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ISOBARIC CONFIGURATION (TANDEM)
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 encap-
sulated pocket of air between them in the same direction.
This arrangement acts like a single speaker which effec-
tively 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.
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.
