# RT60 Graph Question



## Nick (Oct 28, 2009)

Hello and Happy Holidays

I am hoping someone might be able to tell me why my RT60 graph begins at 90hz? I am measuring full range from 20hz and the graph shows no bar for RT60 from 20hz to 90 hz? Could this be a hardware issue, perhaps?

Thanks


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## JohnM (Apr 11, 2006)

RT60 data is calculated for either one octave or 1/3 octave bands, depending on the setting of the controls. The lowest centre frequency is 62.5Hz, which for the 1 octave setting produces a band that goes from approximately 44Hz to 90Hz. If "Use bars on RT60 plot" is selected this will show a bar spanning 44 to 90Hz. If that box is not ticked there will be a line that starts at 63Hz (the band centre) and goes to 125Hz (the next band centre for 1 octave data).


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## SAC (Dec 3, 2009)

What do you think the RT60 calculation, that is based upon behavioral criterion not in evidence in a Small Acoustical Space, will provide that is of use in a Small Acoustical Space?

The RT60 is a Large Acoustical Space calculation, that assumes the presence of a statistically uniform homogeneous reverberant sound field - a phenomena that does not exist in a Small Acoustical Space (at least not at frequencies about which we are concerned).

A Small Acoustical Space is characterized instead by locally variable modal behavior in the LF region and by locally variable specular behavior in the region above the modal behavior (the Schroeder critical frequency typically occurring in the realm of around 250 Hz.) Statistically homogeneous fields do not apply.

Instead, you would more properly use the waterfall/spectrogram to examine the regional modal behavior; and the ETC response to examine the local specular behavior.


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## Nick (Oct 28, 2009)

First, thank you John, for the explanation. It was very helpful. 

SAC - Floyd Toole states that the R60 in a small acoustical space (versus an auditorium) should be less than .5s. The link below also discusses the use of the R60 measurement. The co-author of the article at Real Traps is Nyal Mellor of Acoustic Frontiers. The premise is that the measurement along each octave should not vary too much from the rest. When I got my room below .5 it really widened the soundstage.

http://www.realtraps.com/art_measuring.htm


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## jtalden (Mar 12, 2009)

JohnM said:


> RT60 data is calculated for either one octave or 1/3 octave bands, depending on the setting of the controls. The lowest centre frequency is 62.5Hz, which for the 1 octave setting produces a band that goes from approximately 44Hz to 90Hz. If "Use bars on RT60 plot" is selected this will show a bar spanning 44 to 90Hz. If that box is not ticked there will be a line that starts at 63Hz (the band centre) and goes to 125Hz (the next band centre for 1 octave data).


JohnM,
Like Nick, I had also noticed that when using 1/3 octave bars the 63 and 80 bands are plotted, but they are always plotted with zero value. The 1/1 octave bar for the 63 Hz range is shown normally. This is true both on the main RT60 tab as well as on the tab in the overlay window. See the example below. 

63 and 80 Hz plotted at zero value:








63 Hz 1/1 octave plots normally:








I also noticed that on the “Filtered IR” tab the lowest two 1/3 octave filters listed (63 and 80 Hz) never display. The other 1/3 octave bands and all the full octave bands including the lowest at 63 Hz all display normally. Possibly this is related.


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## fotto (Jan 17, 2010)

Mine seem to be displaying properly. I've attached the measurement...maybe you can use it to help debug.


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## jtalden (Mar 12, 2009)

fotto said:


> Mine seem to be displaying properly. I've attached the measurement...maybe you can use it to help debug.


Thanks for posting this.

I loaded your file on 2 different PCs and it displays just as you showed on both machines. Also the Filtered IR graph at 1/3 octave at 63 and 80 Hz both show normally as well.

None of my measurements (hundreds of them) have ever shown properly like your file does. So there must be something in my measurement settings that is different. I played around, but didn't find the problem. For interest, I attached one of my measurements. I assume it will not display properly on your setup either? 

This is not really a concern for me so long as it doesn't indicate other problems and I don't think it does. 

View attachment 34237


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## fotto (Jan 17, 2010)

I'm willing to give your measurement a try on my system, but get an error trying to access it:

"Invalid Attachment specified. If you followed a valid link, please notify the administrator"

Not sure what the issue is. Did you actually upload it as an attachment or does the hyperlink take you elsewhere?


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## jtalden (Mar 12, 2009)

I'll check out what went wrong.


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## jtalden (Mar 12, 2009)

fotto said:


> I'm willing to give your measurement a try on my system, but get an error trying to access it:
> 
> "Invalid Attachment specified. If you followed a valid link, please notify the administrator"
> 
> Not sure what the issue is. Did you actually upload it as an attachment or does the hyperlink take you elsewhere?


I think this one is working:

View attachment rt60 debug.mdat


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## fotto (Jan 17, 2010)

Ok that's weird, one octave OK but 1/3 octave not working...exactly like you posted. Must be something with the measurement??? It's way beyond my knowledge base.


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## jtalden (Mar 12, 2009)

No problem.
Thanks for confirming it is something in my file.


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## jtalden (Mar 12, 2009)

I have determined that the issue only occurs if I sample at 96 kHz it is fine at 41k or 48k.


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## JohnM (Apr 11, 2006)

jtalden said:


> I have determined that the issue only occurs if I sample at 96 kHz it is fine at 41k or 48k.


Sorry about that, it was a numerical precision problem in the fractional octave filters at high sample rates and low centre frequencies. I've fixed it for the next build.


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## jtalden (Mar 12, 2009)

Thanks John!


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