Lowering Fs of PA bass driver by adding mass - diyAudio
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Old 14th May 2015, 07:36 AM   #1
MrDodo is offline MrDodo  Mauritius
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Default Lowering Fs of PA bass driver by adding mass

I have a 12 inch no-name PA bass driver with no T/S parameters whatsoever. Only 600Watts and 40Hz to 7KHz
I have put it in a sealed 2 cubic feet enclosure to try but Frequency responds starts to drop sharply around 50Hz and going to null around 30Hz.

Is is possible to lower the Fs of the driver by adding mass to the speaker cone of Dust cap, or both.

I don't mind lowering the sensitivity of the driver if it means getting better response @ 30Hz. I can always dial up the power on the dedicated amp.

By adding mass I was thinking about using epoxy glue, wood glue etc (any idea welcome). mostly around the dust cap. Is this completely stupid and will I just damage the speaker beyond repair or will this have a chance to work?

Thierry
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Old 14th May 2015, 08:06 AM   #2
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Might be a good idea to try and get the parameters for it to see if it was ever meant to be used in a sealed alignment. Depending in where you are maybe one of us could measure it for you? I'm in Midlands UK if that helps?
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Old 14th May 2015, 08:07 AM   #3
OMNIFEX is offline OMNIFEX  Jamaica
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It is not common for a 12 inch driver aimed for PA to be used in sealed box unless you are using it for midrange. Why not just measure TS Parameters? You cannot add mass without some means of calculation unless you have lots of time to waste on a lot of trial and errors.
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Old 14th May 2015, 08:23 AM   #4
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What you plan to do is quite irreversible so the advise to first measure your loudspeakers makes eminent sense. Understand what you want to do before you start doing it.

BTW, PA speakers typically don't go very low. 50 Hz is not that bad at all. When you increase the weight of the cone, it is not just the efficiency you will change. You will also increase Qt, which means that your enclosure needs to be larger than it used to be. If you don't at the same time increase the size of the enclosure, you might not notice any change in low frequency response. Another issue is Xmax. If you weigh up the driver, you might improve the bass response somewhat, but at the risk of the driver bottoming out. To reproduce 30 Hz requires a lot more Xmax than 50 Hz for the same SPL.
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Old 14th May 2015, 09:48 AM   #5
AndrewT is offline AndrewT  Scotland
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I roughly doubled the moving mass of my PA speaker to get the new shape of the vented roll off I wanted.
BTW,
I used lead sheet, beaten into a thin foil, stuck on with double sided tape. And retested the T/S params.

In the process I threw away about 6dB of sensitivity in the midrange and upper bass, to maintain sensitivity in the lower bass.
I reported some of my experiments in a Tannoy B950 post.
I considered it a great success going from 101dB/2.83V@1m to ~95dB/2.83V@1m
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Last edited by AndrewT; 14th May 2015 at 09:52 AM.
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Old 14th May 2015, 11:20 AM   #6
MrDodo is offline MrDodo  Mauritius
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Ok, thanks everyone. I thought this "might" work, but I don't want to risk ruining the driver completely. Xmas does not seem to be very high :S but this PA driver has very stiff suspension that wont seem to mind a little weight on the cone, or so I think.
I think it is a 97db driver. I won't mind going lower.
I have an idea that I'll just try sticking some Blue-Tack around the dust cap (and even sticking lead to the blue-tack itself :P) and measure to see if it changes for the better. I can easily remove the Blue-Tack if does not work.
I'll also see if I can gather the courage to understand how to measure T/S for a driver.
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Old 14th May 2015, 01:29 PM   #7
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Please do, gather that courage and get going!
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Old 14th May 2015, 01:35 PM   #8
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A few bits of wire, a known resistor value, a cheap soundcard and something like REW is all you need. Or a DATS or WT-3 system. I've got the later if you are close?
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Old 14th May 2015, 03:04 PM   #9
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Quote:
Originally Posted by MrDodo View Post
Ok, thanks everyone. I thought this "might" work, but I don't want to risk ruining the driver completely. Xmas does not seem to be very high :S but this PA driver has very stiff suspension that wont seem to mind a little weight on the cone, or so I think.
I think it is a 97db driver. I won't mind going lower.
I have an idea that I'll just try sticking some Blue-Tack around the dust cap (and even sticking lead to the blue-tack itself :P) and measure to see if it changes for the better. I can easily remove the Blue-Tack if does not work.
I'll also see if I can gather the courage to understand how to measure T/S for a driver.
You can certainly add mass to a driver to change it's FS and sensitivity. However, if you are at all concerned about frequency response and longevity of the driver you'll want to work to get that mass right at the apex of the voice coil and cone.

Check out the link in my signature...I've actually added mass to a 5.5" woofer in a two way design. A cheap, easy way to make a mass ring is to use heavy gauge copper wire. All the details are in the DIYRM-A design guide at the link.

If you add it to the dustcap I'd be concerned about rocking modes and "oil-canning" at high excursion. The woofer can actually start rocking up and down, and depending on how tight the gap is, you can start making contact between voice coil and the hard parts of the motor.

Once you measure the T/S parameters of the woofer you can use basic algebra to work out how much mass to add to get the response you want. I have a spreadsheet that I can send you if you get stuck.

Scott
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Old 14th May 2015, 05:21 PM   #10
wg_ski is offline wg_ski  United States
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Even if you get the LF response you want, the HF may be useless. If you change the profile or stiffness of dustcap and/or the apex of the cone, the high end response will change. The highs radiate from the center. If you planned on crossing over where you are supposed to (where everything is pistonic, and the dispersion is not narrowing) then it would be fine. If you planned on pushing past this, the resposne you get may be an unpleasant surprise.
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