Same of Values" RT20 > RT30 and RT60 " or" RT30 > RT60 " ?

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I did some measurements, around 28 points, and found that in the 1/1 and 1/3 octave bands, some values of RT20 are larger than RT30 and RT60 results. Some results of RT30 are larger than RT60. What is the reason for these values in 1/1 and 1/3 bonde octaves?

What I know understand that typically RT60 > RT30 > RT20.

I want to know if these phenomene results are correct or not.

By the way, in my full range results, i have RT20 < RT30 < RT60.

I'm asking because I'm confused about the measurements in each 1/1 and 1/3 octave bands.

And does there solution for this situation ?
 
T20, T30, T60 are all RT60 values, just derived from different spans of the Schroeder integral.
 
Yeah i understand.
But I'm confused about how RT20 in 150hz for exemple has values bigger than RT60 or RT30 too.
But in full range every thing okay.
What i ask does this normal results.
Or i need to change same parameters before i measure.
like change IR to 4s or 9s 🤔
 
If you understand, why do you think there should be some fixed relationship between those 3 estimates of the same value?
 
That's what I am asking about, and I mean to correct some informations.

What I understand with the Schroeder integral is that this relationship is not correct.

So, the RT60 = 2 × RT20 is a fault.

This is because at a short time, RT20 of reverberation is not the same as the reverberation time at RT60.

For RT60, it does the average of the short time RT20, the medium time RT30, and the long time of reverberation to have the RT60 in the Schroeder integral.

That's what I understand from searching some articles and your guidelines.

Some articles made me confused. They said it is difficult to measure them. So, we will have these values. But are these values normal or not? That's what I haven't found an answer for. If they are normal, do we need to fix the room to get good results and have RT20 smaller than RT30 and RT60 in the octave bands? Or is it okay if all results, regardless of which is bigger, are within the recommended values?

Is my conclusion correct?"
 
No. T20, T30 and Topt are RT60 estimates derived by a least squares line fit to a region of the Schroeder integral. The slope of the line provides the decay value. In the case of T20 and T30 the figure after the T is the span of that region. T20 uses a 20 dB span, the region from -5 to -25 dB. T30 uses a 30 dB span, the region from -5 to -35 dB. REW's Topt uses a variable span. REW's T60M uses a completely different frequency domain method to estimate the decay.

In a large space above the transition frequency the Schroeder integral has a uniform decay, it is very linear when plotted on a log scale, so all the methods produce very similar results. In smaller spaces, and spaces which couple to other spaces, the decay is much less uniform and the answer changes depending on which part of the integral is analysed.
 
So if I find this phenomenon, like you said, in small rooms, for example:

Should I use Topt and clarity and waterfall results as references after I correct my room with absorption material for each point i measure ?

Or do I need to look at RT60,RT30,RT20 too how they change as well ?

What is your advice
 
RT60 isn't meaningful in small rooms below the transition frequency, as individual modal resonances dominate down there. Above that Topt is probably your best bet, as it can use a later start for the analysis region than T20 or T30. In small rooms the initial drop of the Schroeder integral is often much more than 5 dB so T20 and T30 are less accurate. Waterfalls and spectrograms are helpful to see the effect of treatment.
 
Yeah but i think the most reason is external noise make my measurement too hard sametimes.
Thank you john for clarify informations 🙏
 
In the guidelines, there is a recommendation for room sizes. For rooms under 50 square meters, a reverberation time of 0.3 seconds is needed. This means that small rooms have a maximum size limit of 50 square meters. Rooms above that size are reconsidered large. What i wanna to know the maximum of volume of small romm to know what i should to focus in my measurement with the volume limit of the room .
Best regards
 
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