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Old 22nd October 2007, 02:21 PM   #1
obbelix is offline obbelix  Sweden
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Join Date: Oct 2007
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I have two vented cabinets at 155 litres each and they were built for
JBL 15" drivers some years ago. One driver in each box.
The drivers are now torn so I need two new drivers that will fit in my
cabinets.
I wonder if some of you have any suggestion for drivers that will work in my cabinets? Its possible for me to adjust the vents and damping to suit the new drivers.

There are four vents in each box today but I can adjust it to two if it
would help.
The subs will be used mostly in disco.

Some info about the cabinets as they are today:
Internal dimensions: H:565mm, W:535mm, D:515mm
Total volume: 155,7 liters
Typical damping (3 walls)
4 vents (Diameter: 110mm, Length: 150mm) with one flush end
Boxes built of 18mm birch-plywood.
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Old 22nd October 2007, 02:40 PM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Get Win ISD pro downloaded.
Input the data for your favoured driver and play with vent tuning.
Then try the next driver, until you find a match that suits your needs.

I did it a bit differently.
I had the drivers and box and played with the added mass until I had roughly doubled the MMS and then blocked off one port to drop the Fb down by half an octave. These two mods completely transformed the bass response (at no cost other than time re-measuring the T/S parameters for each series of runs through WinISDpro). Now superb with a droopy reponse falling at about 3db/octave over the two octaves from 100Hz down to 25Hz and then gradually increasing slope below 25Hz.
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Old 22nd October 2007, 05:34 PM   #3
obbelix is offline obbelix  Sweden
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Join Date: Oct 2007
Thanks for your reply!

Unfortunately I don't understand what to look for when comparing drivers in WinISD (What value/graphs).
But I have an offer on two new P.Audio BM-15LF that seems to work ok in the box (the little I know). The adjustment I need to do is plug two of the vents to get the FB 40hz. Is that an ok value or what is recommended for subwoofers?
Can someone, maybe you AndrewT, confirm if those drivers will work or not.
You can find the T/S parameters here (lower part of page):
http://www.paacoustic.com/pdf/BM-SER...-18LF-15LF.pdf

The strange is that if I put the exact same parameters in BassBox 6 I get fb 37Hz and F3 43Hz.
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Old 23rd October 2007, 07:27 AM   #4
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Hi,
all those frequencies 37Hz to 43Hz indicate a bass speaker, not sub-bass.
If you want sub-bass you are looking for an extended response to below 20Hz.
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Old 23rd October 2007, 02:42 PM   #5
bjorno is offline bjorno  Sweden
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Quote:
...I have an offer on two new P.Audio BM-15LF that seems to work ok in the box (the little I know). The adjustment I need to do is plug two of the vents to get the FB 40hz. Is that an ok value or what is recommended for subwoofers?...
Hi,
Yes, Your old tuning was about 42 Hz using 4 ports but with the new drivers and 2 ports gives instead a tuning at 32 Hz without the need for adjusting the lengths.

b

1(1)
Attached Images
File Type: gif bm-15lf_15in-ported_box.gif (50.4 KB, 172 views)
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Old 23rd October 2007, 04:39 PM   #6
obbelix is offline obbelix  Sweden
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Join Date: Oct 2007
I really don't understand very much of the picture, hehe
But if I buy these drivers, plug two of the existing ports in each cabinet, I will get two speakers that sound ok, or?

I forgot to mention earlier that the cabinets today have one Eminence Kappa Pro 15A each and I don't think they sound very well. If you could compare the Eminence driver in existing cabinet and the P.Audio in modified cabinet (two ports, each 150mm long) is there a big differece?

Thanks in advance.
Peter
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Old 23rd October 2007, 06:16 PM   #7
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally posted by obbelix
....... if I buy these drivers, plug two of the existing ports in each cabinet, I will get two speakers that sound ok, or?
probably not.
I expect that you will not be lucky with the first driver you try and may have to go through five or six drivers before you find a good/acceptable match of box to driver parameters.
It will be quicker and cheaper to do this by simulating the end results with some software.

You make the decision to risk your time and money or start learning how to use a simulation programme.
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Old 23rd October 2007, 06:48 PM   #8
prb123 is offline prb123  United States
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Have you looked into reconing the JBLs?
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Old 24th October 2007, 02:54 PM   #9
obbelix is offline obbelix  Sweden
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Join Date: Oct 2007
->prb123
Well, when I removed one of the drivers I unfortunately dropped it on the floor so the basket got a fracture. That's why I thinking of getting new ones.

->AndrewT
You're right, I think I have to learn some software so I can simulate and understand the results. Is there any specific values, (such as fs, qes, qms, qts, vas) that I should look for so I can exclude some drivers just by looking at the t/s?

Also, is there any Fb/F3 that is recommended/common for subwoofers? Maybe the question about t/s is depending on the wanted Fb/F3?
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Old 24th October 2007, 04:02 PM   #10
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Hi,
vented bass drivers work with Qts between 0.2 and 0.45
I'd aim for 0.3 to 0.37.
Fs~=lowest frequency you want.
But you can get then to work quite well down to Fs/1.5.
A low Fs usually means low sensitivity.
I was advised to not aim for a flat frequency response.
I took that advice and it worked for me.
The suggestion was to go for Bessel or similar slow rolloff, since this seems to be less affected by bass boom. That certainly proved to be true.
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