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Old 16th February 2007, 01:00 PM   #1
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Default Box Q value

Hi,

I'm using a program called LSPWLITE to model rreflex box properties for a JX92s.

The box properties involve a Q value which defaults to 15. There is no help text and I'm not sure what to enter here - can somebody explain in basic terms about this value

There are also other box alignment ratios that I'm not sure about.

Thanks.
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Old 16th February 2007, 01:35 PM   #2
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From Boxplot:
<>
Box Parameters:

Alpha: System Compliance ratio.

Alpha determines how big to make the box.
Alpha is the ratio of box volume (Vb) to speaker Vas.
Alpha = Vas / Vb
For a Bandpass enclosure, Alpha only determines the Rear chamber volume. The front chamber volume is determined by H parameter and the basic driver characteristics.


H: System tuning ratio.

H determines what frequency the system will be tuned to, and therefore how to design the vent.
H is the ratio of the speaker free air resonance to the system tuning frequency (Fsb).
H = Fs/Fsb
For Bandpass enclosures the H parameter is not a tuning ratio and does not directly correspond to the tuning frequency. This parameter actually is called the damping factor and determines the basic frequency response shape. In some texts this parameter is labeled S, but it is kept as H in Boxplot for consistency.

Ql or Q7: Q from box leakage losses.

This is a measure of how airtight a box is. Even vented boxes MUST be airtight (except for the vent hole of course).
This is the enclosure Q at Fb from leakage loss. (better sealed boxes have higher Q's) This parameter must be measured after the enclosure is built. Start with 7 as a good estimate. A Ql of 3 or less is unacceptable and must be improved, a Ql of 15 or higher is very good.
Ql is not considered for the Bandpass enclosure type

Fe/Fs:

This is the ratio of the Equalizer resonant frequency to the box frequency.

D

This is the equalizer Damping coefficient. D = 1/Q
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Old 16th February 2007, 01:40 PM   #3
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It's Ql, for losses. 7 is commonly recommended. The last vented sub I did had Ql about 5.8
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Old 16th February 2007, 02:03 PM   #4
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Hi,

Quote:
Ql or Q7: Q from box leakage losses.
Don't forget that driver leaks is embedded in the Ql value and no drivers are without and can be of significance.

b
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Old 16th February 2007, 06:36 PM   #5
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Greets!

Yeah, it's amazing how much loss there can be due to using 'T'-nuts or open cell foam gasket instead of closed cell or around a cheap driver's layered paper gasket........ WRT the latter, over-tightening to compensate usually bends the basket........

GM
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Old 19th February 2007, 10:52 AM   #6
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Thanks for the info - another related question:

The box designer programs (boxplot etc) will allow estimation of the ideal box size. If sufficient wadding is used to increase the virtual box size by 20% (obviously avoiding the port) will this extra value be included in the ideal box size (ie the ideal box is 20% smaller) or does it not effect the ideal size - so the optimal virtual volume is ideal + 20%?
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Old 19th February 2007, 10:55 AM   #7
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You cannot use stuffing to virtually increase a vented box apparant volume as it modifies the operation of the vent.
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Old 19th February 2007, 11:06 AM   #8
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I believe you can, as long as you allow clear passage to the vent - you have to take the virtual volume increase into account when tuning the box though
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Old 19th February 2007, 11:47 AM   #9
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Stuffing reduces the Q of the vent action, it's not just a case of leaving a clear path to the vent.

By virtue of leaving a clear path to the vent, the amount of stuffing you will be able to employ will only have small effect on increasing apparant box volume.
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Old 19th February 2007, 12:39 PM   #10
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Ok, but assuming the virtual volume does increase, is the virtual volume the one to be used in alignment equations of the actual box volume?
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