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|24th June 2003, 10:45 AM||#1|
Join Date: Jun 2003
Location: Parra, Sydney
leakage and Ql in ported systems
When using WinISD/UniBox to design a Ported box, what Ql (leakage) should I put in?
'No leaks'(70Ql)? or 'minor leaks'(7Ql)? WinISD uses 7Ql as default, so every box is supposed to leak somehow?
I know sealed boxes are supposed to have no leaks, what about ported boxes... Major leaks due to port opening?
Or does it mean the leakage from the box besides the port opening.
|24th June 2003, 01:18 PM||#2|
Wizard of Kelts
diyAudio Moderator Emeritus
Port opening does not count as a leak.
Richard Small, of Thiele-Small parameters, in his original paper on ported boxes dealt with Ql's of 10 and 7 as being achievable in practice. So fill in either of those. The numbers come up fairly close to each other anyway.
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|24th June 2003, 01:29 PM||#3|
Join Date: Sep 2001
Ql is the Q-value due to unwanted enclosure and woofer leakage losses
Qa is the Q-value due to enclosure damping material (or absorption losses)
Qp is the Q-value due to port losses
In normal cases Ql is in the range 5-20. Ql is 7 in most cases, but 15 to 30 if the enclosure has very little leakage. Ql may be 7 even with an enclosure with no leakage because the driver unit itself may have leakages.
A box with moderate lining has a Qa of about 30. A heavily stuffed box can have a Qa of 3. Qa is 7 for about a half-filled enclosure.
A perfect port will have absolutely no losses in which case Qp is very large. A good port, one which does not significantly restrict air flow, has about 100 of Q-value.
If the box is completely stuffed with damping material, the QA will decrease. However, the virtual box volume will increase with about 20% (depending on the damping material's properties) -- the driver unit will experience a larger box than Vb. Thermodynamics dictates that the maximum increase in enclosure volume due to enclosure filling is 40%, I think.
There really is no easy way of calculating these Q values without the assistance of measurements. Equations exist for calculating them, but the data needed to be inserted into those equations is next to impossible to acquire (without first taking some measurements... And by the time you're done measuring, you will have already built a test enclosure).
|24th June 2003, 05:20 PM||#4|
Join Date: Jun 2002
Location: USA, MN
I generally agree with Isaac. Measurements of Ql, Qa and Qp are difficult unless you can assume that all the others are negligible when measuring one of them - which is not realistic in most cases.
The values Bullock uses as default:
The "Ql" in programs where they ask you for just Ql is could be called Qlt - or Ql total
Bullock's values give you a Qlt of 7
When running sims, I use:
For vented boxes
For sealed boxes (lined):
or some other combo to get Qlt=~10
For sealed boxes (stuffed):
or some other combo to get Qlt =~5
You can measure the box after it is built and iterate through the values until you find a combination of Q's that give a similar looking impedance curve - which is easier than measuring, but probably not very accurate.....
Qa affects the upper peak in vented boxes
Qp affects the lower peak
Ql affects both and the valley in between.
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