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FAQ: rec.audio.* Rooms 7/07 (part 6 of 13)

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Archive-name: AudioFAQ/part6
Last-modified: 2007/07/12
Version: 2.17

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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.

COPYRIGHT NOTICE
The information contained here is collectively copyrighted by the 
authors. The right to reproduce this is hereby given, provided it is 
copied intact, with the text of sections 1 through 8, inclusive. 
However, the authors explicitly prohibit selling this document, any 
of its parts, or any document which contains parts of this document.

--
Bob Neidorff; Texas Instruments     |  Internet: neidorff@ti.com
50 Phillippe Cote St.               |  Voice   : (US) 603-222-8541
Manchester, NH  03101 USA

Note: Texas Instruments has openings for Analog and Mixed
Signal Design Engineers in Manchester, New Hampshire.  If
interested, please send resume in confidence to address above.

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