🚀 go-pugleaf

Archive-name: AudioFAQ/part1
Last-modified: 1993/6/4
Version: 1.5

1.0 Contents:

    Part 1:
	1.0	Contents
	2.0	Organization
	3.0	Purpose
	4.0	Omissions
	5.0	Credits
	6.0	Errors and Corrections
	7.0	Disclaimer
	8.0	Copyright Notice
	9.0	Speakers
	10.0	Amplifiers
    Part 2:
	10.13	Amplifiers (continued)
	11.0	CD Players, CDs, Turntables, and LPs
    Part 3:
	12.0	High Fidelity Systems
	13.0	Listening Rooms and Houses
	14.0	Recording
	15.0	Mail Order
    Part 4:
	16.0	Wire
	17.0	The Press
	18.0	Retail
	19.0	Miscellaneous
	20.0	Net Protocol

2.0 Organization:

This FAQ is divided into a preamble and a list of subjects.  Each 
subject is described by a list of questions and answers.  Some 
questions have no answers yet.  That's life.  

This FAQ is split into four individual postings.  This FAQ also 
references a frequent posting of sources for audio via Mail Order.  
That list is maintained by nau@SSESCO.com (William R. Nau).  Contact 
Mr. Nau directly for information or corrections to that posting.
The list is also available via FTP in the pub/rec.audio directory
of SSESCO.com.

Lines beginning with "|" are new since the last version of the FAQ.

3.0 Purpose:

The purpose of this FAQ is to address frequently-asked questions as 
a whole, so that rec.audio volume can be reduced.  Towards this end, 
we assembled a list of common questions, and some general answers to 
these questions.  Audio is part science and part art, so some of the 
answers are objectively correct, while others try to open-mindedly 
present both sides of a subject.

4.0 Omissions:

Many valuable things have been left out of the FAQ.  In part this is 
because there is just too much to say about audio.  In part, this is 
because the general reader doesn't need that much detail.  Also, some
things were omitted because they are too controversial or inflammatory.

5.0 Credits:

This FAQ is the work of many people.  Allow me to thank everyone who 
helped now.  We appreciate all of your contributions.  However, all
blame and criticism should go to Bob Neidorff at neidorff@uicc.com.  
Some of the contributors to this FAQ are listed below.  Others have 
made great contributions, but are no less appreciated.

	andrew@research.att.com (Andrew Hume)
	jj@research.att.com (jj)
	neidorff@uicc.com (Bob Neidorff)
	Rick Oakley (no internet access)
	DPierce@world.std.com (Richard d Pierce)
	P.Smee@bristol.ac.uk (Paul Smee)
	gmw1@cunixa.cc.columbia.edu (Gabe M. Wiener)

6.0 Errors and Corrections:

Some errors are intentionally included in this FAQ.  (It's easier to say 
that and later admit we were wrong, than to claim that this is a best 
effort).  If you have a correction to the FAQ, additional information, 
or a new topic for the FAQ, please send e-mail to neidorff@uicc.com 
describing your thoughts in detail.  Please include your e-mail address 
in your submission, so that we can stay in touch.  Every submission will 
be considered for inclusion in the next release of the FAQ.  This FAQ 
will be reposted every month.

7.0 Disclaimer:

Everyone's human.  Nothing is perfect.  The people who wrote the 
information here put varying amounts of research into their work.  
To the best of my knowledge, no one made any contribution or 
comment because of a vested interest.

Audio is a very lucrative and competitive industry, filled with
honest companies, aggressive marketing people, people who stretch the 
truth very thin, excellent products, and lousy products.  We tried 
hard to screen the hype from valuable data.  If we insulted, omitted, 
or otherwise disturbed you, your company, your product, or something 
you feel strongly about, please let me know.

The information here attempts to paraphrase a large portion of the 
information exchanged on rec.audio.  At times, people will make 
recommendations or suggestions to others on rec.audio.  Some of these 
statements are included here because we have been asked to include 
statements of that sort.  These statements, and everything in this 
document is the opinions of various people.  Nothing here is intended 
as recommendation or suggestion.

Further, no matter how it is worded, nothing here should be taken as 
fact.  The authors take no responsibility for any use of this 
information.

8.0 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.

9.0 Speakers:

9.1 What should I listen to when evaluating speakers?
	The most important thing is to listen to recordings that 
	you *know*.  Any good salesman will play you recordings 
	that highlight that particular speaker.  Do not be embarrassed 
	about bringing a stack of CDs with you to the hi-fi shop.

	Do not spend your valuable listening time switching between a 
	dozen pairs every 3 seconds.  If you are shopping at a quality 
	store, the dealer will, from the description of your room, your 
	size requirements, your musical tastes, and your budget, be able 
	to show you a couple of pairs that will be close to what you 
	want.  Spend several minutes listening to each.  When you think 
	you're close, don't be embarrassed about spending half an hour 
	or more listening to the speakers.  You're going to have them in
	your home for a lot longer, and many speakers will cause 
	"listening fatigue" after a short time.  Make sure you really 
	like them before you hand over money.

	One thing to try is well recorded "Spoken Word" records; most 
	people have a very good ability to tell when a speaking voice 
	sounds unnatural, even if they've never heard the person 
	speaking live.  If you play an acoustic instrument, find 
	something that features that instrument solo, or in a small 
	group; make sure it really sounds like it should.  Almost 
	everyone has heard a live piano.  Piano can be very revealing.

	Blues, jazz, folk, or 'easy listening' music with simple 
	instruments and a female vocalist is also revealing.  Well done 
	female singing voices provide a very good test of a system's 
	response.  Try something simple and soft, which will let you 
	hear any noises coming from the system; and something complex, 
	with lots of instruments all happening at once, to make sure the 
	system doesn't go muddy when things get complicated.  And, of 
	course, try a few of your favorites, and see if you like what 
	happens with them.  

	If a sales person suggests some music to listen to, the odds are 
	that it isn't the most revealing.  Sales people tend to suggest 
	things which sound great.  Anything you own and like is good, 
	because you know it and are happy to listen to it carefully.  No 
	matter how good the recording, if you don't like Opera, you 
	won't listen to it as carefully as your favorite, scratchy, 
	1940's rhythm and blues.

	Most important is to listen to something you are familiar with.
	Even if a recording is flawed (and what ones aren't?), how is it
	different from your normal setup?  Some of the most important
	differences are "Gee, I never heard that instrument before!"

9.2 What should I listen for when evaluating speakers?
	When comparing two speakers side-by-side, doing an AB 
	comparison, be extremely careful to match the levels before 
	evaluating.  A slight level difference can make one speaker 
	sound better, even though the difference may not be perceived 
	as a level difference.  Some claim that you will be influenced 
	by a difference of less than 1/2 dB!

	First and foremost, the sound should be natural.  If you listen 
	to vocals, close your eyes and try to picture someone singing in 
	the same room with you.  Does it sound realistic?  Likewise with 
	instruments.  You selected recordings of instruments that you 
	like and have heard live.  Do they sound like what you remember 
	them sounding like live?

	Your very first impression should be something like "what nice 
	sound".  If your initial gut reaction is "gosh, what a lot of 
	detail", the system is likely to be heavy in the treble (often 
	interpreted by beginners as "more detailed") and you'll probably 
	find that annoying after a while.  If your first reaction is 
	"hey, what powerful bass", then the system is probably 
	bass-heavy, rather than ideal.  The most common mistake for 
	beginners is to buy a system with REALLY powerful bass, because 
	it sounds "impressive" at first.  After a while, though, you'll 
	get tired of being thumped on the head by your music.

	Not to say that good bass and treble aren't important.  But your 
	first realization should be that the music is all there, and 
	that it comes together as good music, without one particular 
	part trying to dominate it.  Sit back and listen to it for a 
	bit.  You should be able to pick out the individual instruments 
	if you want.  They shouldn't force themselves on you, and you 
	should also be able to hear the music as a single piece, the sum 
	of its parts, without feeling like each of the instruments is 
	trying to grab your attention away from the others.

	You should check how things sound with the amp turned up, and 
	also with it turned down to a fairly low volume level.  Some 
	speakers which sound very nice at low levels begin to sound 
	confused, like they can't cope, when turned up.  On the other 
	hand, some sound nice loud, but sound thin and bodiless when you 
	turn them down a bit.  With the spoken word or female vocalist, 
	listen for "sibilance", a pronounced 'hiss' at the end of 's' 
	and 'z' sounds.  It shouldn't be there.  Most planar speakers 
	just can't play very loud.  Whatever you hear, do some 
	auditioning at the maximum volume you anticipate ever wanting.

	It is acceptable and sometimes desirable to switch the stereo to
	mono to evaluate naturalness.  Mono is a good test of both the 
	room and the speakers.  The image should be rock-solid dead 
	center, and not move with signal or level.  If it isn't perfect 
	mono, it will be nearly impossible to create a good stereo.

	A speaker in a large box is capable of producing low frequencies
	at higher volumes with more efficiency than a small box, but 
	that doesn't mean that a small box can't have great bass, it 
	just won't be as efficient and can't play as loud.

	Good speakers can "recreate a natural stereo sound stage", 
	placing some instruments to the left of the left speaker, some 
	sounds in the middle, and some to the right of the right 
	speaker.  Poorer speakers make it harder to localize voices.  

9.3 Why use a subwoofer?  Will it help?  One or two?
	One reason to get a subwoofer is to add bass to a feeble system.
	A second reason is to move the lowest frequencies to a separate 
	driver, and thereby reduce a particular kind of distortion 
	caused by the nonlinear mixing of different sounds, called 
	"intermodulation distortion". A third is to increase the power 
	handling ability of the system and the overall reliability.  All 
	are valid reasons, but it isn't so simple.

	To improve the sound of a good speaker system, a subwoofer must
	"integrate smoothly" into the system, extending the bass without
	causing peaks or dips.  Many subwoofers have a crossover that 
	goes between your amp and your main speaker which sends the lows 
	to the subwoofer and sends the higher frequency signals to the 
	main speakers.  This may damage the perfect sound of a good 
	system, it may sound similar, or it may sound better.

	Most good small speaker systems have a bass peak at resonance, 
	which attempts to compensate for the absence of lower bass.  
	Like it or not, this is the only way to make a small system 
	sound realistic. If the small system is done well, the 
	improvement you will get from a subwoofer will be small, but 
	still real and, to many, significant.

	Correctly done, a good subwoofer will enhance the sound of a 
	good small-box system.  Done wrong or haphazardly, anything is 
	possible.  Even a fine large speaker system might benefit from 
	careful addition of a subwoofer.  However, the better the 
	original system, the more likely it will be that a modest 
	subwoofer will do more harm than good.

	Low frequencies travel less directionally than high frequencies, 
	so many people say that only one subwoofer is required for good 
	sound.  This is true to some extent, but not completely true.

	There are a few reasons for getting two subwoofers.  Some feel 
	that you need two subwoofers to accurately reproduce the stereo 
	image, no matter how little low-frequency stereo information 
	there is.  Others feel that two subwoofers are much easier to 
	set up in a room, less likely to excite standing waves in the 
	room, and give smoother sound.  

	A third reason is that two subwoofers can produce twice the 
	sound of one.  Finally, even though subwoofers produce very low 
	frequency sound and very low frequency sound is non-directional, 
	subwoofers also have output at 100 Hz, and sound at 100 Hz is 
	directional, so two subwoofers will give a slightly better 
	stereo image than one.  Assuming, of course, that the two are 
	separated by at least two feet.

	Finally, even though original source signals rarely contain any 
	music with stereo components below 50Hz, there may be some noise 
	component with low-frequency out-of-phase noise.  This unusual 
	noise might add a sense of space to a recording if it is 
	reproduced by a system in which the woofers are very far apart.

	It is still true that a single good subwoofer, correctly added 
	to a system will help the sound but two will probably help more.

9.4 How do you connect a subwoofer to a stereo?
	Many subwoofers contain their own amplifier and crossover.
	For these, take the preamp output and feed it into the subwoofer 
	amp input and also into the main amplifier.

	For other subwoofers, just run them in parallel with your main
	speakers, or combine them into your system with your own bass
	amplifier and crossover.

	Some A/V receivers contain a splitter specifically for use with
	subwoofers.  If you have one of these, you will either want a 
	separate amplifier for your subwoofer or an amplified subwoofer.

	Consult the manual which comes with the subwoofer.

9.5 What do I need for surround sound?
	"Surround Sound" has referred to a number of different products 
	over the years.  Many mass-fi receivers have "Surround Sound" 
	buttons that do little more than muck up the imaging.  

	In recent years the term "Surround Sound" has become synonymous 
	with the surround systems produced by Dolby Laboratories.  Dolby 
	Surround comes in several flavors, such as passive surround 
	(which simply decodes the phase information and sends it to the 
	rear speakers) and the more advanced system called Pro Logic.  
	Pro Logic system uses computer circuitry to route directional 
	information to the appropriate speakers.  

	Generally, one needs at least two more speakers beyond the main 
	stereo pair.  Advanced Pro Logic systems such as the Lexicon and 
	Fosgate can accommodate several more speakers beyond the two 
	additional ones (usually placed in the rear).  Often one can 
	find Pro Logic systems with two front, two rear, two side, as 
	well as a center channel speaker for dialogue.

9.6 I was just approached (accosted?) by a couple of kids driving a van
	that said they had some GREAT speakers to sell. They are 
	overstocks, used by major recording studios and DJs or even 
	hot, and they normally sell for $1000/pr, but they'll let me 
	have them for just $399. Am I getting ripped off?

	Yes, you most certainly are. The speakers these people sell are 
	none of what they describe. They are never used in studios.  
	There might be one or two DJs out there that use them because 
	they can't afford anything else.  They are not overstocks, and 
	in all likelihood, they are NOT HOT!.

	Are they good speakers? No, they're, at best, no better than the
	big boom boxes you find in $400 rack systems in department 
	stores.  They are worth no more than what the kids paid for them 
	($100/pr).

	The speakers go under names like "Acoustic Monitor DB IV",
	"Acoustic Linear," "Pro-Poly," "Audio Reference 4350" and so on. 
	They all "feature" things like "liquid cooled 3" tweeter", 
	poly-cone 12" woofer, fantastic (but impossible) frequency 
	response, 98 db/watt sensitivity, and so on.

	These speaker are made by a couple of manufacturers with the 
	intent of being sold exactly this way.  They cost the kids in 
	the van about $100 a pair, and the kids are given minimal 
	training about what kinds of stories to use, what parking lots 
	are the most likely to generate sales (department store parking 
	lots near colleges in September is a great time for these guys).  
	Anything over and above the $100 dollars the kids paid is pure 
	profit (except for gas money).

	Stay away, you're getting ripped off.

9.7 What speakers should I consider in the $XXX/pair price range?
	This is probably the most commonly asked question on rec.audio,
	and also the most impossible to answer.  The market keeps 
	changing, everyone has different tastes, and no one has the time 
	to listen to even 10% of the products available in any country.  
	Also, many good products are only available in specific regions 
	or countries. 

	If you really want recommendations and are willing to listen to 
	the opinions of others, check the past few issues of Stereophile 
	Magazine.  Although they are strongly biased towards very 
	expensive gear and have their own particular other biases, they
	do steer you to some very good equipment in their 
	frequently-updated list of "RECOMMENDED COMPONENTS".

9.8 Can you build better speakers than you can buy?
	Some people can build better than you can buy.  These people are
	either experts, golden ears, extremely well equipped, inspired, 
	or a combination of the above.

	Some companies have plans available to entice you into buying 
	their drivers: Audio Concepts, Audax, Dynaudio, Focal, KEF, and 
	Scanspeak.  Your success rate with these plans will probably be 
	very good IF your cabinetry skills are very good and IF you 
	follow the plans precisely.  If you deviate (as everyone does), 
	anything is possible.

	Stereophile has published three different plans designed by Dick 
	Olsher which are similar two-way ported systems. A recent one of 
	these was in Stereophile Nov '90, pages 94-127. Audio Magazine 
	published a plan called "The Pitts" by Ken Kantor, in Audio, Nov 
	'88 pages 65-71 continued in Dec '88 pages 73-77.  This plan is 
	a two-way sealed box.

	I have built one published design and one manufacturer's design.  
	I believe that both met my expectations.  They took me a long 
	time to build, taught me a lot, were fun projects, and sounded 
	good when finished.

	I also believe that a commercial system which cost what my parts
	cost will never sound anywhere near as good as the one I build.  
	If you consider $2/hour for my time, however, building is 
	financial suicide.

	Designing your own system is even more a can-of-worms, and 
	should be left to those with either a strong stomach, a very 
	forgiving ear, infinite resources, or excellent guidance.

9.9 Where can I read more about speaker building?
	Speaker Builder Magazine
		Audio Amateur Publications
		PO Box 494
		Peterborough NH  03458 USA
		603-924-9464
	The Loudspeaker Design Cookbook, Fourth Edition
		by Vance Dickason (C) 1991
		ISBN 0-9624-191-7-6
		$29.95 + $2.00 S&H from
		Old Colony Sound Lab
		PO Box 243
		Peterborough NH  03458-0243 USA
		603-924-9464
	High Performance Loudspeakers by Martin Colloms
	Synergetic Audio Concepts Classes and Newsletters
		Syn-Aud-Con teaches classes on Audio and Acoustics
		12370 W. Co. Rd. 100 N.
		Norman IN  47264 USA
		812-995-8212
9.10 Where can I buy speaker drivers?
	A & S Speakers (Broad line)
		3170 23rd Street
		San Francisco CA  94110 USA
		415-641-4573
	Audio Concepts (Their own kits plus drivers)
		901 South 4th Street	
		LaCrosse WI  54602 USA
		800-346-9183
	Phil Baker (Surplus cabinets only)
		546 Boston Avenue
		Medford MA  02155
	Carbonneau
		609 Myrtle NW
		Grand Rapids MI  49504 USA
		616-456-9528
	Drexler Audio Systems (Bandor Speaker Distributor)
		14 Rose Lane
		Rosemont PA  19010
	Gold Sound (Broad line including pro speakers)
		PO Box 141
		Englewood CO  80151 USA
		303-789-5310
	Madisound (Broad line)
		8608 University Green
		Box 4283
		Madison WI  53711 USA
		608-831-3433
	McGee Electronics (Polydax, Pyle, Motorola, EV, MTX)
		1901 McGee Street
		Kansas City MO  64108-1891 USA
		913-491-5287 or 800-876-2433
	Meniscus
		2442 28th Street SW Ste D
		Wyoming MI  49509 USA
		616-534-9121
	Parts Express
		340 East First Street
		Dayton OH  45402-1257 USA
		513-222-0173
	Solen Electronique (Airborne, Audax, Ceratech, Dynaudio, Eton, 
		Lpg, Morel, Peerless, Scan-Speak, Seas, Solen, Vifa)
		4470 Avenue Thibault
		St.-Hubert, QC  J3Y 7T9 Canada
		Voice 514-656-2759
		FAX 514 443-4949
	Speakers Etc.
		1828 West Peoria Avenue
		Phoneix AZ  85029 USA
		602-944-1878
	SRS Enterprises (Pyle, Pioneer)
		318 South Wahsatch Avenue
		Colorado Springs CO  80903 USA
		719-475-2545
	Transducer Technology (Cabasse, Accuton, Vieta, 
			Audiom by Focal, Stage, Tekton)
		4320 Spring Valley Road
		Dallas TX  75244
		214-991-6994
	Zalytron (Broad line including kits)
		469 Jericho Turnpike
		Mineola NY  11501 USA
		516-747-3515

9.11 Where can I buy loudspeaker kits?
	Audio Concepts, Inc. (Wide range of kits.  Catalog available)
		901 South Fourth Street
		La Crosse WI  54601 USA
		608-784-4570
	Fried Products (Parts kits starting $550.  Catalog available)
		(Emphasizes high-end transmission line speakers)
		(Parts kits have plan, crossover, and driver)
		Conshocken Road
		Conshocken PA  19428 USA
		215-277-9004
	Mahogany Sound (Parts kits and Woodstyle kits)
		(Parts kits have plan, crossover, and driver)
		(Woodstyle kits also have 3/4" MDF veneered boxes)
		(Prices $150/pair to $500/pair.  Catalog available)
		(Two way, three way & subwoofer kits)
		2610 Schillingers Rd #488
		Mobile AL  36695 USA
		205-633-2054
	Also see above, under suppliers for speaker drivers.

9.12 How can I improve the sound of my speakers?
	The best way to change the sound of your speakers is to change 
	where you put them.  Ideally, the speakers should be located at 
	ear level, in front of you, squared off between you.  It's then 
	a matter of fiddling with a) the angles, b) the distance apart, 
	c) the distance from you, and d) the distance from the wall.  
	Just moving the speakers around in the room or putting them onto 
	stands can make a major difference. For more on speaker 
	placement, see 13.1 below.

	Other than that, speaker modifications can be a can of worms, or
	can produce very subtle changes, which you might prefer.  For 
	example, you might improve a speaker by adding some cross braces 
	of 1"x1" wood from left to right and from front to back.  This 
	will stiffen the cabinet and reduce speaker cabinet wall 
	vibrations, which probably hurt sound quality.  Alas, this will 
	be most effective with lower-cost and poorly built speakers.

	Along similar lines, some claim success putting lead wire or 
	epoxy putty on thin parts of the speaker to damp out resonances.  
	You can try doing this to the thinner parts of the speaker 
	"basket" or frame, or to the front "baffle" or supporting panel.

	Still another "tweak" is to add sound deadening felt pads to the 
	inside walls of the speaker.  Instead of felt pads some advocate 
	sand-filled latex coatings on the inside walls of speakers.  
	Others advocate ceramic tiles held in place with "thinset". 
	Still others rave about commercial products like AC Glop, 
	Acoustic Magic, and Bostik Sheet.  However, the people who rave 
	about these products tend to be the same people who sell them. 

	Any change along the lines of adding felt, cross-bracing, or 
	putty will have subtle effects on the sound.

	For the brave at heart, you can replace old or cheap drivers 
	with better ones, but the results of this one change can be very 
	dissatisfying if you happen to get the wrong type of driver for 
	that application, and may never sound right, even if you use a 
	similar driver. Speaker system design is still somewhat of a 
	science and somewhat of an art.  Throwing paint on a canvas 
	often makes a mess.

	Whatever change you try, don't "burn your bridge" home.  Be sure 
	that you can undo whatever change you did, just in case.  Many 
	tweaks to good speakers, no matter how well thought through, 
	will correct for one flaw, but create others, or correct a flaw 
	that the designer had cleverly used to his advantage.

9.13 How can I replace/re-cone my old speakers?
	The best chance of success is to buy an identical replacement 
	speaker driver from the manufacturer of the system.

	Second choice is to buy the exact same driver from a 
	distributor.  This is sometimes difficult because it is hard to 
	learn exactly what driver the manufacturer used.  In addition, 
	EVEN IF the manufacturer used stock speakers, they might have 
	used matched pairs or selected speakers by hand for an exact set 
	of specific characteristics.

	There are companies that rebuild drivers, but they charge quite 
	a bit.  I have heard $75 per driver. This is rarely done for 
	anything but very expensive commercial drivers.  Speaker 
	manufacturers will often sell owners the materials that they 
	need to repair a speaker.  If you are handy with delicate 
	things, it is worth a try.

	In addition to speaker manufacturers, there are companies which 
	sells rebuild kits for approximately $30 per pair, containing 
	new foam, a special glue, and instructions.  If you have a blown 
	or distorted voice coil, this still won't help. A few netters 
	have used rebuild kits from this company successfully. Contact:
		Stepp Audio Technologies
		PO Box 1088
		Flat Rock NC  38731 USA
		800-747-3692

	Another vendor of speaker repair parts is:
		Simply Speakers
		P. O. Box 22673
		St. Petersburg FL  33742 USA
		800-767-4041 or 813-571-1245

	Some speaker manufacturers have very good warranties.  
	Electro-Voice warranties all professional products for life.  
	KEF has a similarly broad warranty on their speakers.  Contact 
	the manufacturer first.

9.14 What computer programs can I use to design speakers?
	There are many useful programs available, but none are complete 
	without a good knowledge of speaker design. Further, you will 
	NEED to supplement any program with hand tweaking for the best 
	sound.  Finally, no simulation program is ever useful without 
	good model parameters, and the parameters which manufacturers 
	give you are often imperfect, so many good designers strongly 
	recommend your own lab measurements.  The Loudspeaker Design 
	Cookbook (see 9.9) tells you how to measure a speaker, and also 
	enough theory to feel confident with a good program.

	You can get a lot done with a simple spreadsheet and the 
	equations in a book like The Loudspeaker Design Cookbook.

	For more information on speaker programs for speaker design
	and speaker design hardware, such as measurement systems, get
	the archive "audio.speaker" in directory "dist" on node
	"research.att.com" (also known as 192.20.225.2).

9.15 Can I magnetically shield my speakers for use near a TV?
	You probably will need to buy speakers that are made with an
	integral magnetic shield.  Magnetic shielding is usually done 
	by either shielding the speaker magnet or by cancellation of the
	magnetic field very close to the magnet, or by both.  Shielded
	speakers are NOT built by lining the enclosure with metal.
	While it sounds like a good idea, it doesn't work.  

	A common magnet shield is a mild steel cup around the magnet. 
	This is the cheapest shield, and is usually fairly ineffective. 
	It also will interfere with the speaker's critical magnet gap, 
	so this type of shield can hurt speaker performance by shorting 
	the magnetic field and reducing the magnetic flux density in the 
	gap, which can reduce efficiency and affect the speaker's low
	frequency performance.

	Cancellation is done using a reverse-polarized magnet glued to 
	the back of the main magnet.  If done right, it can almost
	completely cancel the rear stray field.  In some cases it can 
	also increase the magnetic flux density in the gap, which may
	or may not be desirable.

9.16 What are all of these abbreviations people use for speakers?
	Most of these parameters are well documented in the Loudspeaker
	Design Cookbook. (see 9.9)  In summary:

	Fs    Driver free air resonance, in Hz.  This is the point at 
		which driver impedance is maximum.
	Fc    System resonance (usually for sealed box systems), in Hz
	Fb    Enclosure resonance (usually for reflex systems), in Hz
	F3    -3 dB cutoff frequency, in Hz
	
	Vas   "Equivalent volume of compliance", this is a volume of 
		air whose compliance is the same as a driver's 
		acoustical compliance Cms (q.v.), in cubic meters
	
	D     Effective diameter of driver, in meters
	Sd    Effective piston radiating area of driver in square meters
	Xmax  Maximum peak linear excursion of driver, in meters
	Vd    Maximum linear volume of displacement of the driver 
		(product of Sd times Xmax), in cubic meters.
	
	Re    Driver DC resistance (voice coil, mainly), in ohms
	Rg    Amplifier source resistance (includes leads, crossover, 
		etc.), in ohms
	
	Qms   The driver's Q at resonance (Fs), due to mechanical 
		losses; dimensionless
	Qes   The driver's Q at resonance (Fs), due to electrical 
		losses; dimensionless
	Qts   The driver's Q at resonance (Fs), due to all losses;
		 dimensionless
	Qmc   The system's Q at resonance (Fc), due to mechanical 
		losses; dimensionless
	Qec   The system's Q at resonance (Fc), due to electrical 
		losses; dimensionless
	Qtc   The system's Q at resonance (Fc), due to all losses; 
		dimensionless
	
	n0    The reference efficiency of the system (eta sub 0) 
		dimensionless, usually expressed as %
	
	Cms   The driver's mechanical compliance (reciprocal of 
		stiffness), in m/N
	Mms   The driver's effective mechanical mass (including air 
		load), in kg
	Rms   The driver's mechanical losses, in kg/s
	
	Cas   Acoustical equivalent of Cms
	Mas   Acoustical equivalent of Mms
	Ras   Acoustical equivalent of Rms
	
	Cmes  The electrical capacitive equivalent of Mms, in farads
	Lces  The electrical inductive equivalent of Cms, in henries
	Res   The electrical resistave equivalent of Rms, in ohms
	
	B     Magnetic flux density in gap, in Tesla
	l     length of wire immersed in magnetic field, in meters
	Bl    Electro-magnetic force factor, can be expressed in 
		Tesla-meters or, preferably, in meters/Newton
	
	Pa    Acoustical power
	Pe    Electrical power
	
	c     propogation velocity of sound at STP, approx. 342 m/s
	p     (rho) density of air at STP 1.18 kg/m^3

9.17 What are fluid-filled (fluid-cooled, ferro-fluid) tweeters?
	These tweeters are built almost exactly the same as other
	tweeters.  They look and act almost exactly the same, too.
	The only difference is that they have a small, controlled 
	amount of a special fluid inserted into the gap between the
	magnet and the voice coil.

	One big effect of adding this fluid to a tweeter (or to any
	speaker) is that it makes the voice coil capable of dissipating
	more heat.  This means that the speaker can have a lighter voice
	coil, for better performance, or a higher power rating for the 
	same voice coil.  The other big effect of this fluid is to add
	mechanical damping.  The frequency response and transient 
	response of the driver will change, possibly for the better.

	In addition, this fluid may help center the voice coil, may 
	lubricate the voice coil, and may help keep dirt out of the gap.
	This fluid will not increase the magnetic field, concentrate the
	magnetic field or otherwise change the magnetic circuit.  Nor
	will it cushion impact if the voice coil bottoms.

	The fluid used for this purpose is often called "ferrofluid".
	It consists of sub-microscopic particles of magnetic material 
	suspended in special oil.  This fluid stays in the gap because
	of the strong magnetic pull of the magnet.  There is some debate
	over whether these fluids can dry out with time.  Manufacturers
	claim that the oil used is non-volatile.

	It is possible to use ferrofluids in mid-range drivers and
	woofers.  However, as tweeters tend to have the most fragile
	voice coils, tweeters have the most to gain from ferrofluid.
	There are various different fluids on the market, some of which
	have characteristics tailored to tweeters, some to woofers, etc.

	It is very risky to blindly add fluid to a driver.  It may not
	be compatible with the adhesives used in the driver, may not be
	practical with the particular driver layout, and is impossible
	to remove.  Permanent driver damage is possible.

10.0 Amplifiers
	Note: A receiver contains an amplifier, so the following 
	questions apply to both receivers and amplifiers.  In the
	following text, "amp" and "amplifier" are used synonymously.

10.1 What is Biamping? Biwiring?
	Most speakers are connected to an amplifier by one pair 
	of terminals on each speaker. Within these speakers, a 
	crossover distributes the signal (modified appropriately) 
	to each of the drivers in the speaker.

	Some speakers are set up to be either biwired or biamped. A 
	much smaller number allows triwiring and triamping.  The same 
	principles apply but use three sets of wires or three amplifiers 
	instead of two.  Most speakers that support biamping/biwiring 
	have two pairs of terminals and some mechanism for shorting 
	the two pairs together when used in the normal way. This 
	mechanism is most likely a switch or a bus bar. To help 
	the descriptions below, I will refer to these two pairs as 
	LO and HI  (because normally one pair connects to the woofer
	and the other pair connects to the tweeter/midrange).

	Biwiring means that a speaker is driven by two pairs of wires
	from the same amplifier output. One cable pair connects HI to
	the amp, and the other cable pair connects LO to the same amp 
	output that you connected the HI cable to.  Biwiring is 
	controversial; some folks hear a difference, some do not. The 
	most plausible explanation involves magnetic induction of noise 
	in the relatively low current HI cable from the relatively high 
	current signal in the LO cable.  Accordingly, Vandersteen 
	recommends the two cable pairs for a channel be separated by at 
	least a few inches. In any case, the effect appears to be small.

	Biamping means that the two pairs of terminals on a speaker are
	connected to distinct amplifier outputs. Assuming you have two 
	stereo amplifiers, you have two choices: either an amp per 
	channel, or an amp per driver. For the amp per channel, you 
	connect each terminal pair to a different channel on the amp 
	(for example, the left output connects to HI and the right side 
	to LO). In the other configuration, one amp connects to the LO 
	terminals, and the other amp is connected to the HI terminals.

	The point of biamping is that most of the power required to 
	drive the speakers is used for low frequencies. Biamping allows 
	you to use amps specialized for each of these uses, such 
	as a big solid-state amplifier for the LO drivers and higher 
	quality (but lower power) amp for the higher frequencies. 
	When you have two identical stereo amps, some folks 
	recommend distributing the low-frequency load by using an amp 
	per channel.  In any case, whenever you use two different 
	amplifiers, be careful to match levels between them.

	Biamping also allows you to use high-quality electronic 
	crossovers and drive the speaker's drivers (the voice coils) 
	directly, without the series resistance and non-linear 
	inductance of a passive crossover.  Biamping which uses the 
	speaker's crossover is therefore much less desirable.  Replacing
	a good speaker's crossover with an electronic crossover has 
	advantages, but involves some very critical tradeoffs and tuning 
	which is best left to those well-equipped or experienced.
	
	See also section 16.0 below, on wire and connectors in general.

10.2 Can amplifier X drive 2 ohm or 4 ohm speakers?
	Almost any amplifier can drive almost any load if you don't turn 
	the volume up too high.  Tube amplifiers are one exception.  
	Some amps clip if you play them too loud.  This is bad and 
	damages speakers.  Other amplifiers shutdown if they are asked 
	to play too loud.  Many will overheat, with bad consequences. 
	However, in almost all cases, it takes seriously loud sound or 
	low speaker resistance (less than 4 ohms) to do damage.  Running 
	two sets of 8 ohm speakers at once with common amplifiers 
	represents a 4 ohm load.  Four sets of 8 ohm speakers makes a 2 
	ohm load.  Two sets of 4 ohm speakers also makes a 2 ohm load.  
	If you stay sober and don't turn it up past the point where it 
	distorts, you are PROBABLY safe with most amplifiers and most 
	loads.  See 10.3 for more information.

10.3 How do I drive more than two speakers with one stereo amplifier?
	One amp can drive many speakers.  However, there are two limits 
	to this practice.  The first is that you can overheat or damage 
	an amplifier if you drive too low of an impedance to loud 
	listening levels.  Avoid loading any amplifier with a lower 
	impedance than recommended.  Adding two speakers to one amp 
	output loads that output with half the impedance of one speaker.  
	(See also 10.2 above)

	The second is that with tube amplifiers, which are uncommon in 
	today's common system, it is important that the speaker 
	impedance and the amplifier output impedance be well matched.

	When driving two or more speakers from one amp output, always 
	wire them in parallel, rather than series.  Series connection, 
	while safe in terms of impedance levels, can hurt sound quality 
	by raising the impedance that the speakers themselves see. 

	Many amplifiers have connectors for two pairs of speakers.  In 
	general, these amplifiers also have a speaker selector switch.
	These amplifiers almost exclusively connect the speakers in 
	parallel when both are selected.  It is common for these 
	amplifiers to specify 8 ohm speakers only.  The reason that they 
	specify 8 ohm speakers is because the amplifier is built to 
	drive either 4 or 8 ohms, and two sets of 8 ohm speakers loads 
	the amplifier as if it were one set of 4 ohm speakers.  It is 
	probably perfectly safe to connect one set of 4 ohm speakers to 
	an amplifier with two sets of outputs, provided that you NEVER 
	use the second set for any set of speakers.

10.4 How big an amplifier do I need?
	Unfortunately, amplifier power ratings and speaker power ratings
	are almost always misleading.  Sometimes, they are factually 
	wrong.  Speaker ratings are almost useless in evaluating needs.

	More importantly, human hearing is sensitive to the log of the 
	power coming out of the speaker.  This means that the perceived 
	difference between an 80 watt amplifier and a 100 watt 
	amplifier, all else equal, is very small. (under 1 dB!)

	There is a wide variation in the "efficiency" and "sensitivity" 
	of the various speakers available.  I have seen good speakers 
	with under 80 dB per watt efficiency and have also seen good 
	speakers with over 96 dB per watt efficiency, measured one meter 
	from the speaker.  This difference of 16 dB represents a factor 
	of 40 difference in power requirement!

	So the first step in determining amplifier requirements is to 
	estimate relative speaker efficiency.  Other factors include how
	loud you will want to listen, how large your room is, and how 
	many speakers you will drive with one amplifier.  This 
	information will give you a rough starting point.  For an 
	example, a typical home speaker will produce 88 dB at 1 watt.  
	In an average room, a person with average tastes will be happy 
	with this speaker and a good 20 watt per channel amplifier.  
	Someone who listens to loud music or wants very clean 
	reproduction of the dynamics of music will want more power.  
	Someone with less efficient speakers or a large room will also 
	want more power. 

	Past that point, you will have to use your ears.  As with all 
	other decisions, your best bet is to get some candidates, borrow
	them from a friendly dealer, take them home, and listen to them 
	at your normal and loudest listening level.  See if they play 
	cleanly when cranked up as loud as you will ever go, into your 
	speakers in your room.  Of course, it is also important to be 
	sure that the amp sounds clean at lower listening levels. 

10.5 Do all amplifiers with the same specifications sound alike?
	Some say that they do.  Some say that they don't.  Some 
	demonstrated that many amplifier differences can be traced to 
	very slight frequency response difference.  Let your own ears 
	guide you.  If you want to compare amplifiers, you can do it 
	best in a controlled environment, such as your home, with your
	music and your speakers.  Also be very careful to match levels 
	precisely.  All you need to match levels of amplifiers is a high 
	input-impedance digital voltmeter set to AC volts and a test 
	recording or signal generator.

10.6 Is this amplifier too big for that set of speakers?
	There is no such thing as an amplifier that is too big.  Small
	amplifiers are more likely to damage speakers than large ones, 
	because small amplifiers are more likely to clip than larger 
	ones, at the same listening level.  I have never heard of 
	speakers being damaged by an overly large amplifier.  I have 
	heard of 100 watt speakers being damaged by a 20 watt 
	amplifier, however, in really abusive hands.

10.7 Where can I get a cheap low-power amplifier?
	There are very few available.  One source is to buy a cheap boom 
	box and only use the amplifier.  Another source is Radio Shack.  
	A third alternative is to buy a car stereo booster and get a 12V 
	power supply for it.  Finally, you can build an amp pretty 
	easily if you are handy, but it probably won't be that cheap.  
	Mark V Electronics, for example, sells 20 watt amp kits for 
	under $30 and 80 watt amp kits for under $150.  (See 10.14, 
	10.15, and 10.16.)

10.8 Is the stuff sold by Carver really awesome?
	Some people really like it.  Others believe that it doesn't 
	perform up to specifications.  Still others believe that it 
	isn't all that it is hyped up to be. One of Carver's claims to 
	fame is lots of watts per pound of weight.  As with almost 
	everything else, the best policy is to listen for yourself and 
	see what you think.

10.9 What is a preamplifier?
	A preamplifier is an amplifying electronic circuit which can be
	connected to a low output level device such as a phono cartridge 
	or a microphone, and produce a larger electrical voltage at a 
	lower impedance, with the correct frequency response.  Phono 
	cartridges need both amplification and frequency response 
	equalization.  Microphones only need amplification. 

	In most audio applications, the term 'preamplifier' is actually 
	a misnomer and refers to a device more properly called a 
	'control amplifier'.  Its purpose is to provide features such 
	as input selection, level control, tape loops, and sometimes, 
	a minimal amount of line-stage gain.  These units are not 
	preamplifiers in the most technical sense of the word, yet 
	everyone calls them that.

10.10 What is a passive preamplifier?
	A passive preamplifier is a control unit without any
	amplification at all. It is a classic oxymoron, because it has
	no capability to increase the gain of the signal.  It is only
	used with line level sources that need no gain beyond unity.

10.11 Do I need a preamp?  Why?
	The tasks of a preamp are to:
		Switch between various input signals,
		Amplify any phono inputs to line level,
		Adjust the volume,
		Adjust the treble and bass if necessary,
		Present the right load impedance for the inputs, and
		Present a low source impedance for the outputs.

	If you have a turntable, you NEED a preamp with a phono input.
	This is because the turntable has an output which is too
	small for driving amplifiers and because the output of the
	turntable requires frequency response equalization.  You
	can't connect any other source to a phono input other than a
	turntable (phono cartridge).  Also, you can't connect a phono
	cartridge or turntable to any input other than a phono input.

	Microphones also require special preamplifiers. Some microphones
	also require "phantom power".  Phantom power is operating power
	for the microphone which comes from the preamp.  Microphone
	preamps are often built into tape decks and microphone mixers.

	If you only have high level inputs, such as the output of a CD
	player and the output of a tape deck, the main value of a preamp
	is selecting between inputs and providing a master volume
	control.  If you only listen to CDs, it is plausible to skip
	the preamp entirely by getting a CD player with variable level
	outputs and connecting them directly to a power amplifier.

	Some caveats apply. One, the variable outputs on a CD player are
	often lower sound quality than fixed outputs.  Two, some sources
	have high or nonlinear output impedances which are not ideal for
	driving an amplifier directly.  Likewise, some amplifiers have
	an unusually low or nonlinear input impedance such that common
	sources can't drive the input cleanly.  A good preamplifier
	allows use of such devices without sacrificing sound quality.

	Unfortunately, the only way to be sure that a preamplifier is
	of value with your sources and your amplifier is to try one.

	Almost all receivers contain a phono preamp, a volume
	control, and input switching.  Therefore, if you have a
	receiver, you may never need a preamp.

10.12 Should I leave equipment on all of the time or turn it on and off?
	Some gear draws significant electricity, so you will waste money
	and fossil fuel if you leave it on all of the time.  As an 
	example, a common amplifier consumes 40 watts at idle.  High-end 
	gear uses far more electricity, but ignoring that, 40 watts x 
	168 hours x 52 weeks x US $0.0001 per watt hour (rough estimate) 
	is $35/year.  Now add a CD player, a preamp, and a tuner, and it 
	really adds up.

	High-end enthusiasts claim that equipment needs to warm up to 
	sound its best.  If you care about the best sound, give your 
	equipment at least 20 minutes to warm up before serious 
	listening.  Warm up will allow the inside temperature to 
	stabilize, minimizing offsets, bring bias currents up to their 
	proper values, and bringing gain up to operating level.

	Either way, good gear will last a very long time.  Tubes are 
	known to have a finite life, but good tube designs run tubes 
	very conservatively, giving them life exceeding 10 years of 
	continuous service.  Some amplifiers run tubes harder to get 
	more power out, and thereby may be more economical to turn off 
	between use.

	Filter capacitors will fail after enough time at temperature 
	with voltage applied.  They will last longer if turned off 
	between use.  However, like tubes, filter caps can last tens of 
	years of continuous use, as can power transformers, 
	semiconductors, and the like.

	Filter capacitors have a funny problem that justified a simple 
	break-in or reforming when they are restarted after many years 
	of rest.  It involves bringing up the power line voltage slowly 
	with a variable transformer.  For tips on reforming capacitors, 
	consult "The Radio Amateur's Handbook", by the ARRL.

	Semiconductors seem to fail more often because of bad surges and 
	abuse than age.  Leaving gear off may be best for semiconductors 
	and other surge-sensitive gear if you expect power line surges, 
	as come from an electrical storm or operation of large motors.

	Fuses seem to age with temperature and get noisy, but they are 
	so inexpensive that it should not bias your decision.  However, 
	some are inconvenient to change, and may require opening the 
	case and even voiding the warranty.
-- 
Bob Neidorff; Unitrode I. C. Corp.  |  Internet: neidorff@uicc.com
7 Continental Blvd.                 |  Voice   : (US) 603-424-2410
Merrimack, NH  03054-0399 USA       |  FAX     : (US) 603-424-3460