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Old 2nd October 2007, 11:31 AM   #11
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Quote:
Originally posted by rabbitz


The idea was if it's still there when the port was sealed, then it wouldn't be group delay as in a sealed box GD would be very small.
Yep, the GD would change, but so would a number of other things, such as port noise or low frequency extension, which might mean a particular room mode isn't excited as much for example. IMHO there are too many other changes to experimentally determine if group delay is the problem, of if it is something else.

I have seen instances where people plug ported boxes, then proclaim how tight the bass is, only to find out that their boxes are rolling off at 80hz or higher and the tightness is just an outright lack of bass. My point is that in many cases the associations made between what is heard and what is happening can be wrong, so its not a good idea to make assumptions.

edit: That said, stuffing the ports might help narrow down the problem, just not solve it.
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Old 2nd October 2007, 11:46 AM   #12
rabbitz is offline rabbitz  Australia
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Yep... it was only an idea to help trace a problem and not a cure.
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Old 4th October 2007, 03:19 AM   #13
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FYI the sub sounds nicer in other but really inconvenient locations. So leaving it where it was, I played with the delays in the receiver (delaying the other speakers a bit) and it sounds pretty bloody good at the moment. Unfortunately I'm not exactly sure how much it is delaying them by because the settings are changed by specifying the number of metres they are from the listening position.

edit: btw I'm going to live with this for the time being - almost everything will be getting packed up for moving in a couple of weeks anyway, and then I'll have a whole new room to setup (including a massive workshop area )
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Old 4th October 2007, 09:46 AM   #14
bjorno is offline bjorno  Sweden
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Quote:
No stuffing in the box btw. Would that have any effect anyway?Ö
Box volume stuffing is in my opinion often not needed for well-braced ported subsystem that have a bandwidth up to 80 Hz.

Fs/Qes for JBL GT3-12 car driver is about 46-47 and the driver Qts is > 0.5 giving a strong indicator itís a driver designed for closed enclosures but not for unstuffed ported designs if using small volumes.

The driver can only be tamed to sound good in a ported box if sufficient volume is used to bring the driver sensed box Qt down to allow the design to fall within recommended design alignments, or is driven by a negative impedance (positive feedback-) amplifier or by using any of the acoustic damping methods described by R. Small.

Howe to use `Negative impedance`: http://sound.westhost.com/project56.htm

A couple of pictures to consider:

b

1(3)
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Old 4th October 2007, 09:48 AM   #15
bjorno is offline bjorno  Sweden
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2(3)
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Old 4th October 2007, 09:49 AM   #16
bjorno is offline bjorno  Sweden
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3(3)
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Old 4th October 2007, 12:26 PM   #17
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Well TBH some of those graphs are a little beyond me, at least without some indepth reading (which I don't really have time for at present).

I understood the driver doesn't have a great response curve @ 82L (I was actually originally going to buy a cheap JBL GT4-12, which gave a much nicer response), but thought I'd give it a crack anyway - I didn't know it would "sound poor" for the response it did have, if that's what you're trying to illistrate to me.
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