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The Money Seat - ResidentialSystems.com

The Money Seat

Why the Dynamics of a Listening Room Can Significantly Impact Speaker Sound Quality
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Although nothing in the audio reproduction chain affects the sound that we hear more than the quality and accuracy of the loudspeaker, listening rooms play a large role too. Speakers operate within rooms, and the speaker and the room together make up an electro-mechano-acoustical system that takes the amplifiers electrical signal and converts it into the sounds that reach our ears.

Speaker placement affects many things, including sonic imaging or where the sounds appear to be coming from, the speakers frequency response, and what room resonancesoften called standing waves or room modesare activated, and by how much.

The first step is to examine the choice of listening position, which often determines speaker placement. One configuration to avoid is placing the listener near the back wall, where the pressure maximum for several of the rooms low-frequency resonant modes are located, creating boomy, over-accentuated bass with cancellations or near-cancellations at other frequencies.

The money seat should be centered laterally, with the left-front, center, and right-front (LCR) speakers also centered laterally, so that the left speakers spacing from the left wall mirrors that of the right speakers spacing from the right wall. Such symmetrical placement ensures that the influence of wall reflections on the speakers timbre will be the same left and right. Putting the preferred listening position on the centerline of the LCR array optimizes imaging, as the speaker-to-listener distance for the left-front speaker matches that for the right speaker, thus sound arrives from both channels simultaneously.

Its also best to avoid putting the listening position in the exact center of the room, as this will reinforce certain undesired room resonances. A position about two-thirds of the room depth back from the front wall is a good spot. In fact, as a general rule, thinking about placements that involve thirds or fifths of a room dimension is a good plan, as it helps avoid resonances caused by reflections from one wall reinforcing those from the opposite wall. Since we cant always avoid reflections and their impacts on certain frequencies, we strive to spread out their effects evenly across the frequency range. Before getting too involved in placement issues, take into consideration what recommendations the loudspeaker manufacturer has provided for its speakers. Some atypical designs may have atypical placement requirements.

Richard Rives Bird, president of acoustical consultancy Rives Audio, suggests to always start by getting the two main channels right. If you get the left and right channels done very wellvery good stereo imaging for musical programming materialyouve accomplished 90 percent of the battle. The next thing is to get the center channel right. The center speaker carries virtually all of the on-screen dialog for movies, and as such is hugely important to any home theater.

A speakers placement relative to nearby room surfaces affects its frequency response. Most floorstanding and other cabinet-mounted speaker systems are designed for placement well away from room surfaces. If such a speaker is placed close to, or against, a rooms surfacewall, floor, or ceilingthat surface will act as a baffle, resulting in increased output in the low-frequency range as well as produce a dip or notch at some point in the frequency response curve, caused by a portion of the speakers output bouncing off the wall and arriving at your ears out-of-phase with the direct sound. If the speaker was not designed for close proximity to walls or the floor, the result will be exaggerated, boomy bass. Putting the loudspeaker at a corner where two surfaces intersect will only exacerbate the problem.

Speakers designed for bookshelf placement can also create huge sound-quality problems. Floyd Toole, PhD, vice president of acoustical engineering for Harman International, regularly gives a lecture at CEDIA and other conferences titled, The Room and the Loudspeaker. In his presentation, Toole outlines the potential pitfalls of actually placing bookshelf speakers in bookshelves. Each shelf within a bookshelf is a cavity, and placing a speaker in this cavity wreaks havoc on what might otherwise be a well-performing speaker. Beyond this, the close proximity of a wall (or the back of the bookcase, if it has one) results in over-accentuated bass.

Toole doesnt say that bookshelf speakers should never be placed in a bookshelfin fact, hes used such an arrangement in his own home listening roombut he advises that the space surrounding the speaker be filled with books, eliminating the cavity. As the speaker is no longer freestanding, an equalizer, or in some instances, simple tone controls can help control bass reinforcement.

Jeff Cowan, director of training and product support for Boston Acoustics, said that his companys bookshelf speakers intended use is determined by whether or not a wall-mounting bracket was supplied with the system. While many of the firms bookshelf systems come with mounting points for brackets, the systems that were tuned for wall-mounting, and therefore suitable for placement near room surfaces, have the bracket included in the package.

Another common speaker placement mistake is the insertion of high-quality cabinet-type speakers into a wall recess. Doing so causes that over-accentuated bass just mentioned, although equalization can be used to temper that. If a customer wants to hide freestanding cabinet-mounted speakers its best to place the speakers well away from room surfaces and behind walls made of acoustically transparent cloth.

Another option is to use in-wall speakers. Although in-walls used to be associated with low fidelity, theyve since gone upscale, with many vendors offering high-performance in-walls for home theater and other critical listening requirements. Youve got the wall as an infinite baffle to work with, noted Frank Sterns, president of Niles Audio. Obviously, in-wall speakers dont take up any floor space. Aesthetically, you can paint them in, so you dont notice the speakers, and you can locate them easily around a plasma display or behind a perforated screen. So, the practical advantages are huge, compared to putting big boxes out into the room.

For home theater installations and other critical listening spaces, Toole draws the line at in-ceiling speakers. Dont do it, he warned, pointing to the ceiling sound stage issue and the inability to accurately aim the speakers at the listening position, though SpeakerCraft and others are working to address that problem. If a dealers and installers do use them, Toole hopes they will tell their clients about the compromises involved.
Bird, another in-ceiling opponent, did note that going with that option could be a reasonable compromise in surround-channel applications. He advises that the main speakers (left-front, center, and right-front) be either a freestanding cabinet type or in-wall type.

By contrast, Sterns said, Surprisingly, when you put all five speakers in the ceiling, and they all launch from the same plane, the sound doesnt appear to be coming out of the ceiling. It sounds like its hanging in the air, somewhere slightly above halfway up the wall. Which is usually pretty close to where you might mount a plasma display.

Beyond understanding how speaker placement affects a speakers frequency response or timbre, its important to consider how placement affects imaging. For conventional two-channel sound reproduction, a good general rule to follow is that the two speakers and the listening position should form an equilateral triangle. With such a setup, the included angle between the two speakers will be 60 degrees, or 30 degrees to the left and right, respectively, compared with a straight-ahead configuration (see Figure 1).

The International Telecommunications Unions (ITU) Recommendation 775 (figure 2) for 5.1 multichannel audio seeks to standardize the positioning of studio monitors used in recording or mixing soundtracks for movies. Following this speaker layout, the left and right speakers should be positioned 30 degrees, respectively, to the left and right of the center-channel speaker. The two surround speakers should be located somewhere in the range of 100 degrees to 120 degrees to the left and right of the straight-ahead position.
With multichannel soundtracks mixed under these conditions, it makes sense that they are played back under similar conditions. There is, however, a fair amount of debate regarding Recommendation 775. For one thing, optimizing for a single pointwherever the mixing engineer sitsis fine for studio work, but not for home theater, where theres likely to be a larger audience. Dolby Laboratories recommendations for multichannel audio speaker placement suggest placing the left-front and right-front speakers in the range of 22 to 30 degrees off the main axis, with the left-surround and right-surround speakers in the range of 90 to 110 degrees off axis. For 6.1 and 7.1 surround setups, two rear speakers should be added, and placed at 135 to 150 degrees off-axis.

The ITU recommendation calls for all five speakers to be positioned at the listeners ear-level, and defines this as 1.2 meters. Of course, another goal should be to have the height of the LCRs match the midpoint of the projection screen or video display. If this places the speakers above or below listeners ear height, the speakers should be tilted forward or backward, so that listeners are directly on-axis of the front three speakers.

In those cases where a projection screen is used, its preferable to choose an acoustically transparent screen, and mount the center speaker behind the screen, centered horizontally and vertically. For plasma or LCD panels, the center speaker will need to be mounted above or below the panel. Bird cautioned against mounting the speaker below the display, particularly for home theaters with multiple rows of seating, as the direct line of sight between the high-frequency driver will be blocked by those seated in the front.

Dolby Labs and other audio experts recommend that the surround and back speakers be positioned above the listeners ear level, for a more diffuse ambient sound. The ITU recommendation calls for all five speakers to be the same distance from the listening position, which ensures that sound arrival times for the five channels will be matched.

At low frequencies the room really takes control of the sound that is heard. Depending on arrival times, reflections from the room surfaces can add steep peaks and dips to the woofers anechoic responseyet, performance in the bass range is critical. Whether its a music listening room or a home theater, Bird said, if you get the bass right, other things fall into place much easier. But if the bass is wrong, everything is going to be muddied up.

Cowan recommends using the reciprocity technique to find the best location for a subwoofer. Temporarily place the subwoofer in the planned listening position and feed it a test tone signal (Cowan recommends a 45-Hz sine wave). You can then move throughout the room, listening for the best bass reproduction. Avoid points where standing waves cause complete or near-complete cancellation of the subwoofers output as well as excessive peaks. Once youve found the spot where the bass sounds best, thats where the subwoofer should be placed.

While this technique is valuable for finding the best subwoofer placement, it is most useful for optimizing performance for a single listening position. Changing the listening position by just a few feet can sometimes drastically change the result. You can easily get differences of 18 to 24dB at different locations within the room, Cowan said.

Todd Welti, a researcher at Harman International, has conducted and written an extensive study on the use of multiple subwoofers, which can be found on the companys website (www.harman.com). Among Weltis key findings is that multiple subs can give more uniform bass response across a broad range of listening positions than a single subwoofer can. Two subwoofers, if well located, can yield much more uniform bass performance than one subwoofer can across a range of listening positions; four subwoofers are even better, but there is no apparent advantage in incorporating more than that. Four subwoofers, with one placed at the midpoint of each of the four walls, yielded the most uniform bass response.

Alan R. Frank (alanrfrank@earthlink.net) is a former loudspeaker designer.

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