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 visa...@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: stea...@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: neido...@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.
