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Author Topic: Help setting HP filter for bass cab  (Read 6630 times)

john lutz

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Help setting HP filter for bass cab
« on: July 27, 2010, 04:03:10 pm »

What data should I be looking at to find a proper HP for these boxes?

I have some DIY JBL dual 18s w/2241Hs in a vented box tuned to 33hz and have just been using LR24 @30hz.  

Power is CE4000 stereo - so possible 1200w max into nominal 4 ohms.  Is the HP I'm using too conservative or too risky?  

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Duane Massey

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Re: Help setting HP filter for bass cab
« Reply #1 on: July 27, 2010, 09:08:00 pm »

Depends....how loud do you run the rig? What type of program?

I'm very conservative, so I'd run it at 40hz if you're pushing the system very hard. And, as always, don't clip the amp...
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Duane Massey
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Paul O'Brien

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Re: Help setting HP filter for bass cab
« Reply #2 on: July 28, 2010, 11:08:42 am »

john lutz wrote on Tue, 27 July 2010 16:03

What data should I be looking at to find a proper HP for these boxes?



You need to model the boxes in something like WinISD Pro to find the system tuning frequency, and to do that you need the TS parameters for the drivers(available from JBL), and the internal box volume and port dimensions. This software is free to download from here.. http://www.linearteam.dk/default.aspx?pageid=winisdpro
When done properly this software can also show you how much power the speakers can actually take before over excursion.
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Paul O

Silas Pradetto

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Re: Help setting HP filter for bass cab
« Reply #3 on: July 28, 2010, 11:32:38 am »

LR filters are 6dB down at the knee, so you'd be 6dB down at 30Hz. Usually a high pass at box tuning is quite safe, so you're probably fine. Modeling it would obviously be a more accurate way to determine this, but often the physical piece doesn't match the model.

I think Butterworth filters are usually used for high passes because they are only 3dB down at the knee. They stay flatter in their passband.
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Evan Kirkendall

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Re: Help setting HP filter for bass cab
« Reply #4 on: July 28, 2010, 11:42:01 am »

Silas Pradetto wrote on Wed, 28 July 2010 11:32

LR filters are 6dB down at the knee, so you'd be 6dB down at 30Hz. Usually a high pass at box tuning is quite safe, so you're probably fine. Modeling it would obviously be a more accurate way to determine this, but often the physical piece doesn't match the model.

I think Butterworth filters are usually used for high passes because they are only 3dB down at the knee. They stay flatter in their passband.


+1

I'd use a 3rd order BW at your tuning frequency.

As always, keep an eye out for signs of stress during use. Bottoming out, distortion, ect. You may need to play with it a little... What JBL driver are you using?


Evan
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Silas Pradetto

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Re: Help setting HP filter for bass cab
« Reply #5 on: July 28, 2010, 11:44:59 am »

Evan Kirkendall wrote on Wed, 28 July 2010 11:42

Silas Pradetto wrote on Wed, 28 July 2010 11:32

LR filters are 6dB down at the knee, so you'd be 6dB down at 30Hz. Usually a high pass at box tuning is quite safe, so you're probably fine. Modeling it would obviously be a more accurate way to determine this, but often the physical piece doesn't match the model.

I think Butterworth filters are usually used for high passes because they are only 3dB down at the knee. They stay flatter in their passband.


+1

I'd use a 3rd order BW at your tuning frequency.

As always, keep an eye out for signs of stress during use. Bottoming out, distortion, ect. You may need to play with it a little... What JBL driver are you using?


Evan


He mentioned the 2241H so he should be set.
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John Roberts {JR}

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Re: Help setting HP filter for bass cab
« Reply #6 on: July 28, 2010, 12:27:42 pm »

john lutz wrote on Tue, 27 July 2010 15:03

What data should I be looking at to find a proper HP for these boxes?

I have some DIY JBL dual 18s w/2241Hs in a vented box tuned to 33hz and have just been using LR24 @30hz.  

Power is CE4000 stereo - so possible 1200w max into nominal 4 ohms.  Is the HP I'm using too conservative or too risky?  




As others have mentioned the LR alignment is not intended for simple HPF. The LR alignment is basically two Butterworth alignments stacked (added) in series, and the -6dB at tuning is only useful for combining with a symmetrical LPF where the relative phase shift rotates through a full 360' and back to in phase again.

For a single ended HP filter, there in no other bandpass to mate with, so tuning should be based on complementing the box driver needs.

Other speaker guys can give useful specific advise. My experience is more with the filters than boxes and drivers.  

JR
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Paul O'Brien

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Re: Help setting HP filter for bass cab
« Reply #7 on: July 28, 2010, 12:54:31 pm »

I have this software so I ran a simulation quickly just to see for myself. Using a 15 cu ft box(for a pair) tuned to 42hz these drives reach xmax at 900w, the LR24 HPF at 30hz actually does appear to limit excursion below tuning to the same amount that occurs above but it also moves the -3db point a little higher too. These numbers are only valid for this particular combo but as expected it demonstrates that over excursion begins to occur at relatively low power levels. Increasing power handling involves using smaller enclosures and higher low-cut filters.. though that also decreases low freq response.
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john lutz

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Re: Help setting HP filter for bass cab
« Reply #8 on: July 28, 2010, 01:41:51 pm »

Thank you all for replies - plenty for me to digest.  Here is the data I have on these boxes so far.index.php/fa/31645/0/
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john lutz

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Re: Help setting HP filter for bass cab
« Reply #9 on: July 28, 2010, 01:43:26 pm »

john lutz wrote on Wed, 28 July 2010 12:41

Thank you all for replies - plenty for me to digest.  Here is the data I have on these boxes so far.index.php/fa/31645/0/

index.php/fa/31647/0/
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john lutz

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Re: Help setting HP filter for bass cab
« Reply #10 on: July 28, 2010, 01:44:33 pm »

and thisindex.php/fa/31648/0/
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john lutz

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Re: Help setting HP filter for bass cab
« Reply #11 on: July 28, 2010, 01:46:35 pm »

john lutz wrote on Wed, 28 July 2010 12:44

and thisindex.php/fa/31648/0/

and finally:
index.php/fa/31649/0/
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Evan Kirkendall

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Re: Help setting HP filter for bass cab
« Reply #12 on: July 28, 2010, 01:53:44 pm »

John,
Did you get all these graphs with the driver cool? Once you heat up the voice coil and apply some power to it, those graphs can change drastically. You'll get a much more realistic view of what's going to happen to those boxes when you start moving air.



Evan
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Art Welter

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Re: Help setting HP filter for bass cab
« Reply #13 on: July 28, 2010, 02:05:48 pm »

The 2241H has a 35Hz FS, X max of 7.62 mm.
At 1000 watts, the model shows over Xmax from 48-60 Hz.

Tuning so low in the relatively small box is causing low output, almost 10 dB down at FB.

Why would you choose a Fb below Fs?

Have you tried a higher Fb, like 35, 40 Hz, 45 Fb for comparison?

Is the 10.7 cubes for one or two speakers?

Have you checked the actual Fb of the box by sweeping with a sine wave to see where the excursion minima (Fb) is ?
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john lutz

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Re: Help setting HP filter for bass cab
« Reply #14 on: July 28, 2010, 02:21:03 pm »

Thanks Evan.  The Win ISD graphs are predictions before box was built.  The WT3 impedance grarh is an actual measurement of the box at rest (cool).  The predicted tuning was 39hz - the measured tuning is 33hz.  I will remeasure with it heated up a bit.  I'm curious now.  It's all a learning expierience for me.  Once I fully grok the response form JR, Paul, and Silas I think that is basically what I was looking for....thank you all.  
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Art Welter

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Re: Help setting HP filter for bass cab
« Reply #15 on: July 28, 2010, 02:35:21 pm »

john lutz wrote on Wed, 28 July 2010 12:21

Thanks Evan.  The Win ISD graphs are predictions before box was built.  The WT3 impedance grarh is an actual measurement of the box at rest (cool).  The predicted tuning was 39hz - the measured tuning is 33hz.  I will remeasure with it heated up a bit.  I'm curious now.  It's all a learning expierience for me.  Once I fully grok the response form JR, Paul, and Silas I think that is basically what I was looking for....thank you all.  

39 Hz sounds like it would work a lot better than 33 Hz.
Shorten up the ports a bit and you will probably  hear a big increase in low end.
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john lutz

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Re: Help setting HP filter for bass cab
« Reply #16 on: July 28, 2010, 03:30:20 pm »

Art Welter wrote on Wed, 28 July 2010 13:35

john lutz wrote on Wed, 28 July 2010 12:21

Thanks Evan.  The Win ISD graphs are predictions before box was built.  The WT3 impedance grarh is an actual measurement of the box at rest (cool).  The predicted tuning was 39hz - the measured tuning is 33hz.  I will remeasure with it heated up a bit.  I'm curious now.  It's all a learning expierience for me.  Once I fully grok the response form JR, Paul, and Silas I think that is basically what I was looking for....thank you all.  

39 Hz sounds like it would work a lot better than 33 Hz.
Shorten up the ports a bit and you will probably  hear a big increase in low end.


Thanks Art, I was just thinking the same thing (more or less but not as clearly).  This little project came from having 4 drivers and 2 small sturdy boxes laying around - and playing with software.  In real use so far they do OK.  The size is a trade off in output for portability.  I started to wonder if I could get more out of them with proper HP filter and then I realized I didn't actually know what that should be - hence my query.

I think the WT3 impedance sweep tells me actual Fb is 33hz.
Modeling in  Win ISD, I chose a port size to tune to 39hz but the final result has drifted a bit.
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Art Welter

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Re: Help setting HP filter for bass cab
« Reply #17 on: July 28, 2010, 08:04:18 pm »

john lutz wrote on Wed, 28 July 2010 13:30

Art Welter wrote on Wed, 28 July 2010 13:35

john lutz wrote on Wed, 28 July 2010 12:21

Thanks Evan.  The Win ISD graphs are predictions before box was built.  The WT3 impedance grarh is an actual measurement of the box at rest (cool).  The predicted tuning was 39hz - the measured tuning is 33hz.  I will remeasure with it heated up a bit.  I'm curious now.  It's all a learning expierience for me.  Once I fully grok the response form JR, Paul, and Silas I think that is basically what I was looking for....thank you all.  

39 Hz sounds like it would work a lot better than 33 Hz.
Shorten up the ports a bit and you will probably  hear a big increase in low end.


Thanks Art, I was just thinking the same thing (more or less but not as clearly).  This little project came from having 4 drivers and 2 small sturdy boxes laying around - and playing with software.  In real use so far they do OK.  The size is a trade off in output for portability.  I started to wonder if I could get more out of them with proper HP filter and then I realized I didn't actually know what that should be - hence my query.

I think the WT3 impedance sweep tells me actual Fb is 33hz.
Modeling in  Win ISD, I chose a port size to tune to 39hz but the final result has drifted a bit.

Port shape and proximity to the walls of the cabinet can change the Fb quite a bit, as you have found out.

After a few cabinets not performing as they should, I always measure the cone excursion and adjust the port depth accordingly.

Port size is quite important too, and is a “catch 22”. A big port is needed for high excursion, but the pig port chews cabinet size, and you don’t get good LF from a too small box.
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John Robson

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Re: Help setting HP filter for bass cab
« Reply #18 on: September 15, 2010, 05:09:46 pm »

30hz would be a safe place to be at. A lot depends on what you pushing through them and at what level. You want to make sure that you don't go to low that cone is no longer loaded by the box if the cone become unload you can damage the speaker.
I suggest that you use test tones at a low power level and slowly go down from 35hz and watch the cone of the speaker. If the cone stats moving a lot farther at 25hz than it did at 30hz then you know the frequency that the cone is not loaded. John
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john lutz

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Re: Help setting HP filter for bass cab
« Reply #19 on: September 16, 2010, 01:40:09 pm »

John Robson wrote on Wed, 15 September 2010 16:09

30hz would be a safe place to be at. A lot depends on what you pushing through them and at what level. You want to make sure that you don't go to low that cone is no longer loaded by the box if the cone become unload you can damage the speaker.
I suggest that you use test tones at a low power level and slowly go down from 35hz and watch the cone of the speaker. If the cone stats moving a lot farther at 25hz than it did at 30hz then you know the frequency that the cone is not loaded. John


Thank you for input, John.  I'll try the visual sweep.  I did end up shortening the ports to the thickness of the baffle board just to get closer to the design target tuning of 39hz.  Interesting how drastically proximity to cab walls affected the ports.  I know I've read it and heard it, but seeing it and measuring it made it sink in.  

FWIW the result was a lot more output but not as nice a sound.  Now they sound like you might expect, two 18s shoehorned in to a small cab.  Of course that only made me wonder how to find 18s that really like a small box.   Smile    
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Josh Billings

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Re: Help setting HP filter for bass cab
« Reply #20 on: September 20, 2010, 05:52:29 pm »

wait...so what kind of filter is best for a high pass? I've had my subs on a LR24 or 48 for a long as i can remember

-Josh
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Art Welter

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Re: Help setting HP filter for bass cab
« Reply #21 on: September 20, 2010, 06:30:51 pm »

Josh Billings wrote on Mon, 20 September 2010 15:52

wait...so what kind of filter is best for a high pass? I've had my subs on a LR24 or 48 for a long as i can remember

-Josh

BW will have more output at the knee frequency than LR.
24 dB per octave is steep enough for protection.
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