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Old 4th August 2010, 10:40 PM   #1
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Default Question regarding closed box driver resonance

Hi -

I have a question regarding what options I might have for lowering the speaker resonance frequency in a closed box, preferably below Fs.

The reason I want to do this is that I am considering building my own HT surround speaker systems using columnar arrays of FR drivers in the 3-5" range, and, unfortunately, most of them have Fs in the 70-100 Hz range, although several otherwise appear otherwise suitable - I would like to xover to the subwoofers around 50-60hz - I don't want to run as high as 80hz which is barely low enough to avoid hurting imaging, plus a 5 foot column of even 3" drivers should have enough air moving capacity to have good SPL capability that low in frequency.

In this instance, I do not want to go ported, since I plan to use the old activated charcoal box compliance enhancement trick here, and a sealed enclosure is the best way to keep compliance deteriorating atmospheric humidity out. Plus, the closed box will give better LF transient response for better transition to the subs which will also be closed box.

So, I want to move the in box impedance peak to around 50-60hz of a 70-100hz Fs 3-5" speaker and was wondering if there are any well worked out ways to do this short of modifying the driver (or having a custom driver made).

It seems that forcing the speakers to drive a backloaded column of air of constant cross section ought to have the capability to increase the low frequency Mms enough to drop the resonance frequency significantly without killing midrange and HF efficiency, since air is about 1.25mg per liter, a footlong or so guide at the back of each driver (common slot guide would seem to be ok for an array) should conceivably be able to double the effective moving mass of say, a column of 4" drivers, presumably driving down the resonance significantly.

Now, the acoustical resonance Q will drop also, but that's fine since the manufacturers set them highish for most of the little speakers in the first place to make them go thumpety. As long as it stays above about 0.2 overall, that is sufficient for my purposes, since I have a technique using passive components to effectively convert the electrical impedance peak to additional amplitude, and, voila - a 'free' third to half an octave of super easy to drive LF extension, if I can just push the in-box resonant frequency impedance peak down to 50-60 hz.

Thanks in advance for any advice.

Last edited by thoriated; 4th August 2010 at 10:51 PM.
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Old 4th August 2010, 10:58 PM   #2
Did it Himself
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Linkwitz Transform is your dream solution
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Old 4th August 2010, 11:14 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

Linkwitz transform is the common way of doing it,
the "Ace Bass" principle much less, here you mod the amplifiers output
impedance to effectively do the same thing, I've not analysed the real
differences if there are any, they look like just two different approaches.

Sreten.
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Old 5th August 2010, 03:17 PM   #4
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Hi -

Thanks for the suggestions. I've used the 'ace bass' approach before and it has worked well with good quality (KEF) drivers, but a couple slight issues with pure electronic eq, IMO is the increase of power required to maintain the extended bass output at higher levels and most (not all) drivers when run below resonance have greatly increased distortion characteristics. So I figure the more of this capability I can build into the speaker itself, the better the potential performance.
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Old 6th August 2010, 06:26 PM   #5
GM is offline GM  United States
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The only sealed way I know of to do this acoustically is to use a plane wave tube (PWT) to ~completely flatten the driver's impedance peak/phase response, lowering its Qts which in turn raises F3.

GM
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Old 6th August 2010, 08:04 PM   #6
Dr_EM is offline Dr_EM  United Kingdom
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Is that the same as a 1/2 wave closed end transmission line? I'm going to be experimenting with those using a small driver soon, keen to see it in practise
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Old 6th August 2010, 08:19 PM   #7
GM is offline GM  United States
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Correctomundo!

GM
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