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In Reply to: On axis SPL -VS- acoustic power response posted by tomservo on July 28, 2003 at 13:38:14:
Tom, I might have agreed with you a month ago, at least to some extent. While the McBean predictions for the HF limit (which are specified as on-axis response) are a least an octave, if not two, below what I’ve been measuring on-axis from my horns, I had never bothered with off-axis measurements, so the on axis/off axis thing might have explained the difference between what he predicted and what I got.However, I’m branching out from mostly bass guitar cabs to line-array PA cabs, so my latest DR design, the 250a, I measured off axis. The driver is a PAudio SN10MB, basic specs Fs 62, Qts. 3, Vas 30 L, SPL 96 dB- that should be enough to do your own sim. The horn has a 32 Sq. in. throat, 500 Sq in. mouth, length of about 3 feet. I get on axis 105dB at 125 Hz, free-space. I also get 105 dB at 4 kHz. Oh, did I mention that this horn is folded?
Anyway, at 1.6 kHz, where according to Leach’s math I should be well rolled off on-axis let alone off, the on-axis SPL is 104dB, precisely 1 dB down from broadband average. 30 degrees off-axis it’s still 104dB. At 45 degrees off axis it drops to 100dB. If what you are saying were true then I would not be getting these results. I am getting these results. Therefore, the math is wrong. Not your fault, Tom, you didn’t come up with it either.
At 2 kHz the 45 degree off-axis woofer response falls to 98dB, 6dB down from on axis, so that’s where I cross over to the tweeters. At 4 kHz the 45 degree off-axis response is down 12dB, but that’s no worse than most direct radiator mounted tens, and of course moot as the tweeters have taken over fully by then anyway. But the fact of this matter is that no matter how you cut it, on-axis, off-axis, or anything in between, the predicted roll-off that my horns should be suffering from just isn’t there. I get no more roll-off either on or off axis than from a plain baffle.
Now I respect a good equation just as much as the next guy, but when it comes to facts-is-facts, I put a lot more trust in aluminum, neodymium, paper and plywood. Mass roll-off? If there is such a thing it does not function in the way that general opinion assumes.
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Follow Ups
- Re: On axis SPL -VS- acoustic power response - Bill Fitzmaurice 07/28/0315:06:03 07/28/03 (21)
- Re: On axis SPL -VS- acoustic power response - tomservo 18:44:03 07/28/03 (12)
- We're getting a bit off the question, Tom. - Bill Fitzmaurice 05:55:21 07/29/03 (1)
- Re: We're getting a bit off the question, Tom. - tomservo 06:46:21 07/29/03 (0)
- Thank you Tom, could have not said it better! - JLH 20:10:00 07/28/03 (9)
- Re: Thank you Tom, could have not said it better! - Bill Fitzmaurice 06:19:52 07/29/03 (8)
- Disagree - JLH 16:42:27 07/29/03 (7)
- You've got it backwards. The opposite is true. - Bill Fitzmaurice 05:03:47 07/30/03 (0)
- Bends - John Sheerin 21:06:12 07/29/03 (5)
- Re: Bends - hancock 07:21:27 07/30/03 (4)
- Re: Bends - John Sheerin 12:57:53 07/30/03 (3)
- Very interesting - Bill Fitzmaurice 06:06:57 07/31/03 (2)
- Re: Very interesting - John Sheerin 08:00:08 07/31/03 (1)
- Re: Very interesting - Bill Fitzmaurice 08:39:34 07/31/03 (0)
- Re: On axis SPL -VS- acoustic power response - John Sheerin 16:03:37 07/28/03 (7)
- I use three rates of flair over the course of the horn. - Bill Fitzmaurice 16:54:31 07/28/03 (6)
- Re: I use three rates of flair over the course of the horn. - John Sheerin 17:13:54 07/28/03 (5)
- Olsen - Bill Fitzmaurice 05:29:40 07/29/03 (4)
- Re: Olsen - JLH 19:32:07 07/29/03 (3)
- Olsen did what he did - Bill Fitzmaurice 13:34:26 07/30/03 (2)
- Re: Olson, Newton, Einstein and physics (long ramble). - Paul Eizik 21:55:43 07/30/03 (1)
- Hey Paul - Bill Fitzmaurice 05:41:04 07/31/03 (0)
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