Speaker Design
and DJ's Part I of III
by
Larry Mundy
March 2004
The following
article by Larry Mundy will appear in a forthcoming book on the
design and construction of pro-audio cabinets. Larry Mundy retains
ownership of this article and it is republished here with his
permission.
[Editor's Note:
At one time or another, "distortion," "clipping,"
and "blown speaker" are words you will hear in almost
every DJ booth. A fundamental understanding of speaker design
may improve the quality of your sound and ability to move a crowd,
while avoiding system problems. This month's tip is the first
of three parts on speaker design by Larry Mundy.]
Replacing
a Speaker
In the DJ business,
you don't always have the privilege of plugging your setup into
a club or "house" sound system. So you end up buying
(and hauling around) amps and speakers. Sooner or later, you
will probably "blow" a speaker it will put out
horrible, distorted sound, or stop working altogether. Or perhaps
your current amp/speaker combination doesn't have the powerful
sound you want for the larger clubs or halls you've been working,
and you've traced that problem to the quality of your speakers,
but don't have the cash to replace them entirely. In either situation
you may want to replace the drivers (raw, unmounted speakers
are usually called "drivers") in your existing cabinets.
The drivers in
a DJ rig are subject to different demands than most other types.
Home or "hi-fi" drivers are made for playing music
at the relatively low levels appropriate for a home environment.
They would self-destruct quickly at the sound levels you need.
The same is true of car-audio
drivers,
which are designed for efficiency in a small space and not for
projection over a large audience. While it might seem that your
needs are similar to those of performing musicians, many "live
music" drivers are designed for specific instruments, such
as guitar, and have a distinct "sound" that colors
whatever is played through them "guitar speakers,"
in particular, tend to have an exaggerated midrange response
that gives full-range music a "honky" sound. What you
want are drivers that can put out the equivalent of "hi-fi"
sound from the lowest bass to the highest treble, but at sound
levels that will give every person in the room an experience
they hear and feel.
You could write
or call the manufacturer of your speaker cabinet, or ask your
dealer to do the same thing, and pay an inflated "list"
price for exactly the same driver you just blew up. Unless the
failure occurred while your amp was turned to "11",
and you absolutely know you won't do that again, this may not
be the best idea. Many speaker makers market their wares by building
in lots flashy logos and appearance features, and then skimp
on the original-equipment drivers to preserve their profit margins.
In a sealed cabinet, no one sees them anyhow, right? Unless the
manufacturer of your speaker cabinet listed detailed and impressive
specifications for the driver(s) in it, chances are you can vastly
improve things by simply upgrading those OEM ("original
equipment manufacturer") drivers.
What you want
are drivers designed for hard, professional use in large spaces,
usually termed "pro audio" drivers. These are sold
in music stores, electronics-supply houses, or by mail order
or via the Web. Because demand for replacement drivers 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 driver 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.
Drivers are not
complicated to install. They screw or bolt into big holes in
the cabinet, and they connect electrically with two wires. Trained
monkeys can probably do this. So if you educate yourself sufficiently
to select the right drivers in the first place, and can operate
a screwdriver, you may save some money dealing with the giant
electronics warehouse, or via mail order or the web. In the U.S.,
the best-known source of pro audio drivers is probably Parts
Express
(www.partsexpress.com). But first you need to understand some
basic concepts that influence your choice of drivers.
Impedance
All drivers and
speaker systems have a rated "impedance," which is
not quite the same thing as electrical "resistance"
but close enough for this discussion. Usually the safest thing
to do is to replace a blown driver with one of the same impedance,
which should be marked somewhere on the blown unit with a number
designation followed by a little "horseshoe" symbol,
which denotes "ohms." The most common drivers are 8-ohm
designs. But let's understand a little more about impedance before
moving on.
Think of a single
driver hooked to a single amp. The driver'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 sum of
the driver's resistance to electrical input and physical movement
is its "impedance." Impedance is important mostly because
amplifiers expect to be pushing speakers within a given impedance
range. 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 driver. Most driver 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" drivers 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 physically abused. And, you
can usually hear when a driver 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 driver 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 driver 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 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
extremely heavy. Reputable manufacturers 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, over the span of a four-hour gig, and over the entire range
of frequencies.
Hooking multiple
drivers together in the same frequency range (two bass drivers,
two midrange drivers, whatever) can change the impedance of the
overall system. There are charts and discussions on this site
(suggest hyperlink to Shavano here) that will tell you what the
impedance of a given combination of multiple drivers, wired in
a certain way, will be. Once you understand the dynamics of amp-driver
interaction, you can deal with multiple speakers of a given overall
system impedance, just as you would with a single driver.
Distortion
Is it your amp, or your speakers?
Let's say you
have an amp rated to deliver a true 200 watts RMS at 8 ohms.
If you drive an 8-ohm driver rated at 20 watts (a cheapie) 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 driver may emit
a wisp of smoke. You have deformed and/or melted the voice coil
from the excessive amp power and resulting buildup of heat within
the driver. Your driver is now useless junk.
Now let's take
the inverse, an amp rated at 20 watts RMS, and a driver rated
at 200 watts RMS. Doing the same destructive testing, the driver
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 driver gives up first, from trying
to reproduce the "clipped" or horribly distorted sound,
or whether your amplifier simply experiences a meltdown. In the
amp-driver mismatch I've just described, I'd look for the fatal
wisp of smoke from the amp. Now your amp is junk. Fixable junk,
maybe, but still junk.
At low volumes,
either sort of amp-driver 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. Many quality power amps have "clipping
protection" circuits that help protect them against being
overdriven this is a very valuable feature to have in such
an expensive component and should never be turned off with the
back-panel switches in search of that tiny bit of additional
power.
Power Handling
Let's assume
you have a very powerful amp and know the problem is with your
driver(s). What determines the power-handling capability of a
driver? Well, basically, all the elements of its design. A driver
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. Driver
power-handling ratings, generally expressed in watts, are largely
an _expression of how much internal heat the driver can handle
and still keep pumping.
Even the driver
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
driver 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 limited sound output. That's because the weight of the cone,
surround stiffness and heft of the voice coil all absorb amp
power. And feeding that massive, inefficient driver 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.
Contact Larry
Mundy at larry.mundy@comcast.net
Next Month: Part
II
Related Links
Shavano Music Online
Speakerbuilding.com Interview with Joachim
Gerhard
AudioVideo 101
World Studio Group (WSG) Directory of
Related Sites
NAMM
Syn-Aud-Con
Audio
Engineering Society
Mackie
JBL
Pro
US
Speaker
Parts
Express
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