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FAQ:* Speakers 7/07 (part 5 of 13)

( Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Part9 - Part10 - Part11 - Part12 - Part13 )
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Archive-name: AudioFAQ/part5
Last-modified: 2007/07/12
Version: 2.17

See reader questions & answers on this topic! - Help others by sharing your knowledge
12.0 Speakers:

12.1 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!"

12.2 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. 

12.3 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.

12.4 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.

12.5 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.

12.6 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", "Omni
	Audio", 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. The brand names are remarkably similar to reputable firms,
	but different enough to delay law suits.

	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 US $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 the US $100 the kids paid is pure profit.

	Stay away, you're getting ripped off.  For more information on
	these speakers, see:

12.7 What speakers should I consider in the $XXX/pair price range?
	This is probably the most commonly asked question on,
	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".

12.8 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.

12.9 Where can I read more about speaker building?
	Europe's Greatest Speaker Designs
		Solen Electronique
		4470 Avenue Thibault
		St.-Hubert, QC J3Y 7T9 Canada
		Voice 514-656-2759
		FAX 514 443-4949
	High Performance Loudspeakers by Martin Colloms
	Speaker Builder Magazine
		Audio Amateur Publications
		PO Box 494
		Peterborough NH 03458 USA
	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
	The Loudspeaker Design Cookbook, Fifth Edition
		by Vance Dickason (C) 1995
		ISBN 1-882580-10-9
		$34.95 + $4.45 S&H from:
			Old Colony Sound Lab
			PO Box 243
			Peterborough NH 03458-0243 USA
		$30.00 + approx. $3 Shipping from:
			8608 University Green; Box 4283
			Madison WI 53711 USA
		$30.00 + ??? S&H from:
			Parts Express
			340 E. First St
			Dayton OH 45402 USA
12.10 Where can I buy speaker drivers?
	Audio Concepts (Their own kits plus drivers)
		901 South 4th Street	
		LaCrosse WI 54602 USA
		Voice 608-784-4570
	Phil Baker (Surplus cabinets only)
		546 Boston Avenue
		Medford MA 02155 USA
	Bandor Design & Development Studios (Aluminium coned speakers)
		11 Penfold Cottages
		Penfold Lane
		Holmer Green
		Bucks, HP15 6XR United Kingdom
		Tel. (01494) 714085
	DBS Audio (Speaker kits and crossovers)
		PO Box 91, Bury St. 
		Edmunds, Suffolk, IP30 0NF United Kingdom
		Tel (0284) 828926
	Drexler Audio Systems (Bandor Speaker Distributor)
		14 Rose Lane
		Rosemont PA 19010 USA
	Falcon Electronics (Drivers and cross overs)
		Tabor House
		Norfolk, NR14 8JT United Kingdom
		Tel. (0508) 78272
	Faraday Sound (Concrete loudspeaker cabinets)
		248 Hall Road
		Norwich, NR1 2PW United Kingdom
		Tel. (0603) 762967
	Gold Sound (Broad line including pro speakers)
		PO Box 141
		Englewood CO 80151 USA
	Madisound (Broad line)
		8608 University Green
		Box 4283
		Madison WI 53711 USA
	Meniscus (Broad line)
		2442 28th Street SW Ste D
		Wyoming MI 49509 USA
	Parts Express (Broad line)
		340 East First Street
		Dayton OH 45402-1257 USA
	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
	The Speaker Co (Large range of drive units plus speaker kits)
		Unit 9, Waterside Mill
		Waterside, Macclesfield, SK11 7HG. United Kingdom
		Tel. (0625) 500507
	Speakers Etc.
		2728 West Thomas Road
		Phoneix AZ 85017 USA
	SRS Enterprises (Pyle, Pioneer, Eminence, Ultimate, Fane, MG)
		1839 N Circle Dr
		Colorado Springs CO 80909 USA
		Voice 719-475-2545
		FAX 719-475-0359
	Wilmslow Audio (Kits and drive units. KEF, Dynaudio, Audax, SEAS,
			Peerless, Scanspeak, Morel)
		Wellington Close
		Parkgate Trading Estate
		Knutsford, Cheshire, WA16 8DX United Kingdom
		Tel (0565) 650605
	Zalytron (Broad line including kits)
		469 Jericho Turnpike
		Mineola NY 11501 USA

12.11 Where can I buy loudspeaker kits?
	Audiocab (Speaker kits and cabinets)
		9 Skewbridge Close
		Wooten Bassett, Swindon, SN4 7DW United Kingdom
		Tel (0793) 848437
	Audio Concepts, Inc. (Wide range of kits. Catalog available)
		(see 12.10, above)
	Fried Products (Parts kits starting $550. Catalog available)
		(Emphasizes high-end transmission line speakers)
		(Parts kits have plan, crossover, and driver)
		1323 Conshocken Road
		Norristown, PA 19401 USA
		610-277-1014 or 800-255-1014
	IPL Acoustics (Kits using SEAS, Morel, Audax, and Visaton)
		2 Laverton Road
		Westbury, Wiltshire, BA13 BRS United Kingdom
		Tel (0373) 823333
	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
	Tabula Rasa (Wide range of speaker kits)
		1 Silkin Dalton Close
		Broadfield, Crawley
		W. Sussex, RH11 9JD United Kingdom
		Tel. (0293) 531190
	Visaton UK Ltd (Drivers, crossovers, kits, designs, software)
		2 Bentfield Road
		Stansted Mountfitchet
		CM24 8HN
		Tel. +44 (0)1279 817604 Fax: +44 (0)1279 817601
	Also see above, under suppliers for speaker drivers.

12.12 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.

12.13 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

	Two other vendors of speaker repair parts are:
		Parts Express (sells 8", 10", 12", & 15" repair kits)
		340 E First St
		Dayton OH 45402-1257 USA

		Simply Speakers
		P. O. Box 22673
		St. Petersburg FL 33742 USA
		800-767-4041 or 813-571-1245

	Also check out: and for directions on 
	replacing speaker foam.

	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.

12.14 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 12.9) tells you how to measure a speaker, and also 
	gives 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 programs for speaker design and on
	speaker-design hardware, such as measurement systems, get
	the archive "sahfsd**.doc" from directory:
	on "". In addition, there are other interesting
	audio-related files in that directory. Look around.
	That file is also available on in

12.15 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.

12.16 What are all of these abbreviations people use for speakers?
	Most of these parameters are well documented in the Loudspeaker
	Design Cookbook. (see 12.9) 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;
	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; 
	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

12.17 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.

12.18 Should I use spikes under my speakers? Pennies under the spikes?
	Spikes prevent speakers from rocking. They also couple the
	speaker directly to the floor. Spikes will pierce carpet.
	Some spikes will damage carpet. Most will just put a small
	hole in the carpet which is invisible. Putting a heavy 
	speaker directly on carpet will cause a permanent mark on
	the carpet. Spikes can prevent this.

	If you have a pretty hardwood floor, then spikes will definitely
	damage the finish. A rigid disc under the spike will distribute
	the load and lessen the damage. Any coin should work fine. Using
	a coin will not change the speaker/floor interaction. Do not use
	a coin with a carpeted floor. Alternatives to spikes for wood
	floors are Blu-Tack and similar products. (see 12.19) 

	If your floor is extremely rigid, then the spikes will make
	the speaker more rigid. If the floor is more conventional,
	such as a suspended floor or a wooden floor over joists,
	spikes can have a positive or negative effect, depending on
	the resonant characteristics of the floor/speaker system.

	The counterforce resulting from a forward cone motion in a
	speaker may try to move the speaker backwards, but spikes will
	have little or no effect on this. Most audible effects from
	spikes are due to coupling the speaker to the floor, so it
	will be less likely to resonate on its stand. Some argue that 
	in most cases, spikes will have no audible effect at all. 
	Try it for yourself.

12.19 How do you couple speakers to speaker stands?
	Ideally, your speakers should sit flat on the speaker stand 
	or floor. They shouldn't see-saw back and forth if nudged.

	One good way to accomplish this is to use a small dab of 
	putty under each corner of the speaker. There are a few 
	common putties used for this, but all share the properties 
	of being very elastic and staying flexible indefinitely. 
	These putties are inexpensive, removable, and reusable.

	Try either Blu-Tak, which is available in the UK from office
	supply stores for cleaning typewriter elements, Faber Castell
	UHU Hold-It, which is available in the US from office supply
	stores for holding up pictures, DAP's Fun-Tak, which is sold
	in hardware stores for holding up pictures, or Pritt Buddies. 

12.20 What is a Sealed, Ported, Bass Reflex, Acoustic Suspension, 
	Bandpass, and Coupled Cavity Speaker? Which is better?

	All are "direct radiator" enclosures, so called because the 
	sound is produced directly from the driver (the "radiator") 
	without the assistance of a contrivance such as a horn.

	The simplest direct-radiator system. The rear of the driver 
	sees a sealed enclosure, and none of the rear output of the 
	driver contributes to the sound output. Depending upon how 
	stiff the mechanical suspension is vs how stiff the enclosed 
	air in the enclosure is (and that's a function of the size of 
	the box), you can have either an Infinite Baffle enclosure, 
	in which the mechanical suspension is the dominant source of 
	system stiffness and the box is large; or an Acoustic 
	Suspension enclosures, where the air in the box is the 
	dominating stiffness, and the box is small.

	Sealed boxes tend to be the lowest efficiency systems for a 
	given box size and bass cutoff frequency.

	Also the same as Bass Reflex, Ported, or Passive Radiator. 
	Here, an aperture in the box provides a means for the rear 
	output of the cone to contribute to the total output of the 
	system. However, it only contributes over a very narrow range 
	of frequencies. In fact, in a properly designed system, the 
	front output of the cone is reduced at the same time the 
	output of port increases, so the port DOES NOT ADD to the 
	output of the woofer, it REPLACES the output of the woofer at 
	these frequencies. This, if done properly, can significantly 
	reduce distortion and increase power handling at very low 
	frequencies, a region that can be difficult for drivers.

	Vented systems can be up to 3 dB more efficient than a sealed 
	box system that has the same bass cutoff frequency and size.

	These are compound systems in that they have at least two 
	enclosures: one on the front and one on the rear of the driver.
	The enclosure on the front, which looks remarkably like a vented 
	box (because it is), acts as a low pass filter, and, can couple 
	the output of the woofer more efficiently to the outside. They 
	have several useful advantages. For example, the front enclosure 
	can be used as a very effective acoustic crossover, filtering 
	out mechanical noises generated by the woofer, something 
	no electronic crossover can do. For very low frequencies, 
	such an acoustic crossover can be far less expensive and 
	more easily designed than an equivalent electronic crossover.

	They are called "bandpass" because the combination of the rear 
	enclosure and the driver form the high pass portion while the 
	front enclosure forms the low pass section. Making the bandwidth
	of the system narrower raises the efficiency of the system.

	A variation of bandpass and vented systems, they are the results 
	of a designers attempt to solve specific problems. They consist 
	of two or more rear enclosures, each coupled to the next by a 
	vent. Each enclosure/vent combination is another resonant system,
	and the combination is, essentially, a high order, multi-tuned
	resonant system.

	Generally, these systems have quite complex response and are 
	difficult to design. No comprehensive theory on their operation 
	exists like that for sealed, vented and bandpass systems.

12.21 What is the best material to make speaker boxes out of? Why?
	An ideal speaker cabinet material would be very stiff, so that 
	it would not tend to move with variations in box air pressure.
	It would also be very well damped, so that if it ever does 
	deflect from air pressure, it will come back to the original
	position without resonating. It would also have a very high
	resonant frequency (supersonic), so that low frequency box air
	pressure would not cause it to resonate. An attractive material 
	is preferred, and additional credit is given for a material 
	which is easy to cut, glue, and finish. A great material would 
	be cheap, too. Finally, it would be nice if the material were 
	light, because we all have to move our speakers sometimes, 
	and it's hard to appreciate good speakers with a sore back.

	With all of those attributes, it would seem that no 
	material is perfect. However, there are many materials that 
	have enough of the above good attributes to make excellent 
	speaker cabinets. Yet each has advantages and disadvantages.

	In the list of good speaker box materials below, letters are
	used to indicate which attributes the material possesses.

		S = Stiff
		D = Damped
		H = High Resonance
		A = Attractive
		M = Machinable
		C = Cheap
		L = Light
	MEDIUM DENSITY FIBERBOARD (MDF): SDMC This is the most practical
	material for quality speakers. It is harder to find than plywood, 
	but most lumber yards can special order it. It cuts very nicely 
	and has a smooth surface. It takes veneer very well. However, 
	bring a helper when you pick the stuff up. One sheet is very 
	heavy. MDF is harder on tools than common wood, but easier than
	particle board. This is the material that many great speaker
	makers use. US $45 for a 4'x8'x1" sheet. Density: 50 lbs/cu ft.
	POLYCARBONATE (LEXAN): DM A clear or solid-color polycarbonate 
	box can look strikingly good. However, this is not a cheap 
	material. To locate it, look in the classified directory under 
	PLASTICS. US $400 for a 4'x8'x0.5" sheet. Density: 75 lbs/cu ft.
	Acrylic (Plexiglass) is cheaper than Polycarbonate, but weaker
	and poorer damped (not recommended).
	GIBRALTAR (tm): SDA Regardless of the brand, these synthetic 
	countertop materials come in a wide array of colors and look 
	beautiful. They are hard to buy, and different to work with. 
	They take special glue to bond and require wet sanding with 
	very fine paper to finish. You can tap it, but it's too brittle 
	for wood screws. Helicoil inserts are very effective.  Yet an
	experienced builder can complete a cabinet in under an hour,
	from raw material to final finish.  Corian is acrylic mixed with 
	powdered aluminum trihydrate clay filler. Avonite, Gibraltar,
	and Surell are polyester resin mixed with filler. One user
	commented that Corian is easier to use and is easier to make
	invisible seams than the other synthetics. It has been said
	that Corian is actually easier to use than wood, but that
	depends on your equipment and experience level.  Estimated cost
	for Corian is US $20 per 1'x1'x0.5". Density: 100 lbs/cu ft.
	Available from:
		Art Specialties
		74 North Aurora St
		Lancaster, NY 14086
	Ask for their free information pack on working with Corian.
	Note: These product names are registered trade marks and apply
	to specific materials from specific manufacturers.
	MARBLE: SDHA One challenge with marble speaker enclosures is 
	cutting holes for the drivers. A carbide bit on a router will 
	work, but it will dull quickly. Marble is also difficult to glue, 
	so bracing is difficult. But it sure is pretty when you're done!
	US $25 to $45 per 1'x1'x1.25". Density: 160 lbs/cu ft.
	If you have time on your hands and want a great impractical box, 
	try this. Make a simple box out of common plywood. Then glue
	cleats on the outside of the box to space the outside plywood 
	from the common plywood. Glue hardwood-veneered plywood to the 
	cleats and pour sand or lead shot into the spaces between the 
	cleats. It won't be light, but with the filler, it will be 
	extremely well damped. In addition, if you use strong cleats 
	and glue well, the box will be extremely stiff. One person used 
	different size Sonotubes as an alternative to plywood, and 
	filled the space between them with sand. Be sure to sterilize 
	the sand in your oven before putting it in the box.

	(Aerolam): SDHL Airplanes use this material for flooring. Next 
	time a plane crashes in your neighborhood, see if you can get 
	the wreckage for your next speaker project. You can't get a 
	better, light-weight material. Celestion has exploited this for 
	some great products. If you're really ambitious, you can make 
	your own sandwich out of high-quality plywood faces and a thick 
	honeycomb core. You will probably need an epoxy to glue the 
	honeycomb to the plywood. A home-brew sandwich is easier to cut 
	and glue than Aerolam. 
	FORMED CONCRETE: SDHC There are tricks to working concrete, such
	as to cast braces, rebar, and steel-wire right into the mix. 
	Also, some concrete is better damped than other. Remember to oil 
	your concrete forms so that they can be removed. Most concrete 
	speakers use an MDF front panel, but you can pour one if you use 
	cardboard tubes or plywood rings to mold the concrete into the
	shape of a speaker cutout. Alternately, you can make a common
	veneered plywood speaker box and cast concrete inside it for

	Any box can be improved by making the walls thicker, by bracing 
	the walls, and by stiffening the walls. The stiffness of a 
	material goes up as the cube of the thickness, so a slightly 
	thicker material is much stiffer. A thicker panel will also have 
	a higher resonant frequency because the stiffness goes up faster 
	than the mass.

	Consider lining the inside of your speaker with ceramic tile,
	attached with thinset mortar. You can get tile remnants cheaply. 
	They are easy to apply and can be added as an afterthought to 
	an imperfect box. However, be sure to attach all braces before 
	tiling, because it is hard to attach anything to tile.

	Also consider bracing any weak parts of the box. For example, 
	all joints will benefit from a wooden cleat. The back of the 
	box will benefit from stiffeners where the speaker terminals 
	are attached. Most importantly, brace the front panel, or 
	make it out of a double thickness of material.

12.22 What size fuse or circuit breaker should I put in my speaker to 
		protect it from damage?
	Most modern speakers consist of a box containing more two or
	more drivers interconnected through a network of inductors,
	capacitors, and resistors. One fuse or circuit breaker in
	series with that array can't possible protect all drivers.

	Conventional circuit breakers are a very bad choice for speaker
	protection. They add series resistance, series inductance, and
	lousy electrical contacts, all tending to degrade performance.
	Moreover, breakers have a trip characteristic that does not
	match the damage mechanisms of speakers.

	Fuses are a better choice, but still are not very good. This
	is because speakers have complex thermal behavior. Loud
	playing will warm up the voice coil making it more sensitive to
	damage. No fuse takes this into account correctly. A fuse
	will do a better job of protecting tweeters, but is still not

	If you want to protect a speaker with a fuse, use the lowest
	current, fast-blow fuse which will not blow during normal
	listening. This may trip prematurely in a very loud passage,
	or may degrade sound quality, but it is your best bet for fuse
	protection. For a woofer, start with a 1 Amp fuse and work up.
	For a tweeter, start with 100mA and work up.

	There are also cheap tweeter protectors available which contain
	a light bulb and a resistor potted in a small tube. They work
	pretty well, and if you reduce the tweeter network's series
	resistance by a few tenths of an ohm, they are not terrible for
	the sound. But they are audible and not failsafe.

12.23 Why are speakers labeled + and - or Red and Black?
	Speakers make sound my pushing and pulling at the air with the
	motion of their cones or diaphrams. When a positive voltage is
	applied to the red or "+" terminal on a standard speaker, it
	causes the cone to move outwards and push air.

	If you have two speakers side by side and one cone moves out
	while the other moves in, air will move between the two
	speakers but not much sound will escape. The two cone motions
	will cancel eachother. So when you have two speakers close
	together, it is vital that they be wired "in phase", with
	positive voltage going to the "+" terminal of both speakers at
	once. You can do this by wiring the speakers in parallel or
	series. In almost all cases, parallel is preferred. If wiring
	speakers in parallel, the "+" output should go to both "+"
	terminals and the "-" output should go to both "-" terminals.
	If wiring speakers in series, the "+" output should go to one
	"+" terminal. The other terminal ("-") should go to the second
	speaker "+" terminal. The other terminal ("-") of the second
	speaker should go to the "-" output. See the FAQ section on 
	amplifiers for more on series and parallel connections.

	Even if speakers are not side by side, it is good to wire them
	in phase. For very low frequencies, speakers 15 feet apart
	are effectively close together and the same cancellation
	effects mentioned above apply. For higher frequencies, the
	effects are more subtle but still important. One symptom of
	wiring speakers wrong is that the stereo effect is imperfect.
	Instead of a main sound seeming to come from the center, the
	sound of the lead vocalist, for example, may seem to come from
	outside the room. Other odd effects are also possible.

	So when in doubt, always wire "+" to "+".

12.24 What is the best "stuff" to fill a speaker cabinet with?
	The following discussion will focus on practical facts on speaker
	cabinet stuffing and on sealed systems. Theory is limited help 
	in selecting speaker stuffing. Vented system do share a few of 
	these same issues and will also be mentioned, but the goals and 
	physics of stuffing a vented box are different than those of a
	sealed box. 

	NHT speakers use polyester fill. Some use a Danish polyester that 
	mimics the properties of fiberglas very closely. Excluding this 
	special poly, there are two kinds of polyester available: pillow 
	stuffing, and audio-spec polyester.

	Forget common pillow fill. It's cheap and easy to get. If you 
	use enough, it will damp the midrange, and that's a lot better 
	than an empty box but it has little effect on lower frequencies.
	"Mountain Mist Polyester Fiberfill" from Stearns Technical 
	Textiles is a common, inexpensive material that is said to 
	perform as well as audio-spec polyester. Stearns also sells 
	"Fiberloft Premium Grade Polyester" to some speaker makers.
	Mountain Mist is a coarser fiber than Fiberloft, but both are 
	the same composition. We have no information on differences in
	acoustic properties between Fiberloft and Mountain Mist, but
	Fiberloft makes softer pillows and costs more. Both are 
	available from these chain cloth stores:
		Cloth World
		Hancock Fabrics
		House of Fabrics
		Jo Ann Fabrics
		Minnesota Fabrics
	For more information, contact:
		Stearns Technical Textiles
		100 Williams Street
		Cincinnati OH 45215
		513-948-5252 or 800-345-7150

	For lining the walls of a vented enclosure to reduce internal
	reflections, or filling a transmission line to absorb the back
	wave, highly absorptive wool or fiberglas are ideal. However,
	these materials do not provide the desired results in a sealed
	system. They will provide more reflection absorption than
	polyester, but the latter is quite good in this regard in the
	critical midrange. In a sealed system you don't want
	absorption at lower frequencies anyway; you want damping and
	isothermal conversion. (Author's note: I have tried "all-out"
	efforts using fiberglas lining and polyester fill to achieve
	the best of both worlds. I found little practical benefit over
	polyester alone.)

	Most professional designers agree that practical experience,
	combined with trial and error is the best way to get optimum
	stuffing material, quantity, and method for a given design.
	This is why good designers routinely experiment with fill in
	the development of a new system. If you are designing a system
	that differs substantially in shape or volume or source
	impedance (passive crossover) from one of known reference, you
	will need to experiment to get best performance.

	Adjusting the filling is the last step in getting bass right,
	and is used mostly to fine-tune the system Qtc and resonance.
	As increasing amounts of polyester are added to a sealed box,
	the resonance and Q gradually go down. This can be shown
	mathematically to be due in roughly equal parts to the effects
	of simple resistive damping and isothermal conversion. At some
	point, a minimum is reached, and further material reverses the
	trend by taking up volume. An experienced designer can find the
	optimum amount of fill in a few trials by monitoring the
	impedance versus frequency curve as stuffing is added or

	Filling also has the important effect of reducing internal
	reflections, to reduce standing waves and comb filtering.
	However, the amount of filling has comparatively little effect
	on this.

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:
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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