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Speaker Design and how it affects your choices |
NOTE: Larry Mundy is, in his words, "a cranky old coot in north Texas who never made it big in rock and roll, but hasn't given up because he has so few other talents. He has participated in the 'involuntary disassembly' of many speakers and cabinets over the years, and is a proponent of clean, loud sound because he's now half-deaf and can't really hear much else." You can email him with questions, criticism or other comments at lmundy@attbi.com.
NOTE: For those guitar players who use their Vintage Gear, please realize that many top artists also use old stuff too, but they never push the Vintage Gear beyond its operational limits - often 'Miking' the gear (putting a microphone, about 1 foot away from the front of the speaker and running this into the PA system). Running a guitar directly into a PA system often results in a very poor guitar sound since guitar amps and thier associated speaker systems are optimized for a given tone quality (ie. 'coloring the sound' - which is another way to say distortion, but in terms that is pleasing, musically). Guitar Tone is not what is being discussed here, however, the speaker selection for a guitar amp use will follow all of these same guidlines. Taking gear on the road is very hard on it, which should be a concern any time that you are using equipment that is not easy (or impossible) to replace. Please keep these points in mind as you read thru Larrys information - Jens Moller
Choosing a speaker
I've written a couple of articles encouraging musicians to try their hand at building their own speaker cabinets. Unfortunately, the speaker-selection passages in those articles usually say something unhelpful like "buy some good speakers." There are a lot of technical aspects to speaker (and amp) design, most of which you don't have to know in order to assemble something that sounds good and works well.Let's just say you are a performing musician that uses electronic amplification, and you find a need to buy a speaker or two for that do-it-yourself cabinet you're going to build, or more likely, to replace that cheesy speaker in your amp that blew up halfway through your last gig. Happens every day, and the chances are good that this situation applies to you, or you would be watching something interesting on TV instead of reading boring articles like this.
You probably have some idea of the diameter of the speaker you want or need. For a replacement speaker, you know the impedance, measured in ohms, of the speaker you're replacing (this should be written on the dead speaker somewhere; "ohms" may be abbreviated into the little Greek "Omega" symbol). You have some idea what you want it to do, which is to reproduce your dazzling musicianship for an admiring audience, and not melt into fecal matter like the last one did. Beyond that, maybe you're a little confused, and maybe I can confuse you a little further.
Right up front, let me say I'm not an electroacoustical engineer, but I'll bet you're not either. I'm just a guy who wants to play live music, watch drunks try to dance, and hopes to form a meaningful overnight relationship with that blonde in the third row. Having done this for more years than I like to admit, I've blown and replaced quite a few speakers, so perhaps I've learned some things you might find helpful.
There are as many types and sources of "raw," or unboxed/unmounted, speakers as there are hairs in my nose, maybe even more. So where do you start? You have lots of choices:
- Cannibalize a speaker from some other cabinet, hopefully one not
owned and cherished by someone else. Unless it's a pretty good cabinet
with quality speakers, this is a hit-and-miss approach and always has
the drawback of leaving that other cabinet speakerless. The
speakers in your sister's stereo system are not built for the
type of punishment you will give them, and soon you will have not
one, but two blown speakers, as well as a very angry sister. You
are probably going to have to spend some actual money.
- Go to the local electronics-appliance-and-computer-accessory
"superstore" or its small-store equivalent like Radio Shack.
The raw speakers sold there are consumer-grade stuff, and
are probably designed for tabletop radios or low-end car
stereos. They will turn into doorstops in a week.
- Go to a high-end audio store. If they sell raw speakers
at all, they will be replacements for home-stereo-type speakers,
meant to faithfully reproduce a broad spectrum of music in a
cozy home environment. They will be expensive, inefficient,
and very critical of the design of the cabinet they're
placed in. Oh yes, and they will blow themselves inside-out
if you turn your amp past "2".
- Go to the car-stereo store, or car-stereo section of one
of the megastore. There you will find heavy speakers with
big magnets that look promising, but won't work worth a darn for
you. The drawbacks of even the best car-stereo woofers are (1)
they are designed for a completely different acoustic environment;
and (2) they are almost always really stupid-looking. Taking
the first point, car stereo woofers have rather flexible
suspensions and low-impedance voice coils, to wring the most
power out of a little convection-cooled amplifier powered by
a 12-volt system originally intended to power headlights and
spark-plug coils. These speakers are designed to emit sound
at high sound-pressure levels in the confined space of a vehicle
interior. Unless all of your performances occur in the back
seat of a Chevrolet, these are the wrong speakers for you.
Taking the second point, car-stereo woofers are also made to
be admired by a certain small segment of the population that
thinks purple cones with red surrounds and a big logo in the
center that reads "RumbleThrust Earsplitter XRS2000," is the
height of cool. Car stereo manufacturers are also very sneaky
about adding a couple zeroes to the end of their actual power
ratings, so that Junior can say he bought a nine-thousand-watt
amp for his pickup out of last week's grocery-bagging money.
Unless you think the single greatest improvement you could
make to your amp would be to chrome-plate the knobs,
stay away from car stereo stores.
- You could write or call the manufacturer of your amp or cabinet, or ask your music store to do the same thing, and pay an inflated "list" price for exactly the same crappy little speaker you just blew up. Unless the failure occurred while your amp was turned to "11" and your strings were bouncing off the pickups, and you absolutely swear you won't do that again, this may not be the best idea. Many amp and cabinet makers market their wares by building in lots of knobs and flashy logos and appearance features, and then skimp on the original-equipment speakers to preserve their profit margins. In a sealed cab, no one sees them anyhow, right? Unless the manufacturer of your cabinet or amp listed detailed and impressive specifications for the speaker(s) in it, chances are you can vastly improve things by simply upgrading those OEM ("original equipment manufacturer") speakers.
What you want are speakers designed for hard, professional use in large spaces, usually termed "pro audio" speakers. These are sold in music stores, electronics-supply houses, or by mail order or via the web. Because demand for replacement speakers is limited, most music stores don't have a very wide selection - why stock something you don't sell every day? - but some music stores are either well-stocked or can get you what you need in a short time, and if you're squeamish, can even do the installation for a hopefully-small fee. They may even have an experienced technician who can give you good advice on speaker selection. The downside is that they have the overhead of that bricks-and-mortar store to support, and the salary of that trained technician, so you will usually pay more for the same thing than you would if you ordered it from some giant warehouse operation.
Speakers are not complicated to install. They screw or bolt into big holes in the cabinet, and they connect electrically with two wires. Trained marsupials can probably do this. So if you educate yourself sufficiently to select the right speakers in the first place, and can operate a screwdriver or a wrench, you may save some money dealing with the giant electronics warehouse, or via mail order or the web.
Disclaimer for guitar wizards
Okay, the Prozac is kicking in, so back to the subject, in
search of the cleanest, most powerful sound modern speaker
design can give us on a reasonable budget.
Impedance is important mostly because amplifiers expect to be
pushing speakers within a given impedance range, for electrical
reasons beyond the scope of this discussion. If they "see"
greater impedance, they simply put out less power. If they "see"
impedance that's too low, they can try to put out more power
than they are designed for, which heats a lot of expensive
amp parts past their capabilities and can burn them up.
Luckily, before this happens, the whole process is frequently
stopped by a failure of a cheaper component, the speaker. Most
speaker failures occur in the voice coil, where that thin coil
of wire generates a lot of heat (speakers are fairly inefficient).
If there is enough heat to melt through the wire at some point,
you have an open circuit and no sound, so many "blown" speakers
look perfectly fine physically, but just produce no sound. Damage
to the cone itself (other than from careless handling, flying beer
bottles, etc.) is rare, because the cone structure is rarely the
weakest link if it hasn't been stored in a damp basement since the
Eisenhower administration. And, you can usually hear when a speaker
cone is being physically overdriven (i.e., moving back and forth
further than it's designed to), because the resulting distortion is
truly awful (at low frequencies, this is sometimes called "speaker fart"
because, well, you'll know why when you hear it). You can't
always hear when the heat in a voice coil is reaching a critical
point, although if the cone is distorting, that's almost always
a signal that the voice coil is in distress also. It's possible
for a voice coil to deform under high-heat conditions without
actually melting; this results in a voice coil that "rubs" in
the magnet gap - the speaker still sort of works but sounds
horrible, and when the cone is manually pushed back and forth
a scraping sound is heard. A low-power amp being asked to put
out too much power can also distort, a phenomenon called "clipping,"
and send a signal that is very difficult for the speaker to reproduce,
because it's asked to start and stop in a "jerky" motion it was
not designed for. So it's wise to observe manufacturer power ratings
for speakers and amps, and match their capabilities.
Whether a power amplifier (or the power-output section of an
integrated or "combo" amplifier) can handle a low-impedance load
of 4 ohms, or even 2 ohms, is generally a function of the ruggedness
of its design and resistance to overheating. Amps that will handle
low-impedance loads at high wattage levels are almost always
fan-cooled and weigh as much as your average Volkswagen. If
the amp manufacturer is reputable, they will publish ratings
that tell you the minimum safe speaker-impedance load, and the
amp's wattage output into various loads at and above that figure.
In selecting an amp in the first place, you want "RMS" ratings,
which are a true indication of power output, and not "peak" or
"PMPO" or "transient" ratings, which for some manufacturers are
just a handy way of lying to you. Amp and speaker makers catering
to the DJ market are known for this, touting a low-power design
as "1000 watts" because it can reach that level at a certain
frequency, for a millisecond, on a very cold Tuesday. They are
more honest than the car-stereo ratings, but only by a little bit.
A true "RMS" rating tells you what the amp can do on a warm day
in Dallas, over the span of a four-hour gig, and over the entire
range of frequencies.
So, let's say you have an amp rated to deliver a true 200 watts
RMS at 8 ohms. If you drive an 8-ohm speaker rated at 20 watts
with this amp, and turn it up gradually, it will get louder and
louder. Then at some point it will not get much louder and the
sound will become progressively more distorted. Eventually it
will stop working altogether, although the burned-out speaker
may emit a wisp of smoke (you can smell this when it happens).
You have deformed and/or melted the voice coil from the buildup
of heat within the speaker. Your speaker is now magically transformed
into a doorstop.
Now let's take the inverse, an amp rated at 20 watts RMS, and a
speaker rated at 200 watts RMS. Doing the same destructive testing,
the speaker gets louder and louder, and then again starts to distort.
But this time it's not the cone excursion, or the buildup of heat
in the voice coil, that's causing the distortion. Instead, it's
bad stuff happening in the output stage of the amplifier, and it's
a tossup whether your speaker gives up first, from trying to
reproduce the "clipped" or horribly distorted sound, or whether
your amplifier simply experiences a meltdown. In the amp-speaker
mismatch I've just described, I'd look for the fatal wisp of
smoke from the amp. Now your amp is the doorstop.
At low volumes, either sort of amp-speaker mismatch is OK, because
you're operating within the capabilities of the weakest link. It's
when you crank things up that you want to be sure the amp and speaker
are a reasonable match for each other. It's OK, probably even desirable,
for your amp to be capable of greater output than your speaker can handle,
because that helps forestall amp clipping and overheating in what is
almost always the more expensive component of the two. You just won't be
able to set the amp at full throttle without risking your speaker(s). The
overcapacity of the amp in this situation is sometimes called "headroom," but
then so are a lot of other completely unrelated electroacoustic relationships,
so forget I said that. It's also OK for your speakers to be able to handle
greater power than your amp can put out, assuming a proper impedance match,
but again don't expect to run your amp wide open without risking damage
to its internal organs, or at least distorted sound. Since you're
not thinking about amp-speaker matching in the middle of a gig when
you decide that the proper execution of a particular song demands
that it be painfully loud, I'd advise designing a little safety into the
system by making sure the amp can handle the speakers and vice-versa.
What determines the power-handling capability of a speaker? Well,
basically, all the elements of its design. A speaker with a very
flimsy cone may sound great at low levels, but at high levels the
voice coil may be bending or flexing the cone as much as it is
pushing it back and forth. The "surround" (the area around the
cone) and the "spider" (the little accordion-looking thing between
the magnet and the cone) can be stiff or loose, with similar effect.
The size of the voice coil (and hence its surface area), the stuff
it's made of (paper, aluminum, "Kapton") and the diameter of wire it's
wound with are critical to tolerance and dissipation of heat.
Even the speaker frame can help dissipate heat and can help keep
the voice coil aligned in the magnet gap under high-power conditions,
which is why people pay extra for cast-frame (rather than stamped-steel-frame)
drivers. Unfortunately, it is perfectly possible to design a speaker
with a huge voice coil, a flex-free cone, a rugged surround and spider,
and a giant cast frame with ribbed heat sinks, which will absorb
massive amounts of power from the amp, and still have it sound like
crap for reproducing music. That's because the weight
of the cone, surround stiffness and heft of the voice coil can all
take their toll on frequency response; a big, heavy structure just
can't be made to vibrate fast enough to reproduce high frequencies.
And feeding that massive, inefficient speaker with huge wads of power
increases the amount of current that gets turned into heat, in
a vicious circle. There is an easier path to loudness.
I say that because I don't want to give the impression that RMS
output power and speaker impedance are all that determine volume
or sound-pressure levels coming out of the speaker. There is another
variable you need to understand, at least a little, and that's speaker
efficiency. A speaker is a little bit like a pair of magnets, and a
little bit like a toaster. Two magnets in proximity will either
attract or repel depending on their polarity, with very high
efficiency. Attach a big paper cone to one of those magnets and
it will push air around. Reverse the polarity of the magnets
a certain number of times per second, and the pushed air will
reproduce a musical note at that frequency. That's basically how
a speaker works; the alternating current from the amp, traveling
through the voice coil, sets up an electromagnetic field of
constantly-changing "polarity." The voice coil is suspended
in a fixed magnet which repels and attracts the voice coil
according to the input current, and the voice coil pushes air
around accordingly. Very cool.
Speaker efficiency, in it simplest form, is a rating of how much
of the current entering the speaker is turned into sound (and by
extension, how much is lost as heat), as well as how effective the
rest of the speaker is at projecting that sound into the atmosphere.
Because so many speaker-design variables determine efficiency, the accepted
rating system just focuses on sound output levels for a given electrical
input level, a real rubber-meets-the-road test. There are
three variables: the input power in watts, the sound output
measured in decibels ("db") and the distance from the speaker at which this
output is measured (as you know from standing by the side of the highway, the
resistance of air greatly decreases the intensity of sound over distance). The
standard measurement is derived by feeding a speaker one watt of electrical
power, and measuring the output in decibels one meter from the front of
the speaker. This is sometimes called "sensitivity," and sometimes "SPL"
(for "sound pressure level"), and will be expressed like so: "SPL: 89db/1w/1m,"
or since the one-watt, one-meter measurement is fairly standard,
sometimes just "89db."
You will see such ratings both for raw drivers, and for finished,
assembled speaker cabinets. Ignore the latter for now. Cabinet
designs can improve (horn cabinets and ported boxes, at some frequencies
anyhow) or degrade (sealed boxes) the efficiency of a "raw" or unboxed
speaker. No matter what you're going to put your speaker in, it just
makes sense to look for a more efficient driver if you can find one
with the other attributes (power handling, frequency response,
physical size) that you want.
Most decent drivers have efficiency ratings from the low 80's to maybe
110, although it's very rare to see a driver designed for bass use
approach the magic 100 mark, and ratings over that are usually only
achieved by very efficient tweeters. So, how much difference is there
between a speaker rated at 90 db, and one rated at 93 db? Well, duh,
3 db, but how much is that really? Well, one electrical way of looking
at it is that it takes roughly twice as much amp power output to drive
the 90 db speaker to the same "loudness" as the 93 db speaker. In terms
of sound output, a 1dB difference is the smallest change in sound level
that is noticeable, and a 10dB increase is perceived as "double" the
volume, so at a constant amp-current level, a 3 db increase is at
least "noticeably" louder.
The relationship is "logarithmic" rather than "linear." A doubling,
or 10-db increase in volume, with a given speaker, requires 10 times
the power. The price difference between a 100-watt amp and a 1,000-watt
amp is generally hundreds of dollars. The price difference between an
88db SPL speaker and a 98 db SPL speaker can be literally zero. So you
start to see the advantages of shopping for speakers by efficiency
ratings. Many home "hi-fi" speakers have efficiency ratings in the 80's,
because (1) they are designed with flat frequency response as their
primary criterion, frequently at the expense of efficiency; and (2)
people don't listen to them, typically, at levels much above 80-90 db,
in small rooms, so even wimpy hi-fi amps have all the power needed.
Professional-audio speakers (and cabinets), on the other hand, are
designed with more of an eye to power-handling and efficiency, to
wring the highest SPL's out of a given amp/speaker combination, even
at the expense of some anomalies in frequency response over their
range. A good pro woofer should have an SPL rating in the 90's. But
within that general range, and all other things being equal, you will
like the 98 db speaker a lot more than the 91 db speaker in terms of
output. So when you're shopping for speakers for live performance
use, selecting a more efficient speaker is almost like getting a free
amp upgrade. Your amp doesn't have to work as hard, but if it does,
the result is louder sound.
What determines speaker efficiency? Well, for one thing, a light coil/cone
structure that's easy to move around has less physical resistance and
can be more efficient - but if it's too flexible, it can distort at high
power levels, so few pro-audio speakers seek efficiency that way. The
same is true of very flexible surrounds and spiders. That floppy little
speaker in your TV can play pretty loud with the ½ watt or 1 watt the
TV amp feeds it, but fed 10 or 20 real watts it would sound fuzzy or
burst into flames. So the search for efficiency in pro audio speakers
generally focuses on the voice coil - magnet relationship.
Remember, the real work is done by the interaction between the fixed
magnetic field of the speaker's magnet, and the constantly-changing
electromagnetic field generated by the current through the voice coil.
Since that electromagnetic field is not really all that strong even at
high power levels, and decreases with distance as all magnetic fields
do, one way to increase efficiency is to lessen the distance between
the voice coil and the fixed magnet, sometimes called the "voice coil
gap." This is a matter of exacting manufacturing tolerances, and is
one reason for example that JBL or Electro-Voice pro speakers are more
efficient, and cost far more, than equivalent Taiwanese or Chinese
imports with the same magnet weight, voice coil size and so on. You've
heard of manufacturing tolerances measured in the-width-of-a-human hair;
this is one application where that's literally true. And the smaller the
voice coil gap, the greater the risk a deformity in the voice coil will
cause it to rub on one side of the gap or the other, which is also death to a
speaker. The various ways around this (exotic voice coil materials, exacting
coil-wire tolerances, precision machining of the gaps, cast speaker frames to
"center" the voice coil with greater precision, and any number of heat-sink
designs for enhanced cooling capability) add expense and/or the requirement
for more precision machining and assembly. So more popularly-priced speakers
tend to have larger gaps, even at the price of reduced efficiency, for
purposes of reliability.
Usually this is something touted in the specs: "Giant 30-ounce magnet!"
All other things being equal, I'd rather have a larger magnet than a
smaller one, if we're talking about the same magnetic material. The
standard is a strontium or "ceramic" magnet, which usually looks like
a big round slab of heavy, black stuff glued or bolted to the rear of
the speaker and can range from a few ounces to several pounds. Some
people insist their precious Les Pauls sound better through speakers
with Alnico (Aluminum-Nickel-Cobalt) magnets, which are pellet-looking
things, usually with steel "hoops" around them and which are said to
"compress" the sound at the limits of voice coil excursion and add a
type of distortion that is pleasing to some ears. There are newer,
more exotic magnetic materials such as "neodynum" which can create
stronger or more focused magnetic fields from a smaller mass of material.
There is no reliable rule-of-thumb I know of for comparing the actual
magnetic power (measured in "gauss," try to find that in speaker specs
anywhere) of two dissimilar types of magnetic material, and the positioning
of the magnet relative to other components can affect magnet effectiveness
irrespective of its weight and power. But given the same material (let's
stick with the popular and inexpensive "ceramic"), a larger magnet is
usually a hallmark of a better speaker if it's intelligently designed,
which most are. A heavier-magnet speaker usually costs more, not so much
because of the magnet (that stuff is fairly cheap) but because the
larger-magnet speaker is usually a step up in the manufacturer's food
chain and also has a larger voice coil or coil wires, a sexier-looking
sticker on the back, or some other combination of other features that
allows the maker to charge more and still sleep at night. A heavier
magnet can allow the manufacturer to upgrade other important components
and still maintain a reasonable level of efficiency. Other than cost,
the only real downside is that you get to lift those heavier magnets
in and out of trucks and vans for the rest of the life of the speaker.
Watch out for a spec that touts a heavy "magnet assembly," however.
I don't know of a standard which keeps a manufacturer from calling a
non-magnetic plate behind the actual magnet, the screws that hold it
on, or even some decorative doodad glued on the back, part of the
magnet "assembly." If the weight of the "magnet assembly" is almost as
much as the cited shipping weight of the entire speaker, there is likely
some fudging going on.
There are so many other speaker-design parameters and specifications
that without significant schooling and experience in electroacoustics,
and several pots of coffee to forestall the boredom, you might never
be able to choose between the gazillions of speaker designs out
there. Many of the specs you will see (vas, fs, f3, "Q" and so on)
have more to do with how the speaker will interact with its
surrounding enclosure, than how it will handle a high-power input
or how loud it will play with your amp. Enclosure design is a whole
'nother issue; right now we're just selecting raw drivers for their
projected ability to handle that open-B string or that windmilled
power chord. If you are replacing an existing speaker in an existing
cabinet, you don't have the leisure to change the enclosure parameters
much anyway.
Most speaker specs cite a frequency-response range, like "30 - 11,000 hz."
As a spec, this is sort of meaningless, because frequency response can
have dips and peaks, and if those dips in particular are big enough,
the speaker might as well not have any response at all at that point.
It's better to look at a frequency-response curve plotted under controlled
and standardized conditions. If the graph shows that the response at
44 hz (the low "E" on a bass guitar) is 10 db less than the response at
440 hz (the "A" an octave and a half above that), then that speaker is
truly useless for bass guitar, since the one note will be half as loud
as the other. Any fall-off of 3 db or more is significant, and all
speakers fall off at their frequency extremes. If a spec cites
"30-11,000 hz +/- 3 db," that tells you a little more than the simple
range, although still not as much as a graph. For all the reasons
alluded to above, pro speakers have jaggedy frequency-response curves.
What their overall usable response is good for, however, is to judge
whether the speaker will pretty much reproduce the range you want (does
it go low enough for bass guitar? High enough for miked cymbals?), or
whether you need to get into the complexity of a multi-driver setup (i.e.
woofers and tweeters and crossovers, with all the design complexity that
brings). Generally, most bass and standard guitars can get by without
high-frequency help, if the chosen speaker has response to a few thousand
cycles (i.e. the midrange), although some people like to add tweeters in
both instances to capture the higher "overtones". Keyboards, PA's and
electronic drums will need separate high-frequency drivers because they
produce fundamental tones beyond the capabilities of pro woofers.
Indeed, any truly "full-range" speaker cabinets for pro audio
use will inevitably have multiple drivers.
One other spec I at least glance at is "Xmax," which stands for
"maximum excursion" or more simply, how far the cone can move in
and out before reaching its mechanical limits. For deep bass this
can be a useful thing to know, because low frequencies require
moving large amounts of air, and a speaker cone of a given diameter
can only move more air by increasing its cone excursion. Bass
speakers with lots of Xmax tend to have specially-designed surrounds
and voice coils that allow extreme movement, usually at the cost of
upper-end frequency response, efficiency, or both. Again, it all
depends on what sort of sound you're trying to produce, or reproduce.
Okay, let's say I've decided I want to use a 12-inch speaker in a
high-power application, and looking through a catalog, two speakers
have caught my eye. One is a no-name "pro woofer" rated at 190 watts
RMS power handling, with a 2.5 inchce coil, 50 oz. magnet, frequency
response rating of 36-2,700 hz, SPL of 92db, and a Xmax of 3.8mm;
it's $45.00. The other is a recognized brand rated at 300 watts,
with the same-diameter voice coil, a 56-ounce magnet, a frequency
range of 55-5,000 hz, an SPL of 100 db and an Xmax of 2.7mm, for $65.
Power ratings are sort of "soft" and speaker warranties are too,
since there are so many ways to abuse them; I won't make my decision
based on claimed power ratings alone. The voice coils are the same
size, although the specs for the more expensive speaker say the voice
coil former (the tube the coil of wire wraps around) is made of "Kapton,"
which stands up better under heat and may account for the speaker's
higher power rating; the cheaper speaker doesn't specify the voice
coil material, so it might be some exotic material too, but you'd think
they would have said so. The difference between a 50-ounce and 56-ounce
magnet isn't significant, in fact as magnet weights increase, the
differences become less significant. Both speakers have big, healthy
magnets. Neither blurb in the catalog includes a frequency-response
graph, so the ranges cited don't give me much useful information other
than the more expensive speaker seems to have a little more-extended
midrange. The cheaper speaker claims to have lower frequency response
but doesn't tell me how that relates to its average response at higher
levels; it claims to have greater cone excursion, which might be useful
in a bass-heavy application, but the other specs make me wonder if that
comes at the expense of midrange response and/or efficiency. All in all,
these speakers are very similar in all these specifications, and might
sound very similar in actual use, except for their efficiency ratings.
One watt drives the cheaper speaker to 92 db, but it drives the more
expensive one to 100 db - almost twice as loud! For an additional $20,
I can get a speaker which will allow me to use a much smaller amp, or
viewed alternatively, will allow a large amp to work much less hard
and stay cool and undistorted. That's the best $20 I could possibly
spend. And yes, these are actual speakers, from an actual catalog,
that I just picked at random for illustration, although I have to
admit the very high efficiency rating of the more expensive speaker
is fairly unusual, even within that manufacturer's line.
You have settled on the speaker you think you want. Congratulations!
Now you have to get it home and into your cabinet. If you're
ordering online or by phone, try to figure out the shipping weight
and how the seller proposes to ship it. You have two
concerns: shipping cost and safe arrival. That thirty-pound speaker
will not be cheap to ship; there are shipping-cost calculators available
online from UPS, FedEx, the Post Office, etc., or maybe the vendor can
give you a cost estimate. If the total cost with shipping is as much as,
or more than, you can buy it for locally, find the car keys and
you'll have it in your lap tonight.
Most new speakers are packed in sturdy boxes sized especially
for them, and can be safely shipped that way. They're pretty
tough characters; after all, they're going to spend the rest of
their lives being packed around from place to place in other boxes,
right? Just the same, I'd advise purchasing whatever inexpensive
shipping insurance is available. If there is enough force applied
to bend the edge of the speaker frame, as can happen if the box
falls out of a truck to the pavement, the cone will never function
normally again, and the voice coil can become uncentered and rub noisily.
Find that screwdriver and install your new speaker. The speaker
terminals will be marked "+" and "-", or sometimes the "+" side
will just be marked with a red dot. Hook it up the same way your
old speaker was hooked up. If you don't remember, look for one of
the wires to have a "hot" color (in a red-and-black pair, for
example, red is almost always "+"), an identifying stripe in the "+"
wire, or even a raised ridge on one of the wires indicating "+".
Except in arrays of multiple speakers, you won't hurt anything if
you hook the speaker up backwards; it will just operate "out of phase."
Phasing problems can suppress bass response in multiple-speaker setups,
because one cone is moving in while the other is moving out, and vice-versa.
Test the amp-speaker combination and make sure the wires can't wiggle
loose. Reassemble the cabinet, if necessary, and you're done!
NOTE: We do not work on Home or Car Audio. We work only with Pro-Audio applications. We cannot
help you with Home or Car Audio questions.
Questions or Comments about this article? Larry Mundy wants to hear them
Questions? Comments?
.
© 2002 - Shavano Music Online
Before I start talking about speaker design and specifications,
let me say that if you are one of those guitarists in search of
the perfect "vintage tone" you heard on an ancient, scratchy
recording of a long-dead bluesman playing through a low-wattage
tube amp and speakers manufactured 50-plus years ago, I don't
have any advice you'll find useful. But the speaker manufacturers
should kiss your body parts, because they can get you to buy
underpowered 50-year-old designs that cost them three bucks to
make, for the same price as well-designed powerhouses. You've
seen the ads: "Vintage tone, early cone breakup and the very
same sticky blue label that was on Blind Willie Washington's
speaker in this grainy black and white photograph taken in
Memphis in 1949." You are the same players that show up at gigs
with little vintage tube amps that die halfway through the first
song, because those vintage tubes and mazes of hand-wiring and
floppy-coned speakers have all the ruggedness and power of
poisoned butterflies. I have three pieces of advice for you folks:
Fortunately for you folks who don't use a bunch of whiz-bang effects
(and thus probably save hundreds of dollars a year in dead 9V
batteries), major speaker manufacturers like Celestion and
Eminence are now making "vintage-tone" speakers brand new, in
the box, with the benefits of the last 50 years of speaker-building
technology and most of the attributes (light voice coils, "AlNiCo" magnets)
that gave those old speakers their distinctive sound. Some of these
are actually rated to handle serious power. There are also
newer-design tube amps whose circuitry is far more reliable than
you'll find in a 1955 Bassman that's been in someone's basement
since the Cuban Missle Crisis, although the new ones still
have tubes to fuss with. All it takes is money, lots of it in some cases.
Impedance.
All speakers have a rated "impedance," which is not quite the
same thing as electrical "resistance" although they are allied
concepts like, say, velocity and acceleration. There are
discussions, charts, and even a little calculator on this
site that will tell you what the "impedance" of a given combination
of speakers, wired in a certain way, will be. For our purposes,
let's just think of a single speaker hooked to a single amp.
The speaker's voice coil presents a certain electrical resistance
to the amplifier, measured in ohms, because it has a coil of
fairly thin wire (the voice coil) that current passes through,
like trying to pump the contents of a swimming pool through a
piece of aquarium hose - the physical limitations introduce
resistance to free movement of electrons. Then the magnetic
fields created by the current interact with the fixed magnet
structure and move the cone or diaphragm around, which also
takes some effort (in the form of physical resistance, from
the cone mass, its "surround" or suspension and the resistance
of the air it's trying to move). The actual "impedance" a
speaker presents to the amplifier depends on both of these
things and varies with frequency. The "nominal" impedance
of a speaker is not supposed to dip more than 20% over the
covered frequency range - that is, an 8-ohm speaker probably
doesn't present less than 6.4 ohms at any point, although it
may rise to many times that at other points. If you measure
the electrical resistance of an "8 ohm" speaker's voice coil
with an ohmmeter, you'll probably get a result of six-point-something.
If you own an ohmmeter, though, you probably want to read
something more technical than this article, smartass.
Try this:
http://www.epanorama.net/documents/audio/speaker_impedance.html.
Blowing things up
Power handling
Efficiency ratings
OK, now for the not-so-cool part. Not all of the current passing
through the voice coil is actually turned into that useful electromagnetic
field. Much of it, traveling through the resistance of the voice coil wire,
are turned into useless heat. This is how your toaster (or indeed any
electric heating device) works; current passes through a bunch of thin
wires and heats them up. This is great for English muffins, but as we
have seen, it is not so good for speakers.
DECIBELS
POWER REQUIRED
PERCEIVED LOUDNESS
3
2X
1.2X
6
4X
1.5X
10
10X
2X
20
100X
4X
The most cost-effective way to increase efficiency, with all other parameters
optimized, is to strengthen the fixed magnetic field the voice coil moves
in. I've heard this referred to informally as a speaker's "BAM" rating,
which stands for "big-ass magnet". Within certain limits imposed by the
voice coil dimensions and other physical parameters, a giant magnet can
improve efficiency, and even help recover some of the efficiency lost
by widening the voice coil gap. So more popularly-priced speakers sometimes
attain reasonable efficiency while retaining good power-handling
capability, by the simple expedient of adding magnet mass.
More specs
A real-world example
Bringing it home
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