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Old 16th June 2003, 08:12 AM   #1
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Default Tuning at or below Fs

Tuning Freq below Fs, or tuning AT Fs, in a ported speaker. Are there any problems that would arise from this?

In the speaker program when tuning below Fs for a Peerless woofer I get a good flat curve.
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Old 16th June 2003, 01:03 PM   #2
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All right? Tuning at Fs is part of the "classic" bass reflex that Thiele put up as the most desirable alignment! The configuration is: Qts = .383. Volume of Box = Vas, and tuning frequency = Fs, and F3 will equal Fs.

It your Qts is higher than .4, than the box should be larger than Vas and the tuning should be below Fs. These are optimal numbers-you can vary.

While alignments with Vb, (volume of box) less than Vas and tuning frequency abve Fs tend to be sightly preferred, (somewhat less distortion), it is perfectly okay to tune below Fs if you don't push it too far. Qts = 0.53 or so and Vb being 1.5 Vas is perfectly reasonable.

What is your speaker's Qts?
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Old 16th June 2003, 09:02 PM   #3
Vikash is offline Vikash  United Kingdom
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don't know about t.k, but i've got a sub that i want to re-house which has a QTS of 0.130. What would you do with that kelticwiz?
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Old 16th June 2003, 11:45 PM   #4
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Quote:
is perfectly okay to tune below Fs if you don't push it too far
definately.certain speakesr wil distort like mad-some wont

.im gona tune my jbl 10hz below its Fs of 40hz..no lower tho,and only cos i knwo a guy has already tuned his to 25hz.





0.13? =HORN

the peerless XLS has q=0.2,it likes a tiny 50litres ported box(its a 12inch)
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Old 17th June 2003, 06:44 AM   #5
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What is your speaker's Qts?
0.43. You've answered my question anyway. However look at this...

http://www.penhometheatre.com.au/castbooksketc.htm

Its using a 0.43 qts driver.
Its tuned above Fs. All the common bass alignments say a 0.43 Q driver should be tuned below FS. why?
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Old 17th June 2003, 09:41 AM   #6
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Quote:
Originally posted by tech.knockout

Its using a 0.43 qts driver.
Its tuned above Fs. All the common bass alignments say a 0.43 Q driver should be tuned below FS. why?
Quote:
Originally posted by Kelticwizard
While alignments with Vb, (volume of box) less than Vas and tuning frequency abve Fs tend to be sightly preferred, (somewhat less distortion),...
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Old 17th June 2003, 10:22 AM   #7
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ok, what alignment would allow a 0.43 qts driver to be tuned above its Fs?

Or do you just ignore the alignments and tune what you like?
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Old 17th June 2003, 11:52 AM   #8
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There are many different tuning alignments for ported systems. As a matter of fact the number of available alignments vary continuously from an over-damped B2 alignment to a C6 maximum-ripple with 2 pole active bass extension and beyond. A B4 ported alignment is where f3 = fb and Vas = Vb. The alignment you’re asking about will fall more in the QB3 to B2 range. Both are over damped alignments. A further analysis would require more specifics about the driver.

Tuning is always a trade-off of specification and design. It depends on the driver design and what you want from that driver. Bass extension, box size, and efficiency will always be in conflict with each other and you’ll find yourself trading one for the other.
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Old 17th June 2003, 12:16 PM   #9
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alright then. Is it true that in all ported boxes, the smaller the box size, the better the transient response will be?(ignoring driver, etc)
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Old 17th June 2003, 01:00 PM   #10
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This is an example similar to one given by David Weems, who has been explaining how the Thiele-Small numbers work for decades through his book, "Designing, Building and Testing Loudspeakers".

Suppose you have a 10" driver with the following specs:
Fs = 30 Hz
Vas = 2 cu ft
Qts = .38, (or .4, for simplicity)

The "optimal" box will be a 2 cu ft box tuned to 30 Hz. The response will be flat with the F3, (-3 dB point), being 30 Hz.

Suppose we want to use this same driver in a smaller box. Suppose we want to use it in a 1 cu ft box.

Remember, a box half the size will give an F3 one half octave above any given box. So our F3 will be raised half an octave, from 30 Hz to 42 Hz. (To raise a frequency half an octave, you multiply it by 1.4-not 1.5. To raise it a full octave, you multiply it by 2).

Our box will then be:
Vb=1 cu ft
Fb, (frequency you tune the box) = something slightly below 42 Hz
F3 = 42 Hz.
However you will have a +2.3 dB hump in the octave above 42 Hz, and your transient response will be a little worse than the "optimal" box. But-it still can be a good, listenable box. +2.3 dB is not that bad. There is a formula for the correct box tuning, but if you make it 39 or 40 Hz-a little below your F3 of 42 Hz-you should be very, very close.

Another way of looking at that 1 cu ft box is to say that the best Qts for that box is about .32, and the hump is caused by having a Qts too high-.43 instead of .32. But again, the box will be listenable, except for perfectionists.

The fact that you can cut a box volume in half and still produce a system that most people would find perfectly decent should make you understand that that you have a lot of leeway in building ported boxes. If you get things within a reasonable range, it will come out fine. Indeed, variations of 10% or less are almost never audible at all, let alone being bad enough to cause a rejection of the entire configuration.

I find that finding the "optimal" system is a good starting point, then I vary things within an acceptable range to my taste.
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