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-   -   Evil group delay (http://www.diyaudio.com/forums/subwoofers/109293-evil-group-delay.html)

phreeky82 1st October 2007 01:26 AM

Evil group delay
 
I've built a ported 82L box for my 12" I had lying around (JBL GT3-12), and I seem to be getting annoying group delay. It's not massive, but I notice it.

Initially reading around people seem to mention underdamped boxes will give a similar effect, but more of a "sloppy" bass sound. The sound from it actually sounds quite nice, but delayed. I've heard worse.

I've also read people mentioning it shouldn't be audible until at least 20ms or so. WinISD puts GD @ 50Hz somewhere around 6ms and 10ms @ 30Hz, however I'd be curious to know what effect filters and all that rubbish have on it (AVRs filter, the filter on the plate amp that has no bypass switch etc).

Would it be likely those two filters would have significant effect? Would I be right in assuming a typical filter could delay by up to 1 whole wavelength? At 50Hz isn't that around 20ms itself?!?!

None of my mates can hear it, but it drives me mad. One of them has a setup with terrible delay and he hears no problem, so probably not the best person to ask haha. Either that or I'm far more sensitive to it than most I'm gonna try delaying all the other channels from the AVR and see how it goes.

No stuffing in the box btw. Would that have any effect anyway?

noodle_snacks 1st October 2007 02:59 AM

Are you sure what you hearing is group delay, not something that can be attributed to something else?

Svante 1st October 2007 06:00 AM

Quote:

Originally posted by noodle_snacks
Are you sure what you hearing is group delay, not something that can be attributed to something else?
...exactly what I was going to write.

rabbitz 1st October 2007 11:25 AM

Try stuffing the port and listen if it's still there.

phreeky82 1st October 2007 08:29 PM

Not sure, what else would it be? The bass beats sound fairly normal but delayed. It didn't do this with my sealed box.

phreeky82 1st October 2007 10:24 PM

Also I walked around the room a bit listening to some music and I think it might just be the location within the room.

If I'm in a fairly low-volume (low-resonance?) location of the room and turn the sub up a bit more it sounds kind of delayed, but if I'm in a higher-volume location within the room and turn the sub down to match it actually sounds good.

I'll figure it out in a month, we just bought a house so it'll be a whole new setup. I dunno about you guys but I never seem to be happy with what I make - I can see this one going downstairs in the 'workshop' with some basic speakers and building myself a new one for the lounge, hehe :D

noodle_snacks 1st October 2007 10:52 PM

Quote:

Originally posted by rabbitz
Try stuffing the port and listen if it's still there.
The trouble with this is one could still attribute a change in something else to group delay.

Quote:

Originally posted by phreeky82
Also I walked around the room a bit listening to some music and I think it might just be the location within the room.

If I'm in a fairly low-volume (low-resonance?) location of the room and turn the sub up a bit more it sounds kind of delayed, but if I'm in a higher-volume location within the room and turn the sub down to match it actually sounds good.

I'll figure it out in a month, we just bought a house so it'll be a whole new setup. I dunno about you guys but I never seem to be happy with what I make - I can see this one going downstairs in the 'workshop' with some basic speakers and building myself a new one for the lounge, hehe :D

Sounds like your sub is exciting some room modes, and your listening position is in an anti-node. If practical, try experimenting with sub woofer placement (even if only temporarily) and see if the problem is helped. Alternately try making comparisons outdoors, or in some environment where room modes come into play. Next step i'd take would be to find someone with a SPL meter or a mic and measure the room response using roomEQWizard or similar. If your room is particularly bad and optimal placement doesn't help, then the next step could be a BFD or similar parametric eq (it might be more sensible than building another sub and still having problems, assuming the sub itself is fine).

phreeky82 1st October 2007 11:37 PM

Yep I will do, but like I said I'll probably leave it till I move house anyway. I put a small bloody dent on the paint within a few hours of setting it up (agrhrgh), and the side panels are just MDF for the time being (would like to do using real timber for the side panels, but just used MDF for the time being): http://www.clubpoint.net/bluebird/stereo/Img_4325cr.jpg

Underneath: http://www.clubpoint.net/bluebird/stereo/Img_4329cr.jpg (that's just dust etc on the paint and mat)

Note: I just place carpet between panels to remove vibrations as they're bolted on.

Svante 2nd October 2007 06:23 AM

Quote:

Originally posted by phreeky82
Not sure, what else would it be? The bass beats sound fairly normal but delayed. It didn't do this with my sealed box.
Increased energy at low frequencies, combined with room resonaces that takes some time to build up?

rabbitz 2nd October 2007 11:02 AM

Quote:

Originally posted by noodle_snacks
The trouble with this is one could still attribute a change in something else to group delay.

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.


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