AudioBanter.com - View Single Post - Adire Tempest Downfiring Ported Subwoofer Project : Polyfill Concerns

(Stewart Pinkerton)
wrote:

I'll just give a few details about exhaust opening area. My entrance to the
listening room is a simple hole int he floor of 14.5 and 33-inches; in other
words joist width and not quite 3-feet long.

There are 2 basic artifacts (maybe advantages dependibg on your outlook). First
the air trapped in the plenum (containing 8 23mm 15-inch woofers in a floor to
ceiling basement enclosure) gets added to the mass of the system giving a set
of drivers with a 30 Hz Free air resonance at system resonance of 21 Hz when
deployed like this. Accordingly I then need 18 dB of EQ at 12 Hz to makle the
system flat to 12 Hz at 90 dB SPL.

The other effect is a 70 Hz resonance notch apparently caused by the distance
between the top and bottom woofer centers. But the system is rolling off
naturally at 70 Hz anyway so this just sharpens the low pass function.

As for driver piston area and opening area it doesn't seem to be an issue.with
a 2:1 piston area to chute opening ratio. I know of a few other systems where
fairly small chute openings didn't seem to impair in-room response.

I think of such a system as more of a slot-load than a bandpass; but it could
be modeled as a bandpass with a huge rear chamber and a small front with a
short port. Perhaps slot-load is just a special case of bandpass.