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Teeny tiny PA 15" subwoofer
Teeny tiny PA 15" subwoofer
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Old 4th April 2015, 02:12 PM   #1
chris661 is offline chris661  United Kingdom
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Default Teeny tiny PA 15" subwoofer

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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Old 15th April 2015, 12:36 PM   #2
chris661 is offline chris661  United Kingdom
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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
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Old 15th April 2015, 05:35 PM   #3
sine143 is offline sine143  United States
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Aluminum would heatsinking better than steel
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Old 15th April 2015, 05:37 PM   #4
tb46 is offline tb46  United States
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Hi,

A single driver TTL will be only ~75L, if I have the correct driver parameters, and you can squeeze S1 more (e.g.: 1/2 Sd=425cm^2).

Regards,
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File Type: txt 15TTL.txt (988 Bytes, 115 views)
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Old 15th April 2015, 08:25 PM   #5
chris661 is offline chris661  United Kingdom
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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
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Old 15th April 2015, 10:50 PM   #6
tb46 is offline tb46  United States
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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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File Type: pdf fold_2015_Apr15.pdf (12.3 KB, 134 views)
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Old 16th April 2015, 01:31 AM   #7
Brian Steele is offline Brian Steele  Grenada
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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.
Attached Images
File Type: gif 20150415-beyma-01.gif (13.6 KB, 1580 views)
File Type: gif 20150415-beyma-02.gif (11.2 KB, 1569 views)
File Type: gif 20150415-beyma-03.gif (19.2 KB, 1560 views)
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Old 16th April 2015, 02:34 PM   #8
tb46 is offline tb46  United States
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Default 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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File Type: pdf Beyma_15P1200Nd_dimension_question.pdf (34.2 KB, 48 views)
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Old 16th April 2015, 03:37 PM   #9
tb46 is offline tb46  United States
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Hi Y'all,

Here is another fold by kees52, it could done MLTL, TL, B6 (simple vented BR), JBL jet ports... all kinds of options, by moving the internal dividers around. Obviously, it would not have to be anywhere near as high as kees52 made it. It's from this thread:

Lab 12 Based Offset Driver - Mass Loaded - Transmission Line (OD-ML-TL) Design by Bj

There all kinds of folds in that thread.

Regards,
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File Type: jpg fold_by_kees52_MLTL.jpg (237.5 KB, 1541 views)
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Old 16th April 2015, 07:44 PM   #10
chris661 is offline chris661  United Kingdom
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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
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