# which graph shows ITDG?



## Timeking (Nov 14, 2013)

A clear and visible Initial time-delay gap (ITDG) as referenced in Chap 20 of Master Handbook of Acoustics is, at least for a control room, of paramount importance (according to the author). So which graph am I looking at which shows this? If that graphic is the one below, I think I've got some serious problems. Comments?


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## Timeking (Nov 14, 2013)

Follows is an excerpt regarding what I am talking about (i.e., ITDG)


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## jtalden (Mar 12, 2009)

If you zoom your time scale to more like -5 to 30 ms you will be able to better see the timing of the early reflections (the dB scale could use a little expansion also). Did they not give a suggested time for the early reflections? The references I have seen are for 10 ms per Earl Geddes. I am not sure how much agreement there is on this value apparently there are lesser values used. I have seen it stated that early vertical reflections are more negative that lateral reflections. Late arriving reflections are good and apparently help the sensation of space and sound stage.

It is not too convenient to remove floor and ceiling reflections in the home. The reflection points on the side walls near the speakers are often treated or the speakers moved away from them. The "Home Audio Acoustics" forum here should have more authoritative advice.

It's hard to see detail on you chart, but it does suggest you don't have the recommended result. I suspect that none of us that haven't created a dedicated larger size room can fully comply. We do what we can given the constraints we apply.


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## Timeking (Nov 14, 2013)

I totally get doing what you can do, because I am having to deal with a really oddball shaped room. That said, I do think that the early reflection thing is something I should TRY to address. I zoomed in like you suggested, and like the horse on Ren and Stimpy, I have to say "No sir, I don't like it." It is also noteworthy that 10ms equals any surface closer than 5 1/2 feet would have to be addressed. In a room with an 8 ft ceiling, that would include the floor and ceiling, the speakers being somewhat near halfway between the two in my setup. Because I am becoming obsessed with all this, I am thinking I will build a 4ft by 8ft frame out of 1x4s and load that with 3 sheets of Roxul rock wool, try to hang that from the ceiling rafters somehow without the roof caving in or further reducing the resale value of my already depressingly depressed home worth. Yay!!


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## jtalden (Mar 12, 2009)

I just noticed that the trace is labeled as a "Sub". 

I believe this type of analysis is intended for MF and HF as those are freqs that create the imaging problems. You need a full range trace for that. For the SW it is better to look at SPL response and then waterfall to determine if the decay is acceptable.


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## Timeking (Nov 14, 2013)

the title "sub on 70db" is just my note to myself that when I took the measurement that I had the sub on as well as the full range speakers, and had cranked the volume up to 70db with my SPL meter to take the test.


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## Timeking (Nov 14, 2013)

hear is the waterfall and spl


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## jtalden (Mar 12, 2009)

You should be testing one main with the SW at a time for ITDG purposes. 
Actually, that is the best policy for almost all purposes. 

The SPL looks normal, but the waterfall shows a problem but I am not sure is it is just that more than one main is being measured or something else is going on.

Run FL+SW and FR+SW and post the .mdat of the 2 measurements and we can then better identify if there are any problems.


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## AudiocRaver (Jun 6, 2012)

Figure 20-2, which you have posted, is an idealized view of what might be accomplished in an idealized room with ridiculously extensive treatment. In other words, reality probably rarely approaches what that diagram suggests as the ideal. The home theater room in which we perform the HTS speaker evaluation events, while certainly nothing like a control room, is still a fairly well treated room, and only hints at what that diagram looks like. There are almost always, even in a well-designed control room, early reflections coming from around the speaker itself and from the console and surfaces around the listening position. Those reflections, all inside the initial time delay gap, would have magnitudes approaching those of the energy after the gap. Bottom line, the idealized diagram would probably only actually occur in a very large very perfect control room with nothing in it.

As jtalden suggests, zoom in on the first 50 mS of the diagram, then you will be looking for that initial time delay gap to be in the first 10 or 15 mS or so. You will probably only see a slight depression there, not the clean gap suggested in the idealized diagram. I think you are chasing an unrealistic goal.

While a nice empty initial gap would be a worthy accomplishment, you will probably do well to eliminate the main reflections that jump out at you in that gap. There will be some hash, a bunch at the beginning from the surface of the speaker, and some bigger peaks. Focus on the big peaks. Eliminating them will give you the most benefit. Eliminating the hash would be nice, and you may accomplish some of that with absorptive treatment, but you will probably reach the point of diminishing returns very quickly.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Timeking said:


> That said, I do think that the early reflection thing is something I should TRY to address.


 Here’s an excellent article that suggests early reflections aren’t nearly as bad as some have made them out to be.

Regards, 
Wayne


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## Timeking (Nov 14, 2013)

Since you offered, here is my mdat file where I measured R and L separately.

http://www.fflynet.net/greg_graves/index_files/controlroom.mdat

Very informative article! I need to print out and read that through a few times to absorb. Thanks.


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## jtalden (Mar 12, 2009)

I reviewed the measurements. The measuring system is working normally. The waterfall chart now looks normal. The only measuring system issue I see is that a mic calibration file is not loaded. If you intend to EQ the high frequencies a cal file is critical even when using a very good measuring mic. For SW range measurements it is not as critical, but still widely recommended.

*Just some offhand comments on the measurements:*
> Appears to be near field measurements?
> Distortion is unusually high below 120 Hz.
> The R main TW appears to be at significantly lower level than the L main TW.
> Your measurement level is a little lower than recommended at about 65 dB. If your mic is properly calibrated for level, it would be better to measure at 75 dB.
> 4 sweeps are not needed for most work. Single sweeps are recommended unless you need to push the noise floor a little lower for distortion measurements.


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## Timeking (Nov 14, 2013)

> Appears to be near field measurements?

I put the mic where my head would normally be. The mic is a Numark STD272 and don't have a cal file for it. Doubt one exists.

> Distortion is unusually high below 120 Hz.

Is the distortion you refer to those peaks in negative time on the Impulse graph? Despite setting the sub to get a flat response as best I could, I think that it is too loud. Your comment about the distortion supports that idea, it being presumably overdriven. I am in process of trying to set the sub level by ear by listening to various CDs, then when I am happy with that repeat the measurements.

> The R main TW appears to be at significantly lower level than the L main TW.

I can't find out what "TW" refers to. The SPL of right and left looks equal to me.

> Your measurement level is a little lower than recommended at about 65 dB. If your mic is properly calibrated for level, it would be better to measure at 75 dB.

OK. Need to find my ear plugs. I've calibrated the SPL meter with a Realistic (Radioshack) meter.

> 4 sweeps are not needed for most work. Single sweeps are recommended unless you need to push the noise floor a little lower for distortion measurements. 

OK.


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## jtalden (Mar 12, 2009)

You need to change to REW 5.01 Beta 17 to see the distortion panel.

Distortion:
Yes, the pre peak IR spikes are apparently and indicator, but we can see the direct readings in the REW 5.01 "Distortion" panel. The cause may be several things. You may want to investigate this a little.

TW Level:
The TWeeter of R main is lower on level than the L main as shown below:


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