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Old 22nd August 2015, 11:02 AM   #1
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Location: Itupeva, Brazil
Default Need advice:bass reflex not tuned in high power

Hello to all.
After a long time preparing myself , buying measurement equipment and the materials I built a subwoofer for low frequency effects in home theater.

My front and surround speakers are Behringer 2031A which have a nice output from 35 Hz up. They are externaly equalized and delayed to have similar response in all seats.

The subwoffer is a selenium extreme tweve inches which had one of the spidders removed in order to reduce the ressonance from 40 Hz to 24Hz. VAS = 220 liters and qts=0.37 after the modifications. I t was installed in a 200liter bass reflex box with two four inches pvc tubes not flared tuned to 22Hz..

According to my mini dsp usb microphone and REW software the frequency response and distortion are better than I deserve.

The subwoofer is feed from a behringer 3000 dsp, equalized with several deeps to reduce the room ressonances.

The only problem: Ican not reach THX sound pressure levels due to the voice coil hitting the magnetic structure in high intensity passages of movies. Measurements indicate the hitting occurs even at the ressonance frequency of the bass reflex in high power.

My first diagnostic is : the high turbulence of the air in the 4inches tubes is taking the bass reflex out of tuning. Am i correct? Do you have any ideas how tochange the project?

Thanks for your time and help.
Flavio
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Old 22nd August 2015, 12:42 PM   #2
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A pair of 4" ports for a 12" driver should be plenty.

A few things to check...
- are you sure you're causing the driver to bottom out? - the iNuke amps sound like crap when you hit clipping - you could be hearing that.
- are you sure the ports are tuned to 22Hz? Can you check?
- do you have a high-pass filter set to 20Hz to cut off the very low bass?

Chris
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Old 22nd August 2015, 07:02 PM   #3
wg_ski is offline wg_ski  United States
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What is x-max on the woofers? I've got some old Eminence 12's with very similar T/S and an x-max of 8 mm. The models say they run out of gas at *80 watts*. As a woofer in a 3-way in a similar large low tuned cab, a 200 wpc amp will bottom them out with music. They certainly don't have the guts for HT sub duty. Yours might not either. There may have been a reason the original design was only intended to get into the 40's.
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Old 22nd August 2015, 09:14 PM   #4
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Good comparison. The woofer is out of spec now. You can't manipulate all the specs to your desires...
What's the full target specs anyway?!
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Old 22nd August 2015, 11:26 PM   #5
tb46 is online now tb46  United States
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Hi FLAVIO MARINS,

Post #1: "...reduce the resonance from 40 Hz to 24Hz. VAS = 220 liters and qts=0.37 after the modifications. I t was installed in a 200liter bass reflex box with two four inches pvc tubes not flared tuned to 22Hz..."

Vas=220L seems too high. I'll attach an example BR using the TC Sounds Epic 12", that one has Fs=24Hz, and Qts=.4. The Vas is 91.27L. Are your values measured?

Either way, you need to use a steep low cut (high pass) filter below Fb, or your driver will bottom out at just about any high power level signal below resonance.

Your ports may be a little too small, that depends on how much power you use. Try to keep the port particle velocity below 15m/sec in your designs. That would be a reasonable value, 10m/sec would be better. :-)

I'll attach a Hornresp example (which should be Vrc=100L [or less]), you can just enter your T/S parameters, and take a look around.

Regards,
Attached Files
File Type: txt Example1.txt (989 Bytes, 5 views)
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Oliver
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Old 23rd August 2015, 12:41 AM   #6
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I gave a second look at your thread and changing from a 12" to a 15" should be fine.
In top the priority (this if you are in the Brasil controlled market):
Selenium 15WS600
Selenium 15WS600 15" 600W Woofer
http://www.jblpro.com/ProductAttachm..._SpecSheet.pdf
Selenium WPU1505
http://www.selenium.com.br/sitenovo/.../WPU1505_i.pdf
Selenium WPU1509
http://www.parts-express.com/pedocs/...ions-45221.pdf
Selenium 15SW1P (large Xmax, Xlim 9.3mm and 25.0mm. You can use to spec 800 RMS with right align/tuning on this one, maybe smaller box)
http://www.selenium.com.br/sitenovo/...s/15SW1P_p.pdf

The problem with the SELENIUM 15WS600, VB = 200.0 L, FB = 30.0 Hz is that (with most of the Seleniums) you can use only RMS ~80W for Xmax... You can take your cup of venom!!!
If you opt for one of this drivers post it for further development. Have fun.

Last edited by Inductor; 23rd August 2015 at 12:49 AM.
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Old 23rd August 2015, 01:00 AM   #7
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iNUKE NU3000DSP pdf
8 Ω per channel, stereo 440 W
8 Ω 1520 W
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Old 23rd August 2015, 03:32 AM   #8
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Chris,
-Tomorrow morning I will put the iNuke in bridge mode. I was using just on amp .
Loudspeaker impedance curve shows a sadle in 22Hz, while at low power levels- up to 50 Watts. after 50 watts the loudspeakes impedance changes
-I have just a first order 18 Hz High pass.

Thanks for the advice.
Flavio
Quote:
Originally Posted by chris661 View Post
A pair of 4" ports for a 12" driver should be plenty.

A few things to check...
- are you sure you're causing the driver to bottom out? - the iNuke amps sound like crap when you hit clipping - you could be hearing that.
- are you sure the ports are tuned to 22Hz? Can you check?
- do you have a high-pass filter set to 20Hz to cut off the very low bass?

Chris
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Old 23rd August 2015, 03:44 AM   #9
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You have a point. I just looked up in internet for the Xmax, but the manufacturer Xmax specified is the linear Xmax (height of voice coil minus gap height) =14mm. I could not find the mechanical Xmax, and as you said, the original design was 40 Hz...
Thanks!


Quote:
Originally Posted by wg_ski View Post
What is x-max on the woofers? I've got some old Eminence 12's with very similar T/S and an x-max of 8 mm. The models say they run out of gas at *80 watts*. As a woofer in a 3-way in a similar large low tuned cab, a 200 wpc amp will bottom them out with music. They certainly don't have the guts for HT sub duty. Yours might not either. There may have been a reason the original design was only intended to get into the 40's.
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Old 23rd August 2015, 07:33 PM   #10
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the project target is "just" THX ultra 2 spec - 130dB @ 30Hz, 122dB @ 20Hz, 112dB @ 15Hz 1/8 space, 1m with less than 10 % distortion.

In order to Measure this I use a calibrated microphone, which clips at 118 dBSPL, so I measure at 4 meters in the garden ( open field).

Thanks,
Flavio

Quote:
Originally Posted by Inductor View Post
Good comparison. The woofer is out of spec now. You can't manipulate all the specs to your desires...
What's the full target specs anyway?!
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