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RE: Built-in woofers...

Posted by Paul Eizik (A) on October 5, 2023 at 09:30:59

In Reply to: Built-in woofers... posted by eso on October 4, 2023 at 07:39:07:

Eso

Bruce supplied the external dimensions of the Seismic here some time ago (see link). When I helped move one back in the day, the top box section lifted off so we could move it, and the bottom folded horn section separately. That top box section held the JBL 2240 driver mounted to it's reactance annulling back chamber. Bruce used calculus to derive the volume of the back chamber, however there's the old pre-T.S. rule of thumb of: 45/V= the frequency where the box will resonate in Hz, and where V= the square root of the box volume. As Bruce had mentioned at one point (the Show Horn?) that he had to adjust the box volume a bit anyway, even with the calculus, then the old rule of thumb seemed close enough for those of us who are math averse. As a sub will be bandwidth limited at the top end, then Keele's formula for deriving the optimum horn throat size SR (for maximum efficiency) could be used: Sr=0.8Fs*Qes*Vas, where Sr is in sq. inches and Vas is in cu. ft. As I recall the horn throat began at the top of the lower box when the top section was off. It would be interesting how your memories of the Seismic match, if at all, with these numbers. The hard part of reverse engineering the Seismic would be the geometry of the various shelves on the interior of the bottom section which formed the folded horn. Back in the day when the plans for my Olson derived bass horns got lost at one point, I resorted to using an electronic stud finder and chalk to trace out the locations of the internal shelves which formed the folded horn, but luckily the plans were later found. Now if someone had a Seismic plus a stud finder and chalk.... Anyway, any ideas you have on the subject would be interesting. Your current rig is looking great! But of course this can result in jealous feelings in those of us with less attractive horn rigs with higher WAF Z.

Paul

PS the TS specs for the JBL 2241 are: Fs=35 Hz; Qes=0.43; Vas 11^3ft. So the volume of the horn throat for Keele's Sr would be 132.44 sq. in., or a square mouth of 11.50 square. Note edits here due to using wrong Vas in cu. meters initially.

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Edits: 10/05/23 10/05/23 10/05/23 10/05/23

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