21" Horn Subwoofer

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My name is Daan and i do a elektronics engineering Study in the Netherlands.
I have my own Drive-in hobby company and i like working with audio.

I am able to get my hands on a couple of 21" cheap subwoofers with allot of power. 1200 Watt RMS, 98 DB 1w/1m (no cabinet). Iam looking to some sort of plans to get the most power (SPL) out of these woofer in a range of 35/40 Hz to around 120/150 Hz. I have been looking around for horn cabinets but iam a bit blinded by all the difrent types of horns.

My main goal is to get as much sub-low (35/40 to 55/60 Hz) power out of my subwoofer and decent amount of power from the mid-low (so 80 to 150 Hz).
I think about a SuperScoop design from SpeakerPlans.com ( Speakerplans.com ) but what are the pro and cons of this design? and are there any better plans that might suit my need?

All feedback and tips are welcome because everything i can learn now will help me in the future to understand audio engineering better :D

Best Regards, Daan

Edit: These subs will not be used in a inbuild instalation but will be used in allot of difrent rooms and outdoors. (Depending on who hires me or where i organize my own events etc)
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A 21" woofer does not need a horn.-! (And no woodenmaterial-casing, however,-)
If, you have to stonewall. Everything else does not resist, is not rigid - no chance to get a clean sound by listening!
And: regard, think and think and think, how to build in, decoupled and clamped. Exept you want modulate the resonating magnet and basket - most dis-interpret her 20 kg-woofer resonances as clean, deep sub,-)
I have to disagree, otherwise a 30" and 40" subwoofer into horn designs is not 'needed'. yet it is being build using the Powersoft m-system, or the 21" variants of Ricci's Othorn or the Weltersys Keystone.

There is a difference in need and want. And we all want... ;)
Without much calculation, you could put them in a 250L ported box tuned to 35Hz. Any half-decent 21" driver will do something useful with that. If you've genuinely got the power handling you think you've got, then there'd be room to EQ if the driver's Qts is a bit high.

If you're only building two boxes, I'd strongly recommend against traditional horns - they need to be in stacks of 6-8 to start making sense from a size-to-performance point of view.
Tapped horns might be worth a look, but will be very big, and are more sensitive to T/S parameters than ported boxes.

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