12.20 What is a Sealed, Ported, Bass Reflex, Acoustic Suspension, Bandpass, and Coupled Cavity Speaker? Which is better?
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This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
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 driver
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 enclosed
air in the enclosure is (and that's a function of the size of
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 a
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 at
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. They
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 and
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 results
of a designers attempt to solve specific problems. They consist
of two or more rear enclosures, each coupled to the next by a
vent. Each enclosure/vent combination is another resonant system,
and the combination is, essentially, a high order, multi-tuned
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.
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