Posted by claudio f maccarone on April 01, 2002 at 20:44:25:
In Reply to: Re: Bass Cabinet Power Handling Capabilities posted by Sean Naismith on June 13, 2000 at 22:35:41:
: 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
: frequency cancellations.
: 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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