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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.
This arrangement acts like a single speaker
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
recovered with increased amplifier power.
Care must be taken to avoid air leaks
een the two woofers and final speaker
and impedance loads must be
watched.
Before starting tinal 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
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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 construction can take on nearly any shape since we are dealing with long wavelength
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low frequencies. You should avoid odd shapes that are difficult to cut and glue.
Particle board, MDF, or high grade birch plywood are good materials to build boxes with
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thicknesses of 5/8"-314" (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.
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Acoustical damping material (fiberglass batting 2-4" thick) attached to 50% or more of the
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interior walls helps reduce box noise considerably 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
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woofer be damaged for any reason.
S-way binding posts for electrical connection offer reliable connections and easy removal of
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the box for service or theft protection.
When possible, use 4", 5", or 6" diameter port tubes to avoid wind noise from smaller
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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.
ry to always use active (electronic) crossovers built into most power amplifiers.
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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