Audibility of group delay? - diyAudio
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Old 25th April 2003, 09:35 AM   #1
toenail is offline toenail  United States
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Default Audibility of group delay?

I'm working on a ported sub enclosure that has a group delay peak of 32.4ms @ 20.1 hz. Is this considered excessive? Transfer function phase peaks at 179.2* @ 22.0 hz.

I've been reading some interesting articles that suggest the phase and the dalay graphs are simply two different ways to show the same characteristic of output. How audidble is this?

If I understand phase correctly, being close to 180* out of phase at 22 hz means that waveform of frequencies in that range will arrive at a later time than those at say 30* out of phase. This would mean that my perception of those frequencies would be that they were played at a lower db level, though in reality they're not. I'm wondering, could I could actually hear the difference, and what the threshold is for acceptibility of group delay/phase? Bass would sound sloppy/slow due to different frequencies arriving at different times?

Any thoughts appreciated.
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Old 26th April 2003, 04:42 AM   #2
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at adireaudio.;.they said somthing like 30ms is ok

small article on it.
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Old 26th April 2003, 05:36 AM   #3
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i;ve heard confilicting points of view.

view 1.) keep frequency*groupdelay < 400.
view 2.) hearing is based upon the presence of harmonics (and fundamentals), but not the phase relation for them. someone said to test this you could run a square wave input to an allpass filter and get the same sound out, despite the completely different looking waveform.

i'd try for #1 if possible.
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Old 27th April 2003, 02:29 AM   #4
toenail is offline toenail  United States
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I read up on group delay and phase at the Adire site as well as others. While all of them recommend minimizing group delay, most shy away from giving a specific limit. Consensus seems to be in the 25-30ms range for those that did offer a limit.

For grins I fabbed up a box today using specs close to those listed at the beginning of the thread. Initial impression is that there are no ill effects. I was REALLY happy with the increase in response down low, going from an f3 of 40hz to an f3 of 22.5hz. My goal was to use two drivers in a clamshell isobarik ported box for home theater. It ended up working out really well. I'ts amazing how much signal is missed that low on DVD's.
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Old 8th December 2004, 04:28 AM   #5
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BUMP!

I have the same question. How audible is GD really?

Does the perception of it decrease with frequency - does it become less audible down low? Is it more critical above 40 Hz than below.

It's difficult to get deep extension to 20 Hz without getting GD that is fairly high, however I wonder if this is in fact audible, and that the focus shoud be higher up in the freq range.

Can anyone link some articles on this?
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Old 8th December 2004, 10:36 AM   #6
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Haven't had time to read much of it/ vet it, but:

http://www.linkwitzlab.com/frontiers.htm#F

and "My earlier assumption that the group delay of a 4th order LR4 crossover at low frequencies would introduce audible distortion was not correct. Therefore I recommend not to use the LR2 crossover"
from http://www.linkwitzlab.com/filters.htm

and
"The real work that needs to be done is to refine these threshold measurements and extend them further into the low frequency region (where the worst delay occurs in speakers)" - from

http://www.trueaudio.com/post_010.htm

and from http://www.trueaudio.com/win_rc.htm :

"Group Delay Response

The time delay experienced by the audio signal between the speakerís electrical input terminals and the acoustic output is known as the group delay. What does this mean? Well, if the group delay is large enough, it will be audible. The audible threshold for group delay is about 30 milliseconds, although lower is desirable. WinSpeakerz calculates group delay to let you know if your design is within the desired range See Figure 4 for the Titanic system group delay (blue plot). As you can see, our example has a maximum group delay at 23 Hz of 9.2 milliseconds.

Phase Response

The difference in phase between the input of the speaker and the speakerís output is known as the phase response of the system. The phase response of a system should produce a smooth curve with not much in the way of large humps. This would indicate that the group delay is large, approaching the audible limit. Again for our example, the response is smooth, as would be expected by the systemís low group delay (Figure 4, purple line)."

and discussion thread:
http://www.pesupport.com/cgi-bin/config.pl?read=193185

and
http://www.diyaudio.com/forums/showthread/t-39167.html

and
A study of the audibility of different bass alignments by Ingvar Ohman:
http://www.sonicdesign.se/optimum.html

Rod Elliot:
http://sound.westhost.com/pcmm.htm

from Speaker Builder Magazine:
http://www.libinst.com/tpfd.htm



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Old 8th December 2004, 10:58 AM   #7
Pan is offline Pan  Sweden
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Quote:
Originally posted by theChris
i;ve heard confilicting points of view.

view 1.) keep frequency*groupdelay < 400.
view 2.) hearing is based upon the presence of harmonics (and fundamentals), but not the phase relation for them. someone said to test this you could run a square wave input to an allpass filter and get the same sound out, despite the completely different looking waveform.

i'd try for #1 if possible.
This is not correct. It has been prooven that we do indeed hear the harmonics phase relation to the fundamental.

Also allpassed square waves has been audible to listeners as well.

/Peter
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Old 8th December 2004, 11:18 AM   #8
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Thanks Richard, I had a quick flick through those links and it obviously needs more time to look at it than I have now, I will have to look into it more later.
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Old 8th December 2004, 11:22 AM   #9
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Quote:
Originally posted by Pan


This is not correct. It has been prooven that we do indeed hear the harmonics phase relation to the fundamental.

Also allpassed square waves has been audible to listeners as well.

/Peter
Peter, I assume this is research, where are the results?
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Old 8th December 2004, 11:29 AM   #10
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Initial impression seems that it's most likely only audible around 30ms, however this relates to frequency. For LFE use it will not be as important as bass down below 40 Hz is not connected to a wider band inherent in musical signals, so for this purpose it will be "fast" enough. Above 40 Hz it is VERY easy to get low GD, well below 30 ms. Typically it would be less than 10 ms.
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