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13.0 Listening Rooms and Houses 13.1 How should I place speakers in my room? What size room is best? You are after two important, distinct goals: flat frequency response and good three-dimensional image. At your disposal is the room size, the room shape, speaker height, speaker placement, listening position, and room treatments. Even though good speakers are essential to good sound, room effects are also extremely important. In many cases, the differences in room effects will be more noticeable than spending twice as much on speakers! Here are some generally-accepted-as-good guidelines for good sound. If you use these as a starting point, you will be far ahead in terms of getting good sound from your speakers and room. But these are just a guide. Each room and each speaker is a little different. Experiment to see if a change will help. Also, if the manufacturer recommends something different, give that a try, too. Then use what sounds best to you. For smoothest bass response, a listening room should be as large as possible, have dimensions as unrelated as possible, and should be optimally damped. Although nothing is ever ideal, there are a few room dimension ratios that are better for listening rooms: Height Width Length 1 1.14 1.39 1 1.28 1.54 1 1.6 2.33 If your room isn't shaped like that, don't worry. These effects are not major. Also for smooth bass response, woofers should be at distances from the nearest three room boundaries that are as different as possible. In some cases, the line dividing the listening room into left and right halves must be considered a room boundary. Also, for smooth bass response, the listener's ears should be at distances from the nearest three room boundaries that are as different as possible. All of this is essential because a wall near a speaker boosts the bass from that speaker at some frequencies. If a speaker is the same distance from three walls, then some frequencies will be emphasized much more than others, rather than slightly more. For best three-dimensional image, a listening room should have good symmetry about the plane between the two speakers. This means that if one speaker is in a corner, the other speaker must be in a corner. If this symmetry is not right, the first reflection from the wall behind one speaker will be different from the first reflection from the wall behind the other speaker and critical parts of the stereo signal will be damaged. Also, no large object should block the path from speakers to listener or from speaker to speaker. Speakers should be elevated so that tweeters are at listener ear height. The distance between speakers should be no greater than the distance from each speaker to the listener. Finally, the tweeters should be aimed at the listeners. A normal box-shaped listening room with bare walls will have "slap echo" which will reduce intelligibility. A good cure is randomly-placed wall hangings consisting of small rugs spaced an inch or so away from the wall to increase sound absorption. Another cure is convex-shaped art objects on the walls to disperse harmful reflections. If money is available, commercial room treatments such as "Tube Traps" and "RPG Diffusers" are also valuable, but many of the benefits of these exotic devices are available with simpler techniques. As a general rule, in a good room, speakers and listener can be close to room boundaries with minimal adverse effects. In a bad room, a good strategy is to place both speakers and listener as far away from room boundaries as possible. An excellent starting point for speaker placement is to measure the listening room diagonal dimensions. Divide that measurement by three. Put each speaker that distance from a corner, on the room diagonals. I----------------------------------I I I I L I I I I S S I I I I----------------------------------I Place your listening position midway between the two speakers and approximately half way from the speakers to the wall. Be sure that there is nothing in the "triangle" formed by the listening position and the speakers. Try this and then move things 12" (30cm) at a time to see if you can improve the sound. Your ears will be a better guide than any commonly-available instruments. To keep track of what you are doing, take notes. To remember exactly where you put the speaker on the floor, a practical trick is to mark the floor with a sewing needle and thread. Some speakers want to be aimed right at the listener (toed in) while others work best pointed straight ahead. Experiment. 13.2 How do I wire a house for sound? A fundamental principle of physics is that the farther a signal travels, the more the signal will be degraded. Translate this to mean that the shorter the wire, the better. Understanding this, the idea of running speaker cable between every room of the house isn't as attractive as it first seems. If you still decide to wire your house for sound, you should do it at the same time you're wiring for telephone and electricity. It is possible to wire a house after the walls are closed, but it becomes very difficult. It is economical to use common house wire (Romex, UF, NM, etc) for speaker wire in the walls, but this may violate building codes. Check with an electrician or inspector first. It will also confuse future electricians, so label the wire clearly, all along its length. If you want to make your house like a recording studio, it is best to use the techniques of recording studios. When studios run long lengths of sound cable from one room to another, they drive the cable with 600 ohm line amplifiers. They also use shielded, twisted-pair cable. They only connect the shield at one end of the cable. Finally, they use balanced inputs at the other end of the cable. 13.3 Where can I read more about listening room construction and tuning? "Building a Recording Studio" by Jeff Cooper Mix Bookshelf "Handbook for Sound Engineers" "The Master Handbook of Acoustics" by F Alton Everest "Sound Engineering 2nd Edition" by Don and Carolyn Davis; Howard W. Sams & Co. (C) 1990 "Good Sound" by Laura Dearborn Introductory, but clear and accurate "Sound Recording Handbook" by John M. Woram Howard W. Sams & Co. #22583 Excellent General Reference "Audio Technology Fundamentals" by Alan A. Cohen Howard W. Sams & Co. #22678 Overview of Audio Theory "Introduction to Professional Recording Techniques" by Bruce Bartlett Howard W. Sams & Co. #22574 "Modern Recording Techniques" by Hubar and Runstein Howard W. Sams & Co. #22682 "Sound Studio Production Techniques" by Dennis N. Nardantonio Tab Books "The Uneasy Truce Between Music and the Room" F. Alton Everest Audio, February 1993, Pgs. 36-42 "Coloration of Room Sound by Reflections" F. Alton Everest Audio, March 1993, pgs. 30-37 13.4 What is white noise? What is pink noise? "White noise" is characterized by the fact that its value at any two different moments in time are uncorrelated. This leads to such noise having a flat power spectral density (in signal power per hertz of bandwidth), and is loosely analogous to "white light" which has a flat power spectral density with respect to wavelength. Pink noise has flat power spectral density per PERCENTAGE of bandwidth, which leads to a rolloff of -3 dB/octave compared with white noise. There are many reasons for using pink noise in audio testing. One is that music has an average spectral content much closer to pink noise than white noise. Another is that pink noise can be readily measured with constant Q bandpass filters and naturally leads to flat plots on logarithmic frequency scales - which correspond to the equally tempered musical scale. Pink noise is often used with 1/3 octave band filters to measure room acoustics. This idea has merit since 1/3 octave is a convenient number near the limit of our ears ability to detect frequency response irregularities, and because averaging measurements over 1/3 octave bands smooths out the numerous very narrow peaks and dips that arise due to standing waves in rooms. Another term you'll hear about is Gaussian noise - this is noise with a Gaussian amplitude probability density. Gaussian noise has the amazing property that linearly filtering it preserves its Gaussian amplitude density and that sums of Gaussian random variables are again Gaussian. The two terms shouldn't be confused. It is possible to have Gaussian white or pink noise. 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