position and clearance of port in a MLQWT design - diyAudio
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Old 10th February 2005, 12:18 PM   #1
Puggie is offline Puggie  United Kingdom
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Default position and clearance of port in a MLQWT design

I've have a set of MLTQW cabinets modeled from the MJK mathcad sheets using the Audax 4 1/2" units from Maplin. My question relates to port positioning and clearance. I'm planning on using about a 2" diameter port (about 5" long).

Does the port have to be at the end of the line or can I position it part way along the line? how will theis affect things?

How much clearance do I need around either end of the port? I would have liked to put the cabinet on long (18-20mm) spikes and fire the port downwards out of the bottom of the cabinet, would the close proximity of the floor affect things (reflections or air resistance) a single 4 1/2" driver is not going to move air very fast down a 2" port.

Thanks people.
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Old 11th February 2005, 11:32 AM   #2
Puggie is offline Puggie  United Kingdom
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I have a completed cabinet and holesaw ready???
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Old 11th February 2005, 12:27 PM   #3
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I have a pair of MLTQW units using 4" drivers in a bipolar config. The drivers are centered in the cabinets and the port is 3" from the bottom of the cabinet which sits upon the woofer enclosure. I prefer the port facing backwards. Can't give you the science behind this but it sounds good. If you need more info you might try contacting MJK directly he has always responded quickly to my inquiries. Hope this helps.

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Old 11th February 2005, 12:34 PM   #4
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Hi puggie.

I must admit, I am not that familar with the MLTQW design. I suspect though that the port is loading/terminating the TL, and so at the end is probably best. You might get away with a downfiring port, but depending on the height from the floor you might need to fiddle with the length a little.
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Old 11th February 2005, 01:56 PM   #5
GM is offline GM  United States
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Assuming you used the ML-TQWT WS and not the PORTED, then the assumption is a bottom firing vent WRT 1/4WL action, but for ease of programming it assumes that both the driver and vent are radiating from the same point so the sim loses accuracy with increasing frequency, and why the PORTED is more accurate overall.

That said, I prefer boundary loading the vent to maximze gain, so either a bottom firing or near/at the bottom if front or rear positioned, but then I tend to mount the driver in its optimum position along the line. For designs where the driver is near/at the top, moving the vent up somewhat will smooth it out somewhat at the expense of max vent loading.

Anyway, the way I calc bottom firing spacing for a vent, a worst case scenario is where the bottom of the cab has an area based on the vent's diameter, so for a 2" dia. vent, a minimum spacing of a whopping 0.35" is required, with larger bottom areas requiring progressively less before slot loading begins in earnest.

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Old 11th February 2005, 05:47 PM   #6
Puggie is offline Puggie  United Kingdom
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Thanks people, I'll either fire the port out of the base or out of the back of the cabinet as low as I can.

I'll have a play with the ported Mathcad work sheet this evening too.
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