For any kind of
to be practical,
the drivers need to
be mounted in some
kind of enclosure. But
not just any old box will
do, at least in a well-designed
The enclosure is an integral part of the
proper acoustical functioning and overall
performance of the entire system. A poorly
designed or constructed enclosure can degrade
even the best engineered and manufactured
driver. One of the functions of an
enclosure is to control and enhance low frequency
As noted in my last column, a loudspeaker
driver moves back and forth in response
to an input audio signal, producing sound
waves from both its front and rear. The
sound emanating from the rear is out of polarity
(180 degrees out of phase) with that
coming from the front.
If the driver was just hanging in free
space, unenclosed, the sound from the front
and rear would mix at the listener’s position.
Since they are out of polarity with each
other, they would add or null at different frequencies
forming irregularities called comb
filtering in the frequency response. (The
term comb filtering presumably came about
because of the shape of the peaks and dips
as plotted on a frequency response chart,
level vs. frequency.)
With an enclosure, the rear sound energy
could be made to do something constructive.
While there are a different ways to design
an enclosure, we’ll give a general overall
of two common types found in control
SEALED BOX DESIGN
The first is a sealed box design, sometimes
called acoustic suspension. As might
be expected from the name, the box is
sealed top, bottom, and sides, with the drivers
mounted on the front. (Actually a tiny bit
of leakage is necessary and allowed to normalize
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Since the box is sealed, the sound coming
from the rear of the drivers can’t escape into
the outside world causing the dreadful comb
filtering. Often this type of construction includes
sound absorbing material inside the
box, which tends to be more effective at
mid to higher frequencies. Air pressure inside
the box changes as the driver moves in
and out. This is more pronounced at lower
frequencies due to the longer wavelengths.
That cushion of inside air, described as a
“spring” in the literature, helps support the
driver, effectively stiffening its suspension
and altering its resonant frequency.
A sealed box is not that efficient compared
to other designs, since a lot of the energy
the drivers produce is contained or absorbed
inside the box. Frequency response
may not be able to go as low as some other
box designs. On the plus side, this type of
design can come in compact sizes, as its
other common moniker, bookshelf speaker,
can attest, and can provide good transient
response even at lower frequencies.
Another and more common type of box
is the ported enclosure, also called “vented
box” or “bass reflex.” This type has a deliberate
pathway and opening for the rear sound
energy to leave the box in a controlled and
beneficial way, with the port operating within
a defined low frequency (LF) band. You
can feel the air moving out of the port when
the loudspeaker system is operating.
The pathway inside the box and the port
are designed and built in such a way that the
energy emanating from the port is in polarity
with the front sound wave from the LF
driver. The port is tuned to a specific range
of low frequencies where this occurs.
Within this LF range, since the two wave
fronts are in polarity, they reinforce each other,
increasing the bass output. This has the
benefit of increasing efficiency, compared to
the sealed box design, since more acoustic
watts are produced per electrical watt input.
The driver doesn’t need to be driven (so to
speak) as hard to produce a reasonable LF
output. This allows it to operate in its more
linear range, keeping distortion down.
This design may not be as tight as the
sealed box for low-frequency transient response.
While the ported output arrives in
polarity with the direct output from the LF
driver, it has to travel a little longer distance
to get there. So it would be expected that
there would be some sound spreading in the
time domain. This lingering persistence of
sound, often referred to as time smear, tends
to be well-controlled in well-designed
It’s important to remember
to keep the port free and
clear of obstructions when installing
the loudspeakers. Or
in other words, don’t block
the port, or the intended performance
will be degraded.
The loudspeaker enclosure
has to accommodate more
than just the drivers. There’s
the connector panel, internal
wiring, and depending on
the design, passive cross over
networks, and in the case of “powered” loudspeakers,
While the descriptions of these two enclosure
types have been simplified, the actual
design of the boxes is anything but, a
topic for another time.
Mary C. Gruszka is a systems design
engineer, project manager, consultant and
writer based in the New York metro area.
She can be reached via TV Technology.