Something big - P-Audio SD21-1800N - diyAudio
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Old 18th November 2015, 05:03 PM   #1
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Default Something big - P-Audio SD21-1800N

PDF for the driver: http://www.p-audio.co.uk/pdf/SD21_1800N.pdf

I'm working on a compact cabinet for one of these, to be used as part of my PA system. In order to minimise frontal area, I've tried simulating the cabinet where the driver fires out of a pair of slots, one of the slots is in front of the driver, the other being the BR port - see attachments.

With modern music reaching into the 30s, I'd like a cabinet that can reach those frequencies without straining, and also go loud enough for outdoor events for a couple of hundred people. I'm fairly sure this design achieves this, though would appreciate some feedback there. My concern is that the cone may be very unevenly loaded by firing out of that small slot at the top end. Some PPSL designs work in a similar way, with no failures reported, so I suspect it'll be okay.
Bracing such a large panel in front of the driver may prove difficult, though strengthening ribs ought to help. That panel will be removable, though, so that the driver can be mounted.

The side cross-section would be a 60x60cm square, with a pair of full-height 7cm slots. Vent velocity peaks around 19m/s on the sims.

HR sims attached, 80v input used to keep within Xmax, which leaves a bit of wiggle room to EQ the response flat. I'll be using a C.Audio RA3001 bridged, which can put out 100v rms, so there's a bit of headroom there.

I also have a set of four 15" subwoofers that match the output of this design well in the 50-35Hz range, so will be using the 4x15"s at one side and this cabinet at the other. Tops are currently a pair of 12" two-way cabs, to be replaced with some 2x10 tops. Crossover around 100Hz.

Chris
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Old 18th November 2015, 09:31 PM   #2
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i might aim to make the slot it's firing into a bit larger to reduce the possibility of compression issues.
came across a site a while back of a company that only does subs with reflector panels, albeit open on all sides.
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Old 18th November 2015, 09:50 PM   #3
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Quote:
Originally Posted by chris661 View Post
Bracing such a large panel in front of the driver may prove difficult, though strengthening ribs ought to help. That panel will be removable, though, so that the driver can be mounted.
I think you might run into some problems trying to both effectively brace that panel AND make it removable. That was one of the main failings of my POC2 design.
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Old 19th November 2015, 12:11 PM   #4
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Quote:
Originally Posted by snappyturtle View Post
i might aim to make the slot it's firing into a bit larger to reduce the possibility of compression issues.
came across a site a while back of a company that only does subs with reflector panels, albeit open on all sides.
The outlet velocity of the slot comes in at just over 15m/s at 80v drive at 50Hz. I think that'll be okay. The bass reflex port goes a little higher (around 19m/s IIRC), but still not too bad. There are lots of conflicting rules of thumb on vent velocity, but I usually aim for <20m/s.

Quote:
Originally Posted by Brian Steele View Post
I think you might run into some problems trying to both effectively brace that panel AND make it removable. That was one of the main failings of my POC2 design.
Hi Brian,
I was thinking of doing something like this:
Click the image to open in full size.
On the inside of the panel. I would hope that gluing some plywood on the inside (while avoiding where the cone might hit) would stop the panel from 'breathing' in and out with the pressure.

Chris
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Old 19th November 2015, 12:27 PM   #5
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Originally Posted by chris661 View Post
The outlet velocity of the slot comes in at just over 15m/s at 80v drive at 50Hz. I think that'll be okay. The bass reflex port goes a little higher (around 19m/s IIRC), but still not too bad. There are lots of conflicting rules of thumb on vent velocity, but I usually aim for <20m/s.



Hi Brian,
I was thinking of doing something like this:
Click the image to open in full size.
On the inside of the panel. I would hope that gluing some plywood on the inside (while avoiding where the cone might hit) would stop the panel from 'breathing' in and out with the pressure.

Chris
i wouldnt be so much worried about the vent velocity but rather the actual compression on the cone of the driver. if im looking at your hornresp sim correctly it seems that the reflector panel/space etc is roughly a quarter of sd, this might or might not present an issue during long term playback. something to look at anyway.

the plywood ribbing will help and doubling up that panel would stiffen it as well. again, very high impact/energy area. my ms paint skills are awful and describing it may not make sense, but if you were to use an arching rib whose legs hung over at the ends (down in the outlet cavity), you could use a fastener from the cabinet into the brace as well.

im interested to see how it turns out.
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Old 19th November 2015, 01:01 PM   #6
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The Xmax on the SD21-1800N is 14 mm according to the conventional method of adding 1/4 Hg. Optimising the design for 120 V would make more sense. As long as HR says excursion is within 20 mm simulated and your not exclusively playing sinewaves, that will do fine.

Johan
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Old 19th November 2015, 01:03 PM   #7
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The driver used in the Orthorn was this one, IIRC: B&C Speakers
Which survived simulated pressures around twice those Hornresp is reporting for this design. This P.Audio driver isn't quite in the same league as the 21SW152, but its not far off. That said, I might err on the side of caution and increase the slot widths.

Not sure what you mean about the ribs - MS paint or even a pencil sketch would be useful


Edit - Cross-post with Rademakers.
Putting 120v into this design will nudge 130dB in places, which is pretty impressive. I'll run some sine waves through the driver free-air so I can get an idea of when distortion sets in. Port velocity is up at 28m/s, so I'll definitely need to play with that a bit.
Cone pressure stays reasonable - 5500Pa where the Othorn was up over 9000Pa.


Chris

Last edited by chris661; 19th November 2015 at 01:07 PM.
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Old 19th November 2015, 01:37 PM   #8
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Originally Posted by chris661 View Post

Not sure what you mean about the ribs - MS paint or even a pencil sketch would be useful

Chris
crude and quick sharpie will surely do. basic side profile of the top, ribbing, like the picture you posted to repeat every 4-6" +/-
will allow fastener down from the top as well as in from the sides.
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Old 19th November 2015, 05:10 PM   #9
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Ah yes, I see what you mean. It might help, though the maximum amplitude of panel flexing occurs at the centre, so the ribs will be thickest there. More surface area to attach to the rest of the cabinet would be useful, so I'll include that where possible.


I've just had the driver on the end of an EP1500 bridged (up to around 900w into this driver), and can report that it'll do around 30mm p/p travel before the knocking sound of a stretched suspension becomes obvious. Excursion seems to stop pretty quickly after that - some drivers are pretty gradual, but not this one. Some low-order distortion probably starts creeping in as you go past Xmax, but since I was testing free-air at low frequencies (10-30Hz sine waves, plus some music playback), the distortion components would be low enough in frequency and volume that the dipole losses would render them inaudible here.
Its good to know that even a pretty serious "oops" moment is unlikely to kill this driver mechanically.

There was a lot of wind noise from the motor vents, particularly the pole piece vent, so some lining of the BR chamber might be necessary to kill those off. Wind noises started around 10mm p/p and went up from there. The wind noises will probably be masked by the LF output, but since an inch or two of felt won't add much weight, I'll do it anyway.


So...
- bigger amp will yield more output - around 120v looks good, and corresponds exactly to the 1800w thermal rating. Who knew?**
For now, the amp I have will give plenty of output and no worries about excursion.
- ports will need to be bigger, since I'm going from 800w sims to 1800w.


**That said, 120v is more voltage than a bridged RMX2450 will swing, so we're looking at the QSC PLX series, or maybe a Crown XS900. That'd mean spending money when I have quite a lot of this stuff lying around. There's 1.5dB between 120v and 100v, so I might optimise for 100v and call it good, especially when power compression is going to take a chunk out of that 1.5dB.

At 100v input, there's 125dB at 35Hz going up to 128dB at 40Hz and 130dB from 50Hz and up. Not bad.

Chris
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Old 19th November 2015, 09:06 PM   #10
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Originally Posted by chris661 View Post
In order to minimise frontal area, I've tried simulating the cabinet where the driver fires out of a pair of slots, one of the slots is in front of the driver, the other being the BR port - see attachments.
Chris,

According to your sketch, frontal width appears only reduced a few inches compared to a "normal" BR.
Why do you want to minimize frontal area?
Is output sim any better with the slot than if that volume was added to the BR box?

Art

Last edited by weltersys; 19th November 2015 at 09:11 PM.
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