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Old 17th February 2013, 02:38 AM   #1
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Default MCM 21" tuned LOW thoughts?

Evening guys,

As some of you may know, I'm currently using 4 of the MCM 21" woofers in a OB application and while I'm out here in Fort Leonard Wood away at school with nothing today, I decided to play around with them some in WINISD.

Yes, I'm using the specs given by MCM - not actual measured ones. I'll have to try and get measurements sometime or another if I ever decided to give this project a shot - overall, the only difference I could see that the specs would make is the LT eq that I came up with.

Anyway - stuff a 21" woofer into a 7cuft cab with a tuning of 9hz and what do you get? Garbage! Add in a nifty LT (~21db@11hz) and what do you get? An f3 of 16Hz, f6 of 10.8Hz, and f10 of 9Hz. With a max SPL of 101.8db@~69Hz (120Hz/-12db lowpass). Even more fun is the fact that I have 4 of these so with 28cu ft I can hit a theoretical 114db@70Hz,111.5db@20Hz, 107db@10Hz - all without room gain.

My only concern as of right now would be the port resonance at ~120Hz.

What are your thoughts?


Here's some screenshots from WINISD:

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Last edited by siruis815; 17th February 2013 at 02:43 AM.
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Old 17th February 2013, 04:37 PM   #2
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I'd try to go for a 4th order filter below tuning. There's still a lot of boost over 0dB <10Hz.

That said, cone excursion looks reasonable.

Try it and see!

If you have amplifiers that'll go down there, you should be in for a treat with movies and the like.
There's considerable gain at LF from boundry loading and room gain. However, both of those assume solid walls.
At 10Hz, not many walls are solid. I hope your room will stand this: structural damage is a serious concern with a project like this.

Chris
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Old 17th February 2013, 04:47 PM   #3
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Hi Chris,

A 4th order highpass @ 9.5Hz is modeled above - that's the only way I could keep excursion below 5Hz in order. This morning I added a -9db attenuation and tweaked the LT slightly which dropped max gain to ~12db, max amplifier load is down to ~370 Watts and output only suffers about 1db at 11Hz.

I was hoping for some room gain that's why I shot for a non-flat response - maximum output of course is something that I don't think I'll ever get to, but maybe 90-95 db with only 2-3mm of excursion @ 13hz is good enough

The problem now of course is getting the wife to agree and other projects to be finished first.

EDIT: Do you think the cones will hold up to the pressures?
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Old 17th February 2013, 05:49 PM   #4
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Ah yes, I'd mixed up highpass and low pass for a moment there.

I'd expect the cones to hold up fine. BR cabs spread the load (AFAIK) fairly evenly across the cone. THs are a different animal, and they're where big cones get tenderised.

Chris
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Old 17th February 2013, 09:38 PM   #5
Zero D is offline Zero D  United Kingdom
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@ siruis815

Quote:
My only concern as of right now would be the port resonance at ~120Hz
Hi, as you're probably aware, that's due to that LONG port of 4.7 ft x 4 inches ! Maybe you could lower the filter to between 80Hz - 100Hz ? If you'ld like to explore resonances in general further, i can recommend this - Boxnotes - Free Speaker Design Software

Also this page could be helpful,

Quote:
Some suggestions for when the port length is greater than the box size

When a ported subwoofer is tuned low, and uses ports large enough to keep airspeed to a reasonable level, the result can be long ports. Here are a few ideas to help you deal with them..... Long port options for subwoofers
Where did you get BL = 20.13200 as i don't see it in MCM's data ?

I inputted that figure into WinISD v.0.7.0.809 & together with the other parameters i get Sd = 0.1492 which is so close to yours as not to matter.

But Sd = 0.1706 m2 in MCM's data, which would make a difference !

What power did you input into the Signal TAB ?

I made the following observations about MaxPower & Excursion as shown in my screenies.
Attached Images
File Type: png mcm.png (22.2 KB, 50 views)
File Type: png ex.png (8.0 KB, 39 views)
File Type: png maxpow.png (7.5 KB, 17 views)
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Old 17th February 2013, 10:23 PM   #6
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Hey Zero,

I'm using WinISD 0.5 alpha Pro. BL was auto calculated as was SD. FWIW - the actual cone diamater + 1/2 surround = ~268.5sqin (if I remember correctly - based off of 18.5") = 1732sqcm. I'll have to see what that comes out with.

I think I entered around 4W for the signal - but of course that's a 0db gain for the LT. I have since messed with the model again, by changing the HP to 9.8Hz, LT (f0=50,Q0=.88,fp=14,Qp=.54) This drops the gain down quite a bit at the expense of about 2db @10Hz or so but an increase of about 3db above 20Hz. I can live with that.

I wonder what I'd get using the different Sd figure.

EDIT: I'm already getting into 12m/s air speed in the vent using a 4in port - with a 3" port it gets pretty gusty (22m/s). I'm tossing around the idea of a 2"x5" port for a velocity of up to 15m/s.

Last edited by siruis815; 17th February 2013 at 10:28 PM.
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Old 17th February 2013, 10:32 PM   #7
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I just looked at increasing SD which mainly just results in an increase in SPL with little or no impact on the response curve.

EDIT: Thanks for the link to Boxnote!
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Old 18th February 2013, 04:28 PM   #8
Zero D is offline Zero D  United Kingdom
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Quote:
I think I entered around 4W for the signal
And after that it quickly runs out of headroom, due to Excursion limitations ! I have to say that personally, i would rethink using this driver in such a project. But it's good to experiment without making sawdust etc, just to see, what if. Sometimes things do work out

Quote:
I just looked at increasing SD which mainly just results in an increase in SPL with little or no impact on the response curve.
OK.

Quote:
EDIT: Thanks for the link to Boxnote
You're welcome
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Old 18th February 2013, 04:36 PM   #9
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You're very right, it does run into over-excursion issues, however I must state that often the stereo isn't over 100db - which leaves much headroom. This would be fun during movies - lots of potential for 110db at the lowest octaves while being relatively efficient for anything over 30hz - most normal content. For music, these would likely not be used except to maybe annoy the neighbors

In the end - at very good "on paper" experiment.
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Old 19th February 2013, 09:13 PM   #10
abcdmku is offline abcdmku  United States
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I just built a ~12 cu ft enclosure for the MCM 21 inch. It has two 2"x22.5" and 20 inches deep, gets a solid 35hz
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