RT60 Graph Question - Home Theater Forum and Systems - HomeTheaterShack.com

Old 12-25-11, 10:42 PM Thread Starter
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Nick

Join Date: Oct 2009
Location: SF Bay Area
Posts: 63
RT60 Graph Question

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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Old 12-26-11, 10:02 AM
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John

Join Date: Apr 2006
Location: UK
Posts: 6,307
Re: RT60 Graph Question

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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Old 12-26-11, 12:00 PM
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SAC

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Re: RT60 Graph Question

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.

Last edited by SAC; 12-26-11 at 12:08 PM.
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Old 12-27-11, 10:55 PM Thread Starter
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Nick

Join Date: Oct 2009
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Re: RT60 Graph Question

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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Old 12-29-11, 10:23 AM
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Re: RT60 Graph Question

Quote:
JohnM wrote: View Post
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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Old 12-29-11, 12:34 PM
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Floyd

Join Date: Jan 2010
Posts: 68
Re: RT60 Graph Question

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

Attached Files octave debug.mdat (1.73 MB)
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Old 12-29-11, 06:07 PM
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Re: RT60 Graph Question

Quote:
fotto wrote: View Post
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.

Attachment 34237
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Old 12-29-11, 06:36 PM
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Floyd

Join Date: Jan 2010
Posts: 68
Re: RT60 Graph Question

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

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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Old 12-29-11, 07:22 PM
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Re: RT60 Graph Question

I'll check out what went wrong.
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Old 12-29-11, 07:35 PM
Elite Shackster

Join Date: Mar 2009
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Re: RT60 Graph Question

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

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:

rt60 debug.mdat
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