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ISOBARIC WOOFER
CONFIGURATIONS
Increasing in popularity in recent years is
the compound woofer design, more
commonly known as the "isobaric" design.
Isobaric actually means "constant
pressure" which is the case having two
woofers moving an encapsulated pocket
of air between them
in the same direction.
acts like a single speaker
This arrangement
which effectively reduces the required box
volumes for a subwoofer system by 50%
which is substantial if you are space
limited. Unfortunately, the trade-off for
space is the 3 dB loss in efficiency but this
is usually recovered with increased
amplifier power. Care must be taken to
avoid air leaks between the two woofers
and final speaker wiring and impedance
loads must be carefully watched.
Before starting final box assembly some basic construction issues should be mentioned:
Infinite baffle construction in a car is done by simply cutting a piece of wood which will
act as an acoustical divider isolating the front and rear sound of the woofer. This piece
of wood is mounted inside the trunk of the car under the rear deck, or against the back
of the rear seat, and should have some kind of caulking to seal off all residual air leaks.
Box constructiop can take on nearly any shape since we are dealing with
low frequencies. Odd shapes that are difficult to cut and glue should be avoided.
Particle board, MDF, or high grade birch plywood are good materials to build boxes with
thicknesses of 5/8"-3/4" (12-16 mm). Solid wood panels should actually be avoided
because of vibration potential and wood warping over time.
All seams in the box should be glued, screwed, and caulked to prevent wall separation
over time due to vibration and environmental changes within the car.
Cross bracing is important for large boxes to prevent wall vibration.
Acoustical damping material (fiberglass batting 2-4" thick) attached to 50% or more of
the interior walls helps reduce box noise and also increases the box volume seen by the
woofer by lo-20%. Spraying the inside walls with a damping material, such as tar
based automotive undercoating, also helps dampen wall resonances.
The final box should be rock solid and air tight but should also remain serviceable
should a woofer be damaged for any reason.
5-way binding posts for electrical connection offer reliable connections and easy
removal of the box for service or theft protection.
When possible, use 4", 5", or 6" diameter port tubes to avoid wind noise from smaller
diameter sizes. PVC plumbing pipe in 2, 3, 4, 5, and 6" diameters is available at most
hardware stores with "elbow" pipes allowing you to bend the tubes inside the box.
Try to always use active (electronic) crossovers built into most power amplifiers.
Regarding simple vented boxes, you cannot achieve lower frequency response by
simply lowering the port tuning frequency. This will severely overdamp the box
frequency response therefore yielding less bass and lower power handling.
The distance between the woofer and its port should be less than 12" if possible.
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