Teeny tiny PA 15" subwoofer

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November 2018 UPDATE

Here's a link to the plans for the cabinets I built:
https://www.diyaudio.com/forums/subwoofers/272492-teeny-tiny-pa-15-subwoofer-12.html#post4939160

It was done this way to accomodate some different 15" drivers - Faital Pro 15HP1060, which are within 1dB of the Beyma 15P1200Nd drivers. I have 4x of each, and use them interchangeably.

Here's a post I put together of a few different use cases:
https://www.diyaudio.com/forums/sub...pa-15-sub-horn-final-version.html#post5345799

Recommended processing:
Parametric EQ: 40Hz, +4dB, Q=1
Highpass filter: 4th order Butterworth at 36Hz
Lowpass filter: wherever's good for your mains. Not over 160Hz.
You can use an amplifier of up to 2KW into 8ohm without any need for peak limiting.

Real numbers, you're looking at 121dB at 40Hz (accounting for the 2dB of port compression), and around 125dB at 100Hz, with an amp capable of 90v RMS output. ie, a Behringer NU6000.

These days, I run up to all eight on a Crown MA12000i, which works really well.

These subs are a smallish and drop pretty low. They're not the loudest thing in the world, but they are small and light enough to use multiples to get output levels where you want them.



Original post below:



Okay, so I'd like to make some small subwoofers for PA use.

The criteria goes as follows...
- 40Hz LF cutoff
- small
- uses Beyma 15P1200Nd, since I already have 4 of them.

Inspiration comes from here:
Q15 Compact 15" Bass bin - Speakerplans.com Forums - Page 1
However, I'm not happy using a pair of 3" ports for a high-power 15" driver - there'd be a lot of wasted output potential there, when the vent velocity is >50m/s.

I'm a one-man crew with very limited transport space, so THAM15s will have to wait a while.

Playing around in Hornresp, and keeping in mind I'm going to EQ the low end back into shape (lots of power available - Behringer NU6000, 1.2kW@8ohm, 2kW@4ohm per channel), I get this:

Sd: 855
BL: 27.52
Cms: 8.13E-5
Rms: 3.28
Mmd: 162.26
Le: 2.7
Re:5.2

Offset driver design (all lengths are parabolic):
S1=500
S2=500
S3=200
S4=500
S5=650

L12=45
L23=60
L34=60
L45=5

This gives me a ~43Hz tuning, and the port velocity is sensibly low. The driver will stand full swing from an NU6000 without excursion worries (Xmax = 9.5mm, free air the surrounds start locking up at ~40mm p/p, 52mm p/p Xmech).

Folding the design is proving a challenge, though. I'd like a cube-ish shape for maximum stability - these are going to be carted around pubs, clubs and small festivals.


After a bit more playing around, things get really interesting if I go isobaric: >125dB@1m from 40Hz up, still within Xmax!
129dB peaks with 100v per driver, from a 70L box. Not too shabby if you ask me. A pair of isobaric cabs keep up with 4x single-loaded cabs at 40Hz, but lose out everywhere else, by as much as 6dB. Seems like a good deal to me...

Anyway, gotta run. If someone has the time to help fold this, I'd be very grateful and probably get the things built ASAP.
Alternatively, if I've missed something catastrophic, I'm all ears. Drivers ain't cheap...

Chris
 
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... Folding the thing is proving very difficult - fitting an isobaric pair of drivers in there needed a 45 degree angled baffle, and then going from that to a folded tunnel just wasn't happening.

New idea: isobaric T-QWT.

All areas = 400;
L12=20
L23=183
L34=0.1

Thinking I could do a tall thin box, single-fold, with a 45 degree board down the middle. Outer dimensions would come to something like 300x300x1200mm. Very small for drivers this size.

Putting a small flare on the end drops the outlet velocity from ~40m/s down to ~25m/s. Think I'd be adding that, then.

I would be concerned about hitting the inner driver with any real sustained power: it wouldn't have much space to vent into. Could a couple of steel plates with heatsinks inside help much?

Cheers
Chris
 
Wow, Oliver!
Didn't realise that'd give those results. I'll have a play around. One driver instead of two would mean a much lighter box if nothing else. I just tried adding a small flare at the end and the outlet velocity dropped considerably, so I think that'd be worth playing with. Next stop, SketchUp!

I've seen a couple of designs with blips like the one at ~130Hz. There's a group delay spike there. Any idea what's happening?
Its comfortably out-of-band, as I'll be using 12" tops that kick to 80Hz.

FWIW, these drivers are okay to around 40mm p/p, but the suspension puts the brakes on quickly from there.
Again, FWIW, I tried isobaric again and multiplied the cross-sections by about 0.7. Smaller cabinet, and more output around tuning. I really like this design!



Sine, have you had a look at this?
PiSpeakers Forum - Re: Speaker Voice Coil Cooling System - Heat Sink - Photos - Wayne Parham, August 04, 2005 at 00:23:53

Its quite neat, though I'd be reluctant to do that to the Beymas. Maybe I could vent the magnet directly out of the cabinet via a short tunnel...

Cheers, all
Chris
 
Hi Chris,

Post #5: "I've seen a couple of designs with blips like the one at ~130Hz. There's a group delay spike there. Any idea what's happening?"

Bjorno is the master of these designs (plus Martin King, .....), maybe he can add a little more. All 1/4 wave resonators will have length dependent resonances, the little blip will get a lot bigger if you move the tuning of the ~1/3 line length spot (from the throat) that it is set at in the quick simulation from Post #4. You still get big cancellation and reinforcement peaks and valleys. You can reduce these using filling, for a PA speaker I would go very easy on that. People seen to like a little "colour" in their sound. :)

I'll attach a fold suggestion from a fold I did before. Just a fold idea, no dimensions.

Regards,
 

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The problem with these OD/TL designs is that the response at the upper end is very sensitive to driver placement along the line, and sometimes it's very difficult to get the driver in the correct location AND come up with a fold that's transport-friendly (e.g. long and thin is likely not going to work).

I suggest just go with the B6 alignment and choose a different vent arrangement to address the issue you highlighted. Like one with a "butterfly" cross-section, for example.
 

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Beyma 15PS1200Nd Dimensions

Hi Chris,

The other day I tried to make drawing of the 15PS1200Nd, and when I did an overlay drawing, and scaled it I ended up w/ a different depth from the data sheet (.pdf). Could you, please, check the dimensions on one of your drivers?

Regards,
 

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Hi Oliver,
Its 158mm from front gasket to the back of the magnet.

Brian,
I quite like the idea of going isobaric to remove the need to boost around Fb. Should give the drivers a much easier time thermally. Plus, I suspect it'd reduce distortion a little, too.
That said, a small reflex with a big vent is still on the cards, though I'm not sure if its technically a Helmholtz resonator when the port is that big. Good fun to play around with, though.

Haven't been on Hornresp much today, as I've been catching up on Uni work and finally assembling the midbass cabinets (just a Fane Sov.Pro 12-500 in a ported box)

Cheers
Chris
 
Brian,
I quite like the idea of going isobaric to remove the need to boost around Fb. Should give the drivers a much easier time thermally. Plus, I suspect it'd reduce distortion a little, too.

If this is for PA use, I'd recommend against using an isobaric arrangement. A little boost around Fb isn't going to be an issue, particularly if Fb is around 40 Hz or below, unless the music you plan to play through this thing happens to consist mainly of 40 Hz sine waves :). The increased efficiency in the main part of the passband is going to be of considerably more importance compared to the isobaric approach.
 
Yeah, looks like you're right.
Isobaric (grey line) at 4v, drivers in series vs standard loading, 2.83v. Not sure which design this one is, but they all come out fairly similar, with 1x driver having a downward sloped response, and isobaric having peaks at each end.

An externally hosted image should be here but it was not working when we last tested it.


With the drivers I have, I could even make 4 of these, which would go astonishingly loud.

Sometimes they'll be used for live music, other times house parties. For the latter, I'd set more aggressive limiters so the drivers don't cook if someone decides a 40Hz sine-wave track is what we need.
Wonder if the dynamic EQ on the DCX2496 can help here...

Cheers
Chris
 
I use sealed Isobaric for PA. For perspective:

On the plus side, it is visually compact, and wide-range with extended lows.

On the negative side, I use 21,600+ watts for the mains at the moment, and I bring my own assortment of circuit breakers and plug directly into the fusebox. Sometimes infrasonic rumble is only detected by the subtle yet disturbing odor of many cooking voice coils. Consider switching-mode amps; mine have switch-mode power supplies but conventional audio amplifiers, and the stack of amps is nearly as big as the stack of speakers but more difficult to move; but they can be hidden. Bass peaks tend to dim the house lights, as the power line voltage from the service pole dips. Buy a driver distribution franchise from the mfgr, so that you can get the drivers in quantity at wholesale prices.

600 watts into tapped horns is not quite as smooth but considerably louder within a narrow bandwidth. And you can plug the amp into a wall outlet, and carry it without a fork lift.
 
I'd've thought sealed boxes would lose too much SPL compared to tuned designs for PA use. I guess you can get away with really tiny boxes, though.

I'm small-fry in the PA world at the moment. Currently I've a pair of 15" Deltamax tops with these Beyma drivers in. Soon I'll be using 12" tops and some subwoofers. Nothing spectacular, but I'd rather a small, nice sounding system than something huge that just sounds okay. Of course, if, with clever engineering, the small system can keep up with something physically bigger, that'd be a plus.

Just took delivery of another NU6000DSP, so now I can have one per side, plus HF amps. One channel will drive sub(s), the other will do midbass.
I did think about using one NU6k for subs and the other for midrange, but having read they're not too happy with 4ohm a side for prolonged durations, I think one channel at 4ohm and the other at 8ohm should keep them from shutting down.

Now all I need is a little 1U amp that doesn't have a switch-on thump. This old Peavey amp isn't playing too nicely any more. I think it unmutes the outputs before its quite ready - suspect some caps are on their way out.


That's all for now. Looks like I need to go away and contemplate patch panels and figure out which Hornresp sim looks easiest to build.

Chris
 
Hi Chris,

I took the basic fold from kees52 (Post #9), and my Hornresp simulation from Post #4, and ended up w/ the attached drawing. I tried to get it as small as I could. I added a mouth opening flare, as Brian suggested. Also, it should be easy to convert this into a vented arrangement. (If you actually want to do this, and need additional dimensions, I can provide those.)

Ok, get out some cheap wood for a trial run. :)

Regards,
 

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Woah, that looks brilliant.
I suppose, to convert to a ported box, I could cut holes into the panel behind the driver (as discussed in the 12" thread) and see how that comes out.

I've done one OD-TL before, which was an 550 sqcm cross-sectional area, L12=~67, L23=~144cm. It sounds okay, but doesn't work as the sims suggest. IIRC, it shelved down around 6dB from 70Hz to 50Hz, and stayed flat until dropping off around 30Hz.
You can see the impedance plot here. Hornresp suggests the lower impedance peak should turn up at 24Hz, but it looks like it'll peak <20Hz. I didn't add any stuffing, and just ran a brace along the centre of the internal panels.
Adding some filling to the open end of the TL in Hornresp gets me somewhere close to what was measured. It still serves duty when the Deltamax cabinets need some help in bigger spaces, but I'm certain the design above will do better, despite the serious size difference.


Anyone got any thoughts towards the wiggle at the top of the useful frequency range? - there's certainly some kind of "feature" in the group delay plot, though I'll be crossing over in the 80-100Hz range.

I'll draw up a cutsheet for that cabinet of yours, tb46, but it looks like we might be on to a winner. +4dB Q=0.7, F=39Hz parametric EQ gives a wonderfully flat response in free-field. I'm still playing with the DCX2496 dynamic EQ - cutting down the boost at high levels would wring the last bit of SPL out of this little box. :D
Cheers, I owe you a drink if we're ever in the same country.

Chris
 
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Hi Chris,

Yes, to convert it to a ported box you'd have to modify the panels behind the driver, and keep the portion of the bottom duct that you want to use as a port.

As to the wiggle at the top of the useful frequency range, it almost goes away w/ very light filling as shown in the attached simulation. Just a little fiberfill around S1, and a little past the driver in the upper cabinet corner should do it, the more you evenly increase these two points the lower the wiggle, but you start loosing low end SPL (Hornresp gives a total of .055kg Polyfill, w/ .017kg @ S1.).

I got to run, but I'll remember the drink the next time we're in the same country. :) Let me know if you need any more detailed internal dimensions.

Regards,
 

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Hi Oliver,

Struggling a bit with the cutsheet - figuring out the sizes of the internal 5 panels is proving a PITA.
I think I'll keep the 30 degree expansion continuous at the end by chamfering the doubled-up baffle (nice touch).

Need to have a think about how I'll attach a grille, too - I'd like 26mm clearance in front of the driver so the cone never hits anything. I'll probably just stretch the top, bottom and sides forward and put a big square grille over the driver.

Anyway, if you could check those dimensions for me, that'd be wonderful.

Chris
 
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