# Measuring RT60



## aimeenorwich (Feb 12, 2010)

Hi,

My name is Aimee. I am using REQ v. 4, Build 673. I would like to import my impulse responses and measure RT60 (reverb time). I have a few questions. It says online that I can use RoomEQ to read RT60, and REQ has a time vs DB graph in the Impulse section, so, I'm assuming it's giving me RT60. Am I right???

I want to make sure I'm setting the graph parameters correctly and I'm reading it correctly. I've included an example of one of my IRs as an attachment.

1. Should the Y axis be set at DB FS or %FS? And, what does DB FS and %FS actually stand for? When I import my IR in DB FS mode, it never actually goes down to -60. But when I put it in %FS mode, it starts at 90, and eventually goes to zero. So, I was just starting at 90, and reading it 60 below that, to 30. But I don't think %FS is Db anyway... If it is, and I'm reading it correctly, that would put RT60 on this graph at 262ms. I don't think I'm doing this right, because I know with my ears that the RT60 is at least around 1 second.

2. Also, is the dashed line at 500ms on the x axis a marker for easier reading? 

I think I must be setting my graph parameters wrong. Can someone please help me clear this up?

Thanks for any help!

Aimee


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## fbov (Aug 28, 2008)

Using REW 5, so things may be different...

I have an "Impulse" button, and next to it, "Filtered IR".

In Filtered IR, the Controls window gives me a button for Show Data Table

The data table has T20, T30, Topt amoung others, and a filter to let you see discrete frequency ranges. 

There is an "RT60" button at the top, but that shows T20/T30/Topt as a function of frequency, which is a very good way to look at it. 

I also like the spectrogram with the scale set to a 60dB range so any low-level resonances stick out. 

Have fun,
Frank


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## aimeenorwich (Feb 12, 2010)

Thank you Frank. I'm going to download REW 5 and see what I can do.


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## JohnM (Apr 11, 2006)

aimeenorwich said:


> It says online that I can use RoomEQ to read RT60, and REQ has a time vs DB graph in the Impulse section, so, I'm assuming it's giving me RT60. Am I right???


The impulse response can be shown with either a linear (percentage of full scale, % FS) or logarithmic (dB) vertical axis. When viewed with a dB axis you could make an estimate of the slope of the decay of the impulse response and use that to estimate the RT60 time, but REW can do that job for you using the Schroeder backward integration method. The RT60 figure is estimated by looking at the slope of the Schroeder integral over various ranges, for example from -5dB to -25dB (called T20) or -5dB to -35dB (called T30), and extrapolating that slope to estimate the time it would take to decay by 60dB if the rate of decay remained the same.



> Also, is the dashed line at 500ms on the x axis a marker for easier reading?


It shows one edge of the window used to select the part of the impulse response that is used to produce the frequency response.


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