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Rock Block - a space saving sub
Rock Block - a space saving sub
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Old 12th July 2018, 12:05 AM   #21
Brian Steele is offline Brian Steele  Grenada
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Originally Posted by Thijs666 View Post
Thanks. I just tried this and (luckily) there is hardly any difference. Nothing worthy of posting.



Click the image to open in full size.

Could be better, could be worse...
From that graph, it looks like the horn's resonance frequency is a bit higher than predicted, you've got some serious loss going on (unless the impedance response for the measured system included the impedance of the cable, in which case you might want to check the connections), and the horn is suffering from some serious panel flex (the extra blips in the measured curve, and the degraded peak around 100 Hz).

You've got your work cut out for you with this one....
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Old 12th July 2018, 03:55 PM   #22
Thijs666 is offline Thijs666  Netherlands
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Originally Posted by Brian Steele View Post
From that graph, it looks like the horn's resonance frequency is a bit higher than predicted, you've got some serious loss going on (unless the impedance response for the measured system included the impedance of the cable, in which case you might want to check the connections), and the horn is suffering from some serious panel flex (the extra blips in the measured curve, and the degraded peak around 100 Hz).

You've got your work cut out for you with this one....
Serious loss, I expected that much after seeing the THD figures. It was also to be expected with such a large unbraced final section of the horn. I plan on using better plywood for the 'final' version and just hope for the best, loss- and THD-wise . I have been thinking about a sandwich construction for the final section (bottom part), but my guess is that, compared to a solid piece of 25 mm plywood, a similar thickness-sandwich construction isn't any stiffer. And a too thick construction goes against the small pack space goal

Also: it could very well be that my translation from HR to 3D is not completely without flaw, resulting in a shorter hornpath in reality.

Last edited by Thijs666; 12th July 2018 at 03:58 PM.
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Old 12th July 2018, 04:58 PM   #23
weltersys is offline weltersys  United States
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Quote:
Originally Posted by Thijs666 View Post
I'm sorry. I just now understood what you meant . With the wooden braces in between the sides and kept in place by the pressure of the straps... That might work. Though I've tried this on another sub with some serious wood clamps and they just weren't strong enough to keep the brace in place.
A recessed divot or rabbet could be used for brace alignment, ratchet straps would easily provide plenty of force to tighten things up.

For a cleaner look, turnbuckles and bent rods or steel cable with eyelets can be used.
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Old 12th July 2018, 07:18 PM   #24
Cableaddict is offline Cableaddict
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Originally Posted by Thijs666 View Post

.... At the mouth panel flex is about a cm each way at 2000 watts . I've clamped it down at the mouth with two times two opposing clamps (so basically I created two braces) and what it mainly does is bring THD down (by quite a lot, I must add), especially second order, but it doesn't really alter the frequency curve by much. Just listening to the difference I would say it plays louder WITHOUT the braces, that's probably due to the second order harmonics.
I'm still thinking about the best solution for this. Maybe some sort of broomstick with bolts on both ends and a sunken metal dish in the sides of the mouth... Of course any bracing should be quickly installable and removable. That's kind of a challenge!
How about using something like Art's "waveguide" (I'd prefer to call it a mouth extender) but instead of just clamping to the front, it could have thin steel plates "behind" it, which would slide into the current existing mouth by a few inches?

It would be tough to get a really tight fit, and it still wouldn't brace the "middle" of the mouth, but it would surely help, and you'd have an extender to boot. Better throw & possibly a tad better LF extension as well.

One more piece to carry, of course, which maybe defeats your initial plan.
------------

FWIW, I like Art's idea to use ratchet straps, but only for pulling the two pieces tightly against each other. That would be much better than simple latches.
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Old 12th July 2018, 07:19 PM   #25
Cableaddict is offline Cableaddict
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Originally Posted by Thijs666 View Post
Yes, original plans, adapted for 12mm birch ply with LOTS of bracing, loaded with the B&C 18TBW100-8 (18", not 15").
Ah, I understand now.

One of Art's frequency graphs was based on the more expensive B&C 18SW115.
That gives a bit nicer response. See:

Keystone Sub Using 18, 15, & 12 Inch Speakers
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