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Old 19th August 2012, 11:15 PM   #1
AJ34 is offline AJ34  United Kingdom
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Default High resonant frequency sub

Is there any merit in designing a high resonant frequency sub (maybe several large drivers in a small sealed box) and using it entirely below it's resonant freq? I've heard that the freq response of such a design would likely be very smooth - although I appreciate I'd probably need to provide considerable bass EQ (lift) to compensate for early roll-off.

Are there any commercial designs based on this principle?
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Old 20th August 2012, 12:09 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

The short answer is yes, and its very common commercially.
Linkwitz Lab - Loudspeaker Design covers the basics but doesn't
go into the the variable compression commercially exploited.

You need lots of power, comparatively speaking, and in
fact the arrangement is the the only way the drivers
can handle such high powers, do the maths / physics.

rgds, sreten.
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Old 20th August 2012, 12:15 AM   #3
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The Bag End ELF subs.
How they work: what is INFRA?
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Old 20th August 2012, 10:21 AM   #4
breez is offline breez  Finland
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I think there is merit in using a 'too small' sealed enclosure for a subwoofer for the reason of small footprint. As said above it requires more amplifier power and a suitable EQ/filter. From an efficiency standpoint it may be a good idea to try to keep the resonance still in the pass band. At and around the resonant frequency the power efficiency is the best. You can see it in the impedance graph where the peak is many times higher than the nominal impedance. The drive current at and around that frequency will be much smaller.
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Old 20th August 2012, 12:30 PM   #5
breez is offline breez  Finland
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Also, watch out for harmonic distortion at f/2 or f/3 (f = resonance) if your pass band is below f. Any non-linearity in the voltage-current transform in the loudspeaker coil can generate harmonic distortion in the current waveform which will quite efficiently drive the loudspeaker at resonance. Add in the fact that below resonance the loudspeaker is inefficient you may run in to high distortion levels.

You could get around this and linearize the operation by using current-drive. With current-drive the current will be linear and you bypass the poor voltage-current transform of the loudspeaker. You lose all electrical damping of the resonant system (gives you a very high Q of 2 to even 20+!). But given that the resonance is suitably far above your pass band you need not care if you can filter your input signal well enough.
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Old 20th August 2012, 01:52 PM   #6
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As far as I know they are pretty output limited, for obvious reasons. IMHO, if you can spare the space, you are better off with a large ported sub with low tuning and Q. The slow roll off will blend well with room gain, and the low tuning will push the group delay low enough to make it unnoticable.
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Old 20th August 2012, 05:15 PM   #7
AJ34 is offline AJ34  United Kingdom
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Thanks for your interesting replies. Had a look at the Linkwitz Lab closed-box spreadsheet and will try out a few design possibilities when I've got time.

The reason I'm looking into this type of sub is due to space constraints. I'd really like two subs, each acting as a stand for small active studio monitors. Each sub will need to have the same footprint as the small monitors and needs to be 600mm tall. This gives an internal volume of each sub = 20 ltrs approx. Due to dimensions, largest driver I could accomodate would be 200mm or thereabouts. I had thought about using 3 x 200mm bass drivers in each side panel of each sub (12 drivers in all).
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Old 20th August 2012, 09:42 PM   #8
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Quote:
Originally Posted by AJ34 View Post
Thanks for your interesting replies. Had a look at the Linkwitz Lab closed-box spreadsheet and will try out a few design possibilities when I've got time.

The reason I'm looking into this type of sub is due to space constraints. I'd really like two subs, each acting as a stand for small active studio monitors. Each sub will need to have the same footprint as the small monitors and needs to be 600mm tall. This gives an internal volume of each sub = 20 ltrs approx. Due to dimensions, largest driver I could accomodate would be 200mm or thereabouts. I had thought about using 3 x 200mm bass drivers in each side panel of each sub (12 drivers in all).
Sounds like a semi-viable option, but you will need to build them extremely heavy to function as a good stand, like 100+ lbs.
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Old 20th August 2012, 10:56 PM   #9
AJ34 is offline AJ34  United Kingdom
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I would consider using relatively cheap bass drivers because with a fair amount of cone area (12 x 8" units), massive excursions shouldn't be necessary and it seems that big excursions = big money, understandable really. Cheaper, lighter, lower powered drivers would likely be more sensitive too, so as long as they can handle enough power...
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Old 20th August 2012, 11:23 PM   #10
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