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If Xmax shouldn't be exceeded, why have a Xlim?

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fourway hornloaded View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fourway hornloaded Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 2014 at 1:14pm
Ok, again got your point Elliot. Intertechnik just called to confirm that a Dayton DATS will arrive at my doorstep in 8 weeks. So I'll be back on here in august, hopefully with some gained knowledge! Thanks for the replies again. Keep up the good work.
Regards,
Robert
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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 2014 at 11:03pm
You can use the fully functional demonstration version of LIMP until Dayton DATS arives.

http://www.artalabs.hr/

Best Regards,
Elliot Thompson
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fourway hornloaded Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2014 at 12:41pm
For those interested, some kind of 'rectification': after a day of swapping and testing it has turned out that there was a tiny little air gap between basket and baffle. Only at 165 Hz, this leak would make an agressive, loud rattling sound. At all other frequencies it was silent. So on techno, it sounded very much like if the coil were hitting the plate on every beat. The Nd9300 that I got don't have a back gasket. So that's why the problem never arose with PD186, which has a back gasket. Without an air leak, there's hardly any difference noticable, which is what I'd expect with two similar drivers in horns.

So the 18sound was not reaching it's Xlim at all.

Another problem discovered in the process: the chassis holes on the 18sound driver are just about big enough to pass the head of a DIN912 M6 allen screw, if you don't use washers!

The original cardboard box contains a sheet of air tight plastic foam that looks just perfect for cutting out some gaskets ;-).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote b grade Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2014 at 5:12pm
Gasket tape is pretty cheap too. I love that stuff. I try to always own some for problem solving.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andre Vergison Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2021 at 8:12pm
In a closed box the cone excursion at low frequencies often exceeds Xmax when feeding the driver with its rated max. power.

We could make the box smaller, down to the point that Xmax is always safe for all low frequencies. WinISD nicely shows that. A Linkwitz transform can compensate for the worsened transfer magnitude at low frequencies.

The max SPL of such smaller box would not decrease at all, as max SPL depends on frequency, cone surface and Xmax only.

A disadvantage I see is that (much) more power is required to obtain the same SPL in the low regions. The driver has to pump more, so to speak. More power also leads to more coil heating.

But the big advantage is that it would not be possible anymore to overdrive and destroy the speaker by a too large excursion. In addition, the box is smaller and less heavy as well.

What would be against it? I haven't met many such alignments in practice yet...

Andre
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MarjanM Quote  Post ReplyReply Direct Link To This Post Posted: 31 May 2021 at 9:38am
Compensating 3db loss in the low end with more power means twice the power applied to the driver.
You will reach power compression threshold way faster and wont gain anything at the end and risk blowing up the driver because it will start overheating. 
Marjan Milosevic
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https://www.facebook.com/pages/MM-Acoustics/608901282527713
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Post Options Post Options   Thanks (1) Thanks(1)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 31 May 2021 at 10:27am
Conversely, this is entirely possible if you’re very smart about designing the cabinet and the driver together for a desired non-linear response. However, it requires the use of a very accurate lumped element model of the entire loudspeaker, including non-linear and linear distortions plus thermal compression and such.

You also have to be willing to use ‘wrong’ drivers and work with the non-linearities. For example, you can use over-damped suspension or even very low Bl motors to ensure there’s a large resonant peak at your desired cutoff frequency.

It works better for sealed and bandpass enclosures due to the nature of those cabinets’ effect on loading.

The model can then be used alongside measurements of a prototype to develop mirror filter sets which can be applied in real-time via DSP and relatively inexpensive current or pressure sensors to dynamically correct the response of the cabinet. It’s possible to work towards a pre-configured target, while reducing distortion and having an adaptable filter set as level increases.

You might be surprised how many consumer and automotive audio products use a variation of this method, obviously some better than others. Although since Klippel has been publishing papers on how to do it for over 20 years, maybe not. Plenty of those are available to read on the Klippel site, but be warned there’s a lot of equations ahead.

Typically it’s a method applied to small speakers. IPAL and M-Force use a similar method for large drivers and higher output.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andre Vergison Quote  Post ReplyReply Direct Link To This Post Posted: 31 May 2021 at 5:47pm
Marjan what you wrote makes sense, we don't want power compression at all.

The exercise I made was with a Beyma 18LEX1600Nd (which I own) in a 80 liter box. Qtc is then 0,841 and while cone excursion never exceeds 14,5 mm down to 20 Hz.
A box of 140 liter (Qtc 0,709) lets the cone excursion at max power raise up to almost 20 mm at 30 Hz, which still is within Xlim. Throughput increases with 3 db at 30 Hz, from -14 dB up to -11. Max power at 30 Hz is 850 W.

Not too bad, there's at least some degree of protection.

So better pick the 140 liter box?




Edited by Andre Vergison - 31 May 2021 at 5:51pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bob4 Quote  Post ReplyReply Direct Link To This Post Posted: 03 June 2021 at 8:41am
Andre, a friend of mine has a pair of  Beyma loaded 18" DIY subs, they must be close to 30 years old by now. They are very compact CB, with a roughly 50x 50 cm baffle and about 30 cm depth if memory serves me well. 
They work surprisingly well, and have survived decades of abuse. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 03 June 2021 at 12:42pm
Originally posted by Andre Vergison Andre Vergison wrote:

Marjan what you wrote makes sense, we don't want power compression at all.

The exercise I made was with a Beyma 18LEX1600Nd (which I own) in a 80 liter box. Qtc is then 0,841 and while cone excursion never exceeds 14,5 mm down to 20 Hz.
A box of 140 liter (Qtc 0,709) lets the cone excursion at max power raise up to almost 20 mm at 30 Hz, which still is within Xlim. Throughput increases with 3 db at 30 Hz, from -14 dB up to -11. Max power at 30 Hz is 850 W.

Not too bad, there's at least some degree of protection.

So better pick the 140 liter box?






There are quite a few papers and documented lumped element models to calculate the onset of thermal compression for a proposed design, if you're so inclined:
https://www.klippel.de/fileadmin/klippel/Files/Know_How/Literature/Papers/Nonlinear_Modeling_of_Heat_Transfer_03.pdf

If you're not, then pages 16 and 17 are still a good read as to why this is useful. Especially if you have access to a driver to measure a couple of parameters. Sadly I don't think any of the software has implemented this model yet.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote VECTORDJ Quote  Post ReplyReply Direct Link To This Post Posted: 04 June 2021 at 2:42pm
Please keep Us Reconers busy by grossly overdriving  Your woofers.......Thanx, Troy Bernard  VECTORSONICS
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