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Old 7th May 2004, 01:51 AM   #1
netgeek is offline netgeek  United States
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Default Linkwitz transform in vented enclosure??

Has anyone tried using a Linkwitz transform on a sub in a vented enclosure? I'm sure you'd have to come up with some extreme LF cutoff that was set above the "meltdown/destruct" limit - but might there be advantages?

I was thinking about a super-cheap (and limited SPL - say a target of 100 dB MAX - more likely around 85 dB or so) subwoofer using something like the Dayton DVC 10" - PE #295-485. Using WinISD and playing around it seems that this could be done in a sealed enclosure - with Linkwitz - by adding about 12 dB of gain for an F3 of 24Hz. Net requirement ends up being around 88 watts - considering that anything above 100 Hz can reach 100 dB with only 5.5W input.

For some reason these numbers just don't look right. If I input 88 watts to the thing in a sealed enclosure @ 25 Hz, do I exceed the excursion limits (xmax = 6mm)??????

So, I was wondering what happens if I use a vented - but undersized - enclosure (let's say 1.5 cu. ft.) to reduce the excursion, but compensate for the LF roll-off with the Linkwitz transform? Ridiculous idea?

The Dayton DVC is only US$27 so it wouldn't kill me if I fried one or two experimenting - just wanted to see if anyone here had some input beforehand.....

Thanks,
Bill
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Old 7th May 2004, 02:16 AM   #2
RHosch is offline RHosch  United States
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excursion increases so quickly below resonance that a LT circuit would likely cause the sub to self destruct before any useful gain was realized.
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Old 7th May 2004, 02:20 AM   #3
netgeek is offline netgeek  United States
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The Fs on the Dayton driver is 25Hz and I was going to set the enclosure tuning to approximately 20Hz. If I applied a 24dB/octave cutoff (or even a 48dB/octave cutoff) at around 25 Hz - would this be sufficient?
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Old 7th May 2004, 02:29 AM   #4
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Default Re: Linkwitz transform in vented enclosure??

Quote:
Originally posted by netgeek

For some reason these numbers just don't look right. If I input 88 watts to the thing in a sealed enclosure @ 25 Hz, do I exceed the excursion limits (xmax = 6mm)??????

So, I was wondering what happens if I use a vented - but undersized - enclosure (let's say 1.5 cu. ft.) to reduce the excursion, but compensate for the LF roll-off with the Linkwitz transform? Ridiculous idea?


In WinISD you can see the effects of the LT on cone excursion by putting in the amount of watts (for the unboosted region) into the Signal tab and input a Linkwitz transform into the EQ/Filter tab. Then check the cone excursion graph by chosing "cone excursion" from the drop-down on the graph menu bar.

You can also check it via the FRD linktranadv Excel spreadsheet.

Vented will give you the lowest cost and simplest way to go low.

A linkwitz transformed box can give you smaller size, control over the desired f and Q, and low group delay all the way down - but at the expense of lower SPL, larger amplifier requirements and the additional of line level DIY circuit project.
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Old 7th May 2004, 02:52 AM   #5
netgeek is offline netgeek  United States
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Default Re: Re: Linkwitz transform in vented enclosure??

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Originally posted by moving_electron


A linkwitz transformed box can give you smaller size, control over the desired f and Q, and low group delay all the way down - but at the expense of lower SPL, larger amplifier requirements and the additional of line level DIY circuit project.

These are exactly the attributes I'm looking for- very low range (F3 @ 25Hz or so) - admittedly with LOW volume overall. Larger amplifier requirements is a relative thing - if I can reach the volume levels I want with a mere 5 to 6 watts but I need 80 to 100 watts for that last octave - no problem. As for the line-level DIY circuits - also not a problem as I'm willing to put substantial effort into this area via prototyping, measuring, tweaking, etc. - this is where my interests are at the moment. I just don't want to be fighting against basic laws of physics in a vain effort...


Thanks for the feedback!!! I'm hoping I can whack on some experiments and provide some useful results to the folks here in return for all I've learned so far!

Regards,
Bill
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Old 7th May 2004, 03:18 AM   #6
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Default Re: Linkwitz transform in vented enclosure??

Quote:
Originally posted by netgeek

For some reason these numbers just don't look right. If I input 88 watts to the thing in a sealed enclosure @ 25 Hz, do I exceed the excursion limits (xmax = 6mm)??????

The voice coils in parallel are rated at 80w so you would not want to push it to 88 watts anyway.

I was running a single less capable 8" woofer with a similar LT to yours and it was quite useful for most music at a reasonable SPL. It cannot keep up with demanding loud rap or loud HT but it was very useful, cheap and deep. I ran it off of one side of a 80 watt amp. The meters indicate it probably never put out over 60 watts.

I would say try it and play around with it. If you were really going to go for a "less risk" inplementation you would look for a higher excursion subwoofer. But of course this would be more costly.

Your $25 approach sounds like a fine way to try the approach out.
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Old 7th May 2004, 04:02 AM   #7
netgeek is offline netgeek  United States
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Default Re: Re: Linkwitz transform in vented enclosure??

Quote:
Originally posted by moving_electron


The voice coils in parallel are rated at 80w so you would not want to push it to 88 watts anyway.
Right, they claim 80W Rms and 120W peak.

Quote:

I was running a single less capable 8" woofer with a similar LT to yours and it was quite useful for most music at a reasonable SPL. It cannot keep up with demanding loud rap or loud HT ....
.
I'm all for "reasonable" SPLs and don't consider loud rap or HT to be "music" or even an approximation..i.e. will forego listening to these "genres"...

Quote:

I would say try it and play around with it. If you were really going to go for a "less risk" inplementation you would look for a higher excursion subwoofer. But of course this would be more costly.

Your $25 approach sounds like a fine way to try the approach out.
Let's "let the games begin" - and bring on the smoke!!!

Thanks for the input,
Bill
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Old 7th May 2004, 12:03 PM   #8
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Default Re: Re: Linkwitz transform in vented enclosure??

Quote:
Originally posted by moving_electron
In WinISD you can see the effects of the LT on cone excursion by putting in the amount of watts (for the unboosted region) into the Signal tab and input a Linkwitz transform into the EQ/Filter tab.
What EQ/Filter tab?? I can't see it anywhere.
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Old 7th May 2004, 12:49 PM   #9
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You posted the same thing twice! I've answered in the other thread of the same name...
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Old 7th May 2004, 02:07 PM   #10
Ron E is offline Ron E  United States
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You don't want to do LT in a vented box. The 12db/octave boost would destroy the driver in in a hurry.

The proper thing to do isn a vented box is tune low and apply boost at the tuning frequency with a 2nd order highpass filter. This is known as a 6th order alignment.

The only free software I know of that does 6th order alignments is Robert Bullocks old DOS Boxmodel programs. With that program you can see the effects on excursion and MaxSPL.
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