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FAQ:
rec.audio.* Speakers 2/99 (part 5 of 13)
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 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 don female singing voices provide a very good test of a system'
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, o 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. N 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 i 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 A 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 perceive as a level difference. Some
claim that you will be influence 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 i the same room with
you. Does it sound realistic? Likewise wit 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 o 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 i "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 you first realization should be that the
music is all there, an that it comes together as good music, without one
particular part trying to dominate it. Sit back and listen to it for bit.
You should be able to pick out the individual instrument 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 i trying to grab your attention away from the others.
You should check how things sound with the amp turned up, an 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 speaker 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 t mono to evaluate naturalness. Mono is a
good test of both the room and the speakers. The image should be
rock-solid dear 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, i 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 "intermediation distortion".
A third is to increase the power handling ability of the system and the
overall reliability. Al 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 than goes between
your amp and your main speaker which sends the low 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 good small-box
system. Done wrong or haphazardly, anything i possible. Even a fine large
speaker system might benefit fro careful addition of a subwoofer. However,
the better the original system, the more likely it will be that a modes
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
fee 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 t 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 lo frequency sound and very low
frequency sound is non-directional subwoofers also have output at 100 Hz,
and sound at 100 Hz i 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 an 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 i 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 bas amplifier and crossover. Some A/V receivers contain a
splitter specifically for use wit subwoofers. If you have one of these,
you will either want 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 product over the years. Many mass-hifi
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 an Fosgate can accommodate several more speakers beyond the
two additional ones (usually placed in the rear). Often one ca find Pro
Logic systems with two front, two rear, two side, a well as a center
channel speaker for dialogue.
12.6 I was just approached (accosted?)
by a couple of kids driving van that said they had some GREAT speakers to
sell. They are overstocks, used by major recording studios and DJs
or eve hot, and they normally sell for $1000/pr, but they'll le 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 i
studios. There might be one or two DJs out there that us them because they
can't afford anything else. They are no 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 s 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 i the van about US $100 a pair, and
the kids are given minima training about what kinds of stories to use,
what parking lot 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 o
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
keep 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 region or
countries. If you really want recommendations and are willing to listen t
the opinions of others, check the past few issues of Stereophil Magazine.
Although they are strongly biased towards very expensive gear and have
their own particular other biases, the 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, an 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 Dic Olsher which are similar two-way ported systems. A recent
one o these was in Stereophile Nov '90, pages 94-12.. Audio Magazines
published a plan called "The Pitts" by Ken Kantor, in Audio, No
'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 part 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, an 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 Design
Solen Electronique 4470 Avenue Thibaul St.-Hubert, QC J3Y 7T9 Canada Voice
514-656-275 FAX 514 443-494
High Performance Loudspeakers by Martin Collom
Speaker Builder
Magazine Audio Amateur Publication PO Box 49 Peterborough NH 03458 US
603-924-946
Synergetic Audio Concepts Classes and Newsletter
Syn-Aud-Con teaches classes on Audio and Acoustic
12.70 W. Co. Rd. 100 N. Norman IN 47264 US 8 12.995-821
The Loudspeaker Design Cookbook, Fifth Edition by Vance Dickason
(C)
199 ISBN 1-882580-10- $34.95 + $4.45
S&H
from Old Colony Sound La PO Box 24 Peterborough NH 03458-0243 US
603-924-946 $30.00 + approx. $3 Shipping
from Madisound 8608 University Green; Box 428 Madison WI 53711 US
608-831-343 $30.00 + ???
S&H
from Parts Express 340 E. First S Dayton OH 45402 US 800-338-053
12.10 Where can I buy speaker drivers?
Audio Concepts (Their own kits plus drivers
901 South 4th Stree LaCrosse WI 54602 US Voice 608-784-457 http://www.audioc.co
Phil Baker (Surplus cabinets only 546 Boston Avenu Medford MA 02155)
US Bandor Design & Development Studios (Aluminium coned
speakers) 11 Penfold Cottage Penfold Lan Holmer Gree Bucks, HP15 6XR
United Kingdo Tel. (01494) 71408
DBS Audio (Speaker kits and crossovers) PO Box 91, Bury St. Edmunds,
Suffolk, IP30 0NF United Kingdo Tel (0284) 82892
Drexler Audio Systems (Bandor Speaker Distributor) 14 Rose Lan Rosemont PA
19010 US
Falcon Electronics (Drivers and cross overs) Tabor Hous Mulbarto Norfolk,
NR14 8JT United Kingdo Tel. (0508) 7827
Faraday Sound (Concrete loudspeaker cabinets) 248 Hall Roa Norwich, NR1
2PW United Kingdo Tel. (0603) 76296
Gold Sound (Broad line including pro speakers) PO Box 14 Englewood CO
80151 US 303-789-531
Madisound (Broad line) 8608 University Gree Box 428 Madison WI 53711 US
608-831-343 http://www.itis.com/madisound
Meniscus (Broad line 2442 28th Street SW Ste Wyoming MI 49509 US
616-534-912.
Parts Express (Broad line) 340 East First Stree Dayton OH 45402-12.7 US
513-222-017
Solen Electronique (Airborne, Audax, Ceratech, Dynaudio, Eton Lpg, Morel,
Peerless, Scan-Speak, Seas, Solen, Vifa) 4470 Avenue Thibaul St.-Hubert,
QC J3Y 7T9 Canada Voice 514-656-275 FAX 514 443-494
The Speaker Co (Large range of drive units plus speaker kits Unit) 9,
Waterside Mil Waterside, Macclesfield, SK11 7HG. United Kingdo Tel. (0625)
50050
Speakers Etc.2728 West Thomas Roa Phoneix AZ 85017 US 602-272-669
SRS Enterprises (Pyle, Pioneer, Eminence, Ultimate, Fane, MG) 1839 N
Circle D Colorado Springs CO 80909 US Voice 719-475-254 FAX 719-475-035
Wilmslow Audio (Kits and drive units. KEF, Dynaudio, Audax, SEAS Peerless,
Scanspeak, Morel) Wellington Clos Parkgate Trading Estat Knutsford,
Cheshire, WA16 8DX United Kingdo Tel (0565) 65060
Zalytron (Broad line including kits) 469 Jericho Turnpik Mineola NY 11501
US 516-747-3515
12.11 Where can I buy loudspeaker kits?
Audiocab (Speaker kits and cabinets) 9
Skewbridge Clos Wooten Bassett, Swindon, SN4 7DW United Kingdo Tel (0793)
84843
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 Roa Norristown, PA 19401 US 610-277-1014 or
800-255-101
IPL Acoustics (Kits using SEAS, Morel, Audax, and Visaton) 2 Laverton Road
Westbury, Wiltshire, BA13 BRS United Kingdom Tel (0373) 82333
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 #48 Mobile AL 36695 US
205-633-205
Tabula Rasa (Wide range of speaker kits) 1 Silkin Dalton Clos Broadfield,
Crawle W. Sussex, RH11 9JD United Kingdo Tel. (0293) 53119
Visaton UK Ltd (Drivers, crossovers, kits, designs, software) 2 Bentfield
Roa Stansted Mountfitche Esse CM24 8H U Tel. +44 (0) 12.9 817604 Fax: +44
(0) 12.9 81760 E-Mail visaton@visaton-amc.demon.co.u 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 a ear level, in front of you, squared off between you. It's the 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, o can produce very subtle changes, which you might
prefer. For example, you might improve a speaker by adding some cross
brace 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 o 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 th 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, o putty will have subtle effects on the sound. For
the brave at heart, you can replace old or cheap driver with better ones,
but the results of this one change can be very dissatisfying if you happen
to get the wrong type of driver fo that application, and may never sound
right, even if you use similar driver. Speaker system design is still
somewhat of 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. Man tweaks to good speakers, no matter how well thought through
will correct for one flaw, but create others, or correct a flap 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 distributor. This is
sometimes difficult because it is hard t 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 se of specific characteristics. There are companies that
rebuild drivers, but they charge quit a bit. I have heard $75 per driver.
This is rarely done fo anything but very expensive commercial drivers.
Speaker manufacturers will often sell owners the materials that the 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 blow or distorted voice
coil, this still won't help. A few netter have used rebuild kits from this
company successfully. Contact Stepp Audio Technologie PO Box 108 Flat Rock
NC 38731 US 800-747-3692. Two other vendors of speaker repair parts are
Parts Express (sells 8", 10",12., & 15" repair kits 340
E First S Dayton OH 45402-12.7 US 513-222-0173 Simply Speaker P. O. Box
2267 St. Petersburg FL 33742 US 800-767-4041 or 813-571-12.5 Also check
out: http://www.decware.com/surround.htm
and http://www.les.safety.net/stepp.html
for directions o 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 manufacturer 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 o 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 a integral magnetic shield. Magnetic shielding
is usually don 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 lo frequency performance. Cancellation is done
using a reverse-polarized magnet glued t the back of the main magnet. If
done right, it can almost completely cancel the rear stray field. In some
cases it ca also increase the magnetic flux density in the gap, which ma
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 a which driver
impedance is maximum.
Fc System resonance (usually for sealed box systems), in H Fb Enclosure
resonance (usually for reflex systems), in H F3 -3 dB cutoff frequency, in
HZ
Vas "Equivalent volume of compliance", this is a volume o air
whose compliance is the same as a driver' acoustical compliance Cms
(q.v.), in cubic meter
D Effective diameter of driver, in meter
Sd Effective piston radiating area of driver in square meter
Xmax Maximum peak linear excursion of driver, in meter
Vd Maximum linear volume of displacement of the drive (product of Sd times
Xmax), in cubic meters.
Re Driver DC resistance (voice coil, mainly), in ohm
Rg Amplifier source resistance (includes leads, crossover etc.), in ohm
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;
dimensionles
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 o stiffness), in m/ Mms
The driver's effective mechanical mass (including air load), in k
Rms The driver's mechanical losses, in kg/
Cas Acoustical equivalent of Cm
Mas Acoustical equivalent of Mm
Ras Acoustical equivalent of Rm
Cmes The electrical capacitive equivalent of Mms, in farad
Lces The electrical inductive equivalent of Cms, in henries
Res The electrical resistive equivalent of Rms, in ohm
B Magnetic flux density in gap, in Tesla
l length of wire immersed in magnetic field, in meter
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/
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 an 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 th same voice coil. The other big effect of this
fluid is to ad 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, ma 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. No 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 doubt over whether these fluids can dry
out with time. Manufacturer claim that the oil used is non-volatile. It is
possible to use ferrofluids in mid-range drivers an 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 no be compatible with the
adhesives used in the driver, may not b 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 o 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 us a coin with a carpeted floor. Alternatives to
spikes for wood floors are Blue-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 o the resonant characteristics of the floor/speaker system. The
counterforce resulting from a forward cone motion in speaker may try to
move the speaker backwards, but spikes will have little or no effect on
this. Most audible effects fro spikes are due to coupling the speaker to
the floor, so i 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 o putty
under each corner of the speaker. There are a few common putties used for
this, but all share the property of being very elastic and staying
flexible indefinitely.
These putties are inexpensive, removable, and reusable. Try either Blue-Tak,
which is available in the UK from office supply stores for cleaning
typewriter elements, Faber Castel 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 drive 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 enclose air in the enclosure is (and that's a function
of the size o 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 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 a 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. The 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 an 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
result of a designers attempt to solve specific problems. They consist of
two or more rear enclosures, each coupled to the next by vent. Each
enclosure/vent combination is another resonant system and the combination
is, essentially, a high order, multi-tune 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 doe 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 n 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 speakers makers use. US $45 for a
4'x8'x1" sheet. Density: 50 lbs/cu ft.
POLYCARBONATE (LEXAN): DM A clear or solid-color polycarbonat box can look
strikingly good. However, this is not a chea material. To locate it, look
in the classified directory unde 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 wit very fine paper
to finish. You can tap it, but it's too brittle for wood screws. Helicoil
inserts are very effective. Yet a experienced builder can complete a
cabinet in under an hour from raw material to final finish.
Corian is acrylic mixed wit powdered aluminum trihydrate clay filler.
Avonite, Gibraltar and Surell are polyester resin mixed with filler. One
use 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 S
Lancaster, NY 1408 800-724-400. 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: SDAM. 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 cleat and
glue well, the box will be extremely stiff. One person use different size
Sonotubes as an alternative to plywood, an 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 HONEYCOM (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 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 cu 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 us cardboard tubes or plywood
rings to mold the concrete into th 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 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 t 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 th box will benefit from stiffeners where the speaker terminal are
attached. Most importantly, brace the front panel, o make it out of a
double thickness of material.
12.22 What size fuse or circuit breaker
should I put in my speaker t protect it from damage?
Most modern speakers consist of a box
containing more two o more drivers interconnected through a network of
inductors capacitors, and resistors. One fuse or circuit breaker i 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, an lousy electrical contacts, all tending
to degrade performance.
Moreover, breakers have a trip characteristic that does no 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. Lou
playing will warm up the voice coil making it more sensitive t damage. No
fuse takes this into account correctly. A fuse will do a better job of
protecting tweeters, but is still no 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 by pushing and pulling
at the air with the motion of their cones or diaphragms. 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 motion will cancel each other. So when you have two speakers close
together, it is vital that they be wired "in phase", wit
positive voltage going to the "+" terminal of both speakers a
once. You can do this by wiring the speakers in parallel o 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 on
"+" 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 o amplifiers for more on series and parallel
connections. Even if speakers are not side by side, it is good to wire the
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 o 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 fro 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 o these same issues and will also be mentioned, but the goals an
physics of stuffing a vented box are different than those of sealed box.
NHT speakers use polyester fill. Some use a Danish polyester that mimics
the properties of fiberglass very closely. Excluding thin 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 t perform as well
as audio-spec polyester. Stearns also sell "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 i acoustic properties
between Fiberloft and Mountain Mist, buy Fiberloft makes softer pillows
and costs more. Both are available from these chain cloth stores Cloth
Worl Hancock Fabric House of Fabric Jo Ann Fabric Minnesota Fabric For
more information, contact Stearns Technical Textile 100 Williams Street
Cincinnati OH 4521 513-948-5252 or 800-345-715 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 fiberglass 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 wan absorption at
lower frequencies anyway; you want damping an isothermal conversion.
(Author's note: I have tried "all-out efforts using fiberglass 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 i 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 show mathematically to be due in
roughly equal parts to the effect 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 o 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. |
