(Reprinted from issue 60 of UHF Magazine. To purchase the issue, click here. Or click here to subscribe to UHF)

Acoustics for Surround Sound

by Paul Bergman

Getting proper acoustical results with two speakers is tough enough. So how easy is it when you have 5.1 speakers?

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If it can be of some consolation, the management of many a recording studio has been facing much the same conundrum you are: how to take a room designed to play two-channel stereo recordings and upgrade it to 5.1 channel surround. The equipment to be added is hardly a problem, but fitting all of it into the room and making it work is another matter.
     Let us look first at one of those studios, partly because it is a milieu with which I am familiar, and partly because you can see how difficult the task is even if (almost) unlimited funds are available.
     A typical studio control room is much wider than it is deep, in good part because its floor space is taken up by a mixing console the breadth of a pickup truck. The main monitor speakers are usually built into the wall just above the window through which the engineer and producer can watch the performers. These custom-built loudspeakers are typically a part of the studio installation package, with two very large woofers, a midrange and an imposing horn tweeter. Unlike domestic loudspeakers, these monitors do not require the aid of a subwoofer, since they are capable of reproducing low frequencies at Richter scale levels.
     A second, much smaller, pair of speakers is probably sitting atop the mixing consoles. Known as "nearfield monitors," these speakers are meant to let the producer check what the mix sounds like played through speakers that a consumer well might have at home.
     Only the smallest studio today dares remain with just two channels, however. Not only are video and film projects done in surround sound (as they have been for some years), but it is widely believed that tomorrow's home music medium will also need to be recorded in 5.1 channel surround, and it isn't too early to prepare.
     Let us look at the front of the studio first. It is tempting to install a third monitor like the other two dead centre, but it is more than likely that the video monitor will already be installed there. If the video monitor is lowered, it will hide the studio window (as in the photo on this page), or it will be all but invisible behind the console.
     As for the rear speakers, it is certainly possible to install small monitors on the rear wall, if the space is not already taken up by the door and possibly the equipment racks, but small monitors may not suffice, because panning a sound between the front and the rear will result in a major change in tonal balance. I shall add, finally, that a good studio is designed with controlled acoustics, almost certainly not conceived with the idea that loudspeakers would be operating at the back and would need to mesh seamlessly with the front monitors.
     Yet let us not feel too sorry for the studio owners, who can no doubt recoup the outlay with a few lucrative contracts (without which, of course, they will go broke). Let us now look at how to manage the same miracle in the confines of your home. We should perhaps begin by reviewing some established principles of acoustics for home audio. I can assure you that, new though 5.1 channel surround may be, it does not call for the reinvention of the principles of physics.
     Let us, then, examine some basic guidelines.
     a) Beware of the rear wall. It was once common to place the loudspeakers against the wall, since they would take up less space in that way; however most audiophiles know that this is inadvisable.
     b) Beware of early reflections. These are reflections of sound from surfaces close to the loudspeaker, typically the side walls and the floor (and, in the case of tall speakers, perhaps the ceiling as well).
     c) Beware of obstacles with straight edges, such as coffee tables or cabinets.
     d) Don't worry about the back wall. It seems intuitively probable that rear wall reflections will greatly alter what you hear, however the human ear is an odd device, and this is not so.
     e) Don't be overimpressed by symmetry.
     f) Don't listen from too far away. In a small room you are inevitably close to the speakers, but even in a large room you should place yourself at not too great a distance from them

Installing the main speakers
     Begin with the main (left and right front) speakers, perhaps even before you bring in the TV set. Use the rules I have outlined to determine their placement, and check the results by listening. Of course the speakers should sound "good," which is to say they should not seem bass-heavy, disembodied or shrill. Then, two tests may be performed... (see the print version)

Adding the rear speakers
     A studio will want to have rear speakers which, if they are not actually identical to the front speakers, will at least have identical tonal balance through the most important part of the audible range, from about 100 Hz through 5 kHz. A producer relies on what comes out of those speakers for decisions touching recording and remixing, and acting on incorrect information leads quite naturally to unwise decisions. You, of course, are the end user, and the only decision which depends on what you hear is whether to go on watching a particular film.
     Of course, it is ideal to install the same speakers at the rear as you have at the front, but there may be several reasons to hesitate. This choice can be costly, if we assume that you have chosen first class speakers for the front, but even if the funds are available the space may not be. Unless your room has been laid out for the specific purpose of serving as a home cinema, chances are that the seats are placed against the rear wall, or very close to it. If that is the case, there is no space for rear speakers, or at least no space to position them for optimum performance.

The bottom end
     Finally, there is the matter of the subwoofer, considered a necessity on most home theatre systems, though usually not in the studios in which soundtracks are produced. This topic would require its own article, and so I shall be brief.
     It is unfortunately untrue that a subwoofer can be placed simply anywhere in the room. If you are using two subwoofers, they should be as close as possible to the speakers with which they are associated, namely the left and right speakers. If you are using a single subwoofer, you may be coping with conflicting requirements...

The bottom line
     You are now nearly ready to watch a movie. You may want to verify that the volume setting of each channel is correct, by running the pink noise generator that is built into many receivers and outboard sound processors. Pink noise readily reveals alterations in tonal balance. If all is well, that final test will confirm that your hard work has been worthwhile.

(To read the the entire--much longer--article, including the details of the procedures and all of the illustrations, order the print version of UHF No. 60)

PARTIAL TEXT: Reproducing Extreme Lows, Acoustics for Surround Sound, Monitor Audio Silver 9, Klipsch RB-5, Coincident Triumph Signature, Mirage BPS150i, Audiomat Solfège, Gossip
FULL TEXT: The Digital Radio File, Reference 3a MM De Capo, State of the Art

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