Infinite Baffle Subwoofer
Here is an IB subwoofer that I recently installed. The manifold housing the drivers sits behind the house and vents it's back wave into the back yard. Since the sub only operates below 50Hz, the output off the back of the drivers is not offensive, and so far has not been responsible for any neighbour complaint. It uses eight 18-Sound 18LW1400 drivers that have a linear excursion of around +/-10mm and damage limited excursion of +/-25mm. This gives the infinite baffle sub 20 liters of linear displacement and over 50 liters of damage limited displacement. With the 3000W available to the drivers, this is enough displacement to achieve outputs in excess of 112dB at 10Hz, 124dB at 20Hz and 130dB at 30Hz in a half space environment. In room these figures would be quite a bit higher, but maximum output testing has not been carried out for fear of breaking the glass windows in the room. Needless to say, at normal to high levels the bass is very clean and free from any compression or chuffing noises. The opposing drivers cancel out structure borne vibration very well.
The drivers actually have significantly less resistance to the backwave than can be seen in the pictures. The top drivers have a 50mm by 600mm gap down the edge nearest the air conditioner, and through the louvers on the doors. The back and bottom drivers, which face into a 150mm deep hole, vent between the ground and the box through an 800 by 75mm gap and between the house and the box through a 75mm gap which is 800mm or so high. The total area available for the backwave to travel out of the box is just under 75% of the drivers ECA. Granted the airpath into the room has less resistance, even though that path is only around 40% of total ECA, but chuffing is not an issue at any level tried so far. I suspected this subwoofer would work very well after reading ThomasW's article on the 12 Shivas project, which has an outlet of 26% ECA and higher excursion drivers than I used, with no audible chuffing. ThomasW's 12 Shivas project has 15 liters of peak to peak linear excursion.
A Behringer DSP1124P FEEDBACK DESTROYER PRO is used as a dual 12 channel parametric equaliser to flatten the response of the IB and also the four 12" Lambda Acoustics drivers that cover from 50Hz to 100Hz.
The innocent looking box hiding in the fernery behind the house actually houses the manifold of an infinite baffle subwoofer.
It looks a little less innocent with the lid and doors open :)
Here is the view from inside the house. Not much to be seen, except through the window.
Here is a closer view of the wooden AC return grill that covers the outlet of the manifold. This grill is around 600mm square and is free from rattles and any chuffing noises.
Here I am wiring up the drivers in the manifold. One of the drivers has been removed so I can work inside. My analogue watch stopped while I was in there, so the job took no time at all!
And a view of the eight drivers inside the manifold.
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