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ISOBARIC WOOFER CONFIGURATIONS
Increasing in popularity in recent years is the com-
pound woofer design, more commonly known as the
„Isobaric" design. Isobaric actually means „constant
pressure" which is the case having two woofers mo-
ving 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 spa-
ce 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.
SUBWOOFER ENCLOSURE CONSTRUCTION
Before starting final box assembly some basic construction issues should be mentioned:
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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.
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Box construction 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.
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Particle board, MDF, or high grade birch plywood are good materials to build boxes of thicknesses
of 19 to 25 mm.
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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.
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Cross bracing is important for large boxes to prevent wall vibration.
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Acoustical damping material (sheep wool or fibre-glass batting 5 to 10 cm 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 10 - 20%. The application of self-adhesive damping mats to the inside walls also helps to
eliminate wall resonances.
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The cables running from the amplifier to the subwoofer should be as short as possible with a cable
cross section of at least 4 mm2.
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The final box should be rock solid and air tight but should also remain serviceable should a woofer
be damaged for any reason.
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When possible, use 10 to 15 cm diameter port tubes to avoid wind noise from smaller diameter
sizes. PVC plumbing pipe in various diameters is available at most hardware stores with „elbow" pipes
allowing you to bend the tubes inside the box.
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Try to always use active (electronic) crossovers, such as those built into our Velocity amps, for
example.
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ISOBARIC CONFIGURATION (TANDEM)
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