RT60 Model timings

[Flexifools]

Hi

I have just started using REW so am pretty new. I have got most of the basics however when using RT60 decay it seems a bit odd...

So you add the calculate RT60 Model from the controls which creates a straight line on the graph as to where the signal will decay 60DB. however in 2 other places on the chart it has the model T60 which shows a different number.

I did check the help files but they did not suggest an obvious difference.

Can someone shed some light on why they are different and which one should be used for room measurements?

Picture attached with the different values circled

thanks

Andy

 

[John Mulcahy]

The upper graph shows the best fit line. The corresponding decay time comes from the slope of that line, not from where it intercepts the graph axis.

 

[Flexifools]

Hi

thank you for your reply, so when measuring the room i am guessing that the figure is the correct time to quote rather than the straight line. sorry i am just a little confused as, taking this chart as an example, i cannot see where the programme has got the time of 0.233 from. Or is this using estimates?

Thanks

 

[John Mulcahy]

As mentioned above, the RT60 time is calculated from the slope of the line. The RT60 figure is the time it takes for a line of that slope to drop by 60 dB.

 

[steve138]

I too was a bit confused but after seeing this post I am getting somewhere and if you set up your room model, the lower Waterfall chart includes the calculated room modes, which is nice and will be usefull when I start measuring my actual room and not just working on speakers in my basement as I am at the moment.
I would not have thought to look at the line slope value in the inset of the upper chart. But I would be aiming for an in-room RT60 of about 350ms

 

[Flexifools]

Hi

thanks for your response, I understand that. However using the above example above you can see that the DB drop off in total is only about 48DB which happens at about 230ms. It then levels out and stays at -48 DB. Therefore how is REW calculating the RT60 of a curve that never actually goes -60DB? Is it just using the first say 150MS and drawing the line based on the first part and assuming the drop from there. (hence the line) but if so what is the other figure for?

Sorry for any confusion in my questions. The software is great, i just know i will get asked these questions so want the best answer i can give them.

Thanks

 

[John Mulcahy]

You can find out more about the meaning of RT60 and the ways it is classically calculated in the help. The approach taken to obtain a higher resolution view for the RT60 decay graph is explained in the help for that graph.