Rotary Subwoofer Install Blog

October 18, 2006

How does backvent space affect front vent output?

Filed under: Uncategorized — bassment @ 5:56 pm

Early on in the design phase, we fretted about the rear volume space, and how it might affect output. Clearly I would not be able to vent outside to the infinite volume there. Neighbors would burn my house down.

Clearly I only had the following spaces to back vent to:
– garage above media room, about 1.2x the volume of the media room, or the front side vent. This garage volume would be accessed by a 7-8 sq ft opening in the media room bath ceiling, which was next to one of the fans. So the opening cross section is quite small, but it’s all we had. Hence our concern.
– office and bathroom volumes were also directly available as back vents, via the ceiling grills shown below. Recall that the twin TRW’s are sitting up above the ceiling of the media room bath and office rooms, which are both adjacent to to the media room, separated by a heavy IAC STC55 door.

I wanted to see what I was missing by not venting to the outdoors, and also, if I left the garage door open, would that make any productive difference. So I ran three tests:

1) normal configuration, garage doors closed, garage windows closed, media room office door to stairwell closed.
2) media room office door open. This connects the office to the outdoor stairwell, about a good a vent as the TRW’s are ever likely to get.
3) normal but with the garage twin door open. I thought this might make a difference, because in normal config, the garage doors slap around quite a bit when the TRW’s are pumping. Hence, the hypothesis went, if I opened the doors, maybe more venting would increase the output.

SO here are the results:

output spl.jpg

Clearly, the output is helped in one frequency range by more infinite baffling in the rear, and hurt in the other band. So it’s a toss up. Hopefully Bruce will come in here and comment on what he thinks is going on.

By the way, the SPL readings above were using Spectral Pro software, with a calibrated mic and the software calibrated SPL-wise using an RS meter at higher frequencies. I trust the SPL level calibration makes sense down to DC. I did record RS digital SPL meter measurements alongside with the above Spectral Pro readings, and the RS meter just pretty much indicated 75-80dB between 3 and 18Hz, no matter what the Spectral Pro had to say. So it seems to me that the RS meter is sort of useless below 20Hz.

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