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Subject: FAQ: rec.audio.* Speakers 7/07 (part 5 of 13)

This article was archived around: 15 Jan 2009 06:01:55 GMT

All FAQs in Directory: AudioFAQ
All FAQs posted in: rec.audio.tech, rec.audio.opinion, rec.audio.misc, rec.audio.marketplace
Source: Usenet Version

Archive-name: AudioFAQ/part5 Last-modified: 2007/07/12 Version: 2.17
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: http://bigsun.wbs.net/homepages/o/m/n/omniaudioscam/ http://www.frii.com/~rjn/audio/whitevan.htm 12.7 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". 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 603-924-9464 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 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 603-924-9464 $30.00 + approx. $3 Shipping from: Madisound 8608 University Green; Box 4283 Madison WI 53711 USA 608-831-3433 $30.00 + ??? S&H from: Parts Express 340 E. First St Dayton OH 45402 USA 800-338-0531 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 http://www.audioc.com 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 Mulbarton 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 303-789-5310 Madisound (Broad line) 8608 University Green Box 4283 Madison WI 53711 USA 608-831-3433 http://www.itis.com/madisound/ Meniscus (Broad line) 2442 28th Street SW Ste D Wyoming MI 49509 USA 616-534-9121 Parts Express (Broad line) 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 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 602-272-6696 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 516-747-3515 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 205-633-2054 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 Essex CM24 8HN UK Tel. +44 (0)1279 817604 Fax: +44 (0)1279 817601 E-Mail visaton@visaton-amc.demon.co.uk 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 800-747-3692 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 513-222-0173 Simply Speakers P. O. Box 22673 St. Petersburg FL 33742 USA 800-767-4041 or 813-571-1245 Also check out: http://www.decware.com/surround.htm and http://www.les.safety.net/stepp.html 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: usenet/rec.audio.high-end/Software on "ftp.uu.net". In addition, there are other interesting audio-related files in that directory. Look around. That file is also available on ftp.graphics.cornell.edu in /pub/rahe/software 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; 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 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. SEALED BOX: 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. VENTED ENCLOSURES: 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. BANDPASS: 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. COUPLED CAVITY: 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). CORIAN (tm), FOUNTAINHEAD (tm), AVONITE (tm), SURELL (tm), 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 800-724-4008 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. PLYWOOD SHEETS SPACED AND FILLED WITH SAND OR LEAD SHOT: SDAMC 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. ALUMINUM SHEETS SPACED AND FILLED WITH ALUMINUM HONEYCOMB (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 stiffening. 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 perfect. 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 http://www.palaver.com/mountainmist/ E-mail: stearns@fuse.net 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 removed. 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. COPYRIGHT NOTICE The information contained here is collectively copyrighted by the authors. The right to reproduce this is hereby given, provided it is copied intact, with the text of sections 1 through 8, inclusive. However, the authors explicitly prohibit selling this document, any of its parts, or any document which contains parts of this document. -- Bob Neidorff; Texas Instruments | Internet: neidorff@ti.com 50 Phillippe Cote St. | Voice : (US) 603-222-8541 Manchester, NH 03101 USA Note: Texas Instruments has openings for Analog and Mixed Signal Design Engineers in Manchester, New Hampshire. If interested, please send resume in confidence to address above.