterior grade doors. Try to seal the room off as much as possible. If it leaks air it will leak sound.

While there is a lot known about room acoustics and the speaker/room relationship, the science is not complete. There are still many things yet to understand. Nevertheless, given everything discussed, with some composure and diligence you should be able to bring even the most pesky rooms under control. Make small corrections then listen, then correct again.

© A Plus Electronics, Inc. 2004
4845 Steeds Crossing Circle
Park City, KS 67219 USA
Phone: 316-265-0366
E-Mail: tech@apluselectronics.com