Posted by Sean Naismith on June 13, 2000 at 22:35:41:
In Reply to: Bass Cabinet Power Handling Capabilities posted by jcor on May 22, 2000 at 11:29:16:
Although they may look simple, cabinets are complex creatures. You could easily fill a
book (or two) with the science of speaker enclosures, but here's a brief explanation of
the major terms you're likely to encounter.
Nearly every enclosure can be placed in one of two categories: sealed or ported. A
sealed (also known as "infinite baffle") cabinet is just that: there are no holes from which
the air inside the enclosure can escape. The "springiness" of the trapped air provides a
higher level of damping, or controlled speaker-cone movement; the downside is that a
sealed enclosure isn't very efficient and therefore takes more power to drive than a
ported one. Sealed cabinets also generally produce less bass response, because some
of the lows stay trapped inside.
In a ported (also known as "bass-reflex") enclosure, one or more tube ducts--usually
made of plastic or cardboard--are placed either at the front or rear, extending into the
box. The size and placement of the ports are critical to the tuning of the box; the ports
are designed to reinforce the front-radiated air with a controlled amount of in-phase
back-radiated air for better bottom end. This design forms what's known as a
"Helmholtz resonator." A simple form of resonator is a bottle; when you blow across
the top, a tone is produced, the resonant frequency depending on the size of the bottle.
The same is true with ported enclosures: the greater the internal volume, the lower the
resonant frequency. Compared to a sealed cabinet, a ported enclosure is more
efficient, has a lower cutoff frequency (for better lows), and has more acoustic output
(volume). A vented cabinet is a variation on the ported design, using a horizontally
mounted vent. Thiele and Small parameters, named for two engineers who invented a
computer-based enclosure-design program in the '70s, have exploited the venting
principle to its fullest.
Both sealed and ported designs may also incorporate an
internal baffle, a wooden shelf or set of dividers that
modifies the internal air space and reduces component
interaction. A cabinet may also contain acoustical batting,
an absorbent material made of cotton, foam, or fiberglass;
this dampens high end that might otherwise lead to
The speakers are also known as "drivers," because they're
essentially pistons that pump the air. Larger or multiple drivers set more air into motion.
Most speakers have the following main components: a chassis, a diaphragm (cone), a
magnet assembly, and a voice coil. At the end of the diaphragm is an attached tube
called a former; enameled copper wire is wound on the former to make a coil. The
former and coil are suspended in the magnet assembly's gap, which is a small opening
in the middle of the magnet around the pole piece. When alternating current from an
amplifier is applied to the coil, it moves back and forth in the gap, which in turn moves
the diaphragm, causing air to be set into motion.
Another type of driver, found in two-way 4x10s, is the high-frequency tweeter. A
tweeter is often a mini-horn that can extend the upper frequency response of an
enclosure up to and beyond the 8kHz range. Because high frequencies can be
reproduced without a lot of driver excursion (low frequencies require more cone
movement), a horn tweeter doesn't have a cone; instead, the coil is wound onto a
former at the end of a rigid dome made of plastic or another synthetic material. This
dome is housed in a magnet assembly similar to that of a regular speaker, and the
magnet and dome are coupled with a horn to help project the highs. Other types of
tweeters are sometimes used, including ones that are just small drivers.
A 4x10 with a tweeter also incorporates a passive, high-level crossover, which
divides the incoming signal into separate low- and high-frequency components. A
crossover contains a series of capacitors, resistors, and inductors, and it must be able
to withstand the high output from the power amp. The frequency at which the
crossover divides the sound is called the "crossover point"; in 4x10s, this is generally in
the 1kHz-4kHz range. A crossover might also have an attenuator, which lets you
adjust the tweeter's output level.
The power handling of an enclosure can be expressed by three different figures:
continuous, program, and peak. Continuous power (often called RMS, or root mean
square), refers to the average amount of steady, uninterrupted signal a speaker can
take. This is the rating you should look for--not the program rating, which is
determined by subjecting the speakers to variable power levels. (Program ratings are
often double that of the continuous rating.) Even more inflated is the peak rating; it's the
maximum amount of short-term, high-powered peaks a driver can endure.
The usable range of frequencies an enclosure can produce is called its frequency
response. The frequency response of a typical 4x10 might be 40Hz-12kHz @
+/-3dB; this means that between those two extreme frequencies, the response of the
speaker to any particular frequency doesn't vary by more than 3dB above or below an
average (flat) level. The designation "+/-3dB" is called the "tolerance level"; a smaller
number indicates flatter, or more true, response between the two extreme frequencies.
Technically speaking, impedance is the opposition of a circuit to the flow of alternating
current, which involves a complex sum of resistance and reactance. In simpler terms,
impedance is the amount of load placed on a signal. Impedance gets tricky, though,
because a loudspeaker's impedance constantly changes depending on the frequency it's
reproducing. Impedance is expressed in ohms, and a typical rating for a 4x10 is 4
ohms or 8 ohms. It's important to pair enclosure impedance ratings with those of your
power amp; mismatches can lead to power losses or--even worse--amplifier
transformer failure. (For some useful tips on correctly matching amps and speakers,
Of course, there are many more factors involved in enclosure design, construction, and
performance. For more info, check your local music store or library for books and
manuals that delve deeper into this complex subject.
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