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Old 8th December 2004, 11:52 AM   #11
Pan is offline Pan  Sweden
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I have no notes at hand and no links but IŽll post if I find it.

/Peter
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Old 8th December 2004, 02:35 PM   #12
wimms is offline wimms  Estonia
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Quote:
Originally posted by paulspencer
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?
Audibility of group delay is fundamentally limited by 0.5 or 1 period of the frequency you consider. For 20Hz this is 25-50ms. For 1kHz its 1ms. In addition human ear temporal resolution comes into play, especially at higher frequencies, then what matters is signal envelope delays. It doesn't go below 1ms.

Don't have any good links handy, but this is interesting discussion http://www.trueaudio.com/post_010.htm
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Old 2nd March 2012, 02:55 PM   #13
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I'm about to design a subwoofer. Is it okay if the group delay is kept below the time of one cycle? I don't want slow and sloppy bass.
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Old 2nd March 2012, 03:59 PM   #14
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Originally Posted by Rullknufs View Post
I'm about to design a subwoofer. Is it okay if the group delay is kept below the time of one cycle? I don't want slow and sloppy bass.
To avoid "slow and sloppy bass" the subwoofer needs to be time aligned with the woofer playing above it (or with it) at the crossover point.

If the sub/top alignment is off by more than about 1/4 wavelength, frequency response dips and peaks will occur. If the woofer is late by one complete wavelength at the crossover, the summed response will be smooth, but the bass will sound "slow".

Time alignment with a ported top cabinet will require a different delay than a sealed top cabinet.

The output of a folded horn sub that has a 10 foot path length (9.944’) happens to be one wavelength behind at 100 Hz, it will acoustically sum OK crossed at that frequency, but will sound slow unless the tops are delayed by about 8.85 milliseconds, assuming the sub and the top cabinet had flat phase response at 100 Hz.

Some sub designs have erratic phase response within their passband, even though aligned at the crossover, they still may sound poor.

If the phase response simply shows a progressively increasing degree angle towards the low frequency cutoff, the bass won’t sound "slow and sloppy ".

Art Welter
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Old 2nd March 2012, 04:54 PM   #15
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Click the image to open in full size.

The grey line is the ported CSS SDX10 cabinet which I first thought of building. The yellow one is a closed CSS SDX10 cabinet. The orange line is the "top" speakers I first thought of building and the brownish line is a closed version "top" speaker.
Filters added as to keep the drivers from bottoming out. XO between subs and "top" speakers is not determined yet, just put 60Hz as an example.

Would the group delay change if I use miniDSP as XO? (which I intend to do).
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Old 2nd March 2012, 05:21 PM   #16
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Originally Posted by Rullknufs View Post
Would the group delay change if I use miniDSP as XO? (which I intend to do).
All DSP has latency, but the latency is across all bands, so is not a problem.

The top and bottom filters add some delay, how much depends on the particular DSP and the filters applied.

The simulations you posted (too large) are only predictions, always best to measure and adjust delay times as needed.
Delay times can be accurately set simply using a dB meter (easier with an RTA if you have one) and a sine wave tone set at the crossover point.
Set the meter on the floor with the sub and top cabinet at equal distance to the meter, if indoors, the sub and top should be set next to each other as far away from walls as possible and the meter placed about a meter from the front of each cabinet.
With the sub speaker's polarity reversed the output between it and the top speaker will be at minimum when the delay is set properly. The sub then gets put back to proper polarity.
Care must be made that you don't set the delay an extra wavelength off, usually only a few milliseconds are required to align a front loaded ported cabinet, if you end up with 12 or 14 ms, you have delayed the tops too much.

One can physically time align cabinets using the same method, but since the sub may end up some distance closer to the mic (listener) position, best to do it outside to avoid the problem of reading reflected sound levels.

Art
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Old 2nd March 2012, 05:29 PM   #17
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Thanks for the information.
I was planning on building a measuring mic with the Panasonic WM-61A. Will that be enough to measure?
Also, is time adjustment possible with the miniDSP?
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Old 2nd March 2012, 05:34 PM   #18
system7 is offline system7  United Kingdom
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Group delay is a subject I studied as an engineer on telephone and data lines and at college purely for interest. So I'll tell you what I know.

Group delay is the rate of change of phase wrt to frequency in a channel. It describes the delay in the flow of information in a channel. Having built allpass opamp filters (which have flat frequency response but time delay), we could never actually hear ANY difference whatsoever, but it ought to be something you minimise. A filter with a high "Q" has more group delay than a filter with a low "Q". It makes sense that if energy is stored by a high "Q" electrical or mechanical filter, the time release of that energy is smeared and delayed. Ist. order RC filters introduce a constant delay at the bass end, 2nd. order allpass RCL adds a sort of ringing to the waveform.

High "Q" and steep slopes should be avoided. This equates to big closed boxes, even Acoustic Suspension, for fast bass response, and low "Q" Linkwitz-Riley filters for your crossovers. I won't opinionate about this too much, but here's the graphs. Group delay is really unavoidable in real time, but spatial filtering or digital techniques can avoid it altogether. In real time it is worth noting that smooth transitions at speaker and cabinet edges also minimise ringing.
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File Type: jpg Highpass_lowpass.JPG (65.2 KB, 196 views)
File Type: jpg GroupDelay.JPG (121.7 KB, 204 views)
File Type: jpg GroupDelay2.JPG (93.3 KB, 197 views)
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Well, there it is! Best regards from Steve in Portsmouth, UK.
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Old 2nd March 2012, 06:03 PM   #19
system7 is offline system7  United Kingdom
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I should add a surprising finding from antenna theory. You actually don't want a loudspeaker that behaves as a rigid piston. The fact is a square or sharp circular aperture radiates very badly into the far field. Far better is the Gaussian aperture with a smooth rolloff at the edges. This is because the Gaussian aperture is its own inverse in the far field, so is mathematically perfect. It's the same thing of sharp transitions ring, smooth ones don't.
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File Type: png Gaussian response.png (11.2 KB, 226 views)
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Well, there it is! Best regards from Steve in Portsmouth, UK.
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Old 2nd March 2012, 06:11 PM   #20
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Quote:
Originally Posted by Rullknufs View Post
I was planning on building a measuring mic with the Panasonic WM-61A. Will that be enough to measure?
Yes

Quote:
Originally Posted by Rullknufs View Post
Also, is time adjustment possible with the miniDSP?
Yes, up to 7.5ms iirc.
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