Amplitude FDW Optimisation based on RT20/RT30 Analysis

[whitesilo]

At https://my.digitalroomcorrection.hk/aco ... correction, a method to determine suitable amplitude FDW values for Acourate macro1 based RT30/RT20 analysis is referenced, written by nicolaievalentin on an earlier Acourate news group:

Anyways, to accomplish this, one would need to use both the TD-Functions Reverberation time calculations with 1/24th octave resolution and the excellent STransform wrote by Uli again with 1/24th octave. In both, one would need to look up to reverberation times in general but mostly in the 20-200Hz bass range as well as 1000Hz. While TD-Functions will give a general idea it is actually the STransform which will reveal all the information needed to make the judgement. It is well known that in smaller to medium rooms RT60 is not the right tool to judge the reverberation tim e and more like RT30 or even 20 would be closer to reality. Based on this in STransform one could actually change the view to show from the font where the -30db “sits” in the general picture. Knowing that a pulse’s 20Hz frequency has a 50 ms cycle and for instance a 100Hz has a 10ms cycle one can actually define the proper FDW windows for both bass region and mid-highs based on the actual reverberation times. For example in my case picking the 100Hz frequency, STransform shows that I have a decay time of about 80ms for a -30db figure (RT30) which in turn means that my optimum window in time would be 8 for bass(8×10=80ms). 1000Hz has aprox. a 3ms decay so here the window would be 3. To further confirm I generated a set of filters with 15/15 to which I listened to and one with 8/3. The 15/15 filters while presented me with a good sound as Uli has said created a too much dependency on the listening position not to mention the amount of dynamic range cut which was in the range of 6db. However, by far most annoyingly the sound was too reverberant, with too much echo which for sure was not representative of the original recording. Also, all IACCs reported a decrease which is a clear sign of correcting something else than the direct sound. Soon as I switched to the 8/3 filters all was good, the sound was incredible, IACCs increased with the exception of the last one which Uli advised is normal to decrease as we get further away from the direct sound.

I can understand some of what he is saying but not all and can't find the original post or find in Acourate a function name 'STransform'.

Can anyone help to explain this procedure and offer a broad outline of the steps to perform it?

[Ecwl]

STransform is downloadable on the Acourate website under Infocenter, Download.

Very interesting. Thanks for sharing. I’ll have to try this out for my system.

[UliBru]

It is ok to use half the reverberation time of the RT60 value to get the RT30 value.

[whitesilo]

Probably just user error but I am struggling with both methods.

STransform method
I loaded Pulse48L.dbl, set the correct filter length, sample rate and 1/24th smoothing and hit 'Transform' and it produced this:

L-ST30.jpg (1.39 MiB) Viewed 437 times

Is this correctly loaded? If so, do I just look at the -30dB time for 100 and 1000Hz or do I need to look at the times when it is 30dB less than at time=0? And what is T=0, is it 0 or +0.04?

RT60 Method
If I run RT60 in Acourate with 1/24th smoothing, I get RT60 times for 100/1000Hz of left 415/263ms and right 356/275ms. If I halve these and then divide them by the appropriate cycle times of 10/1ms, I get 21/132 and 18/138 which seem rather high:

LR-RT60.jpg (1.06 MiB) Viewed 437 times

Tim

[UliBru]

1. you can edit the chart parameters - select the axes and change the minimum value of the height axis to -30 dB. So you can better measure the time at a given frequency, IMO best in top view.

But obviously the STransform is different to the RT60 or RT30 time. So using the RT30 time derived from the reverberation chart seems to be the wrong time as indeed 275 ms @ 1 kHz means 275 cycles. Which is too much for FDW.
Thus it is IMO wrong to talk about RT30 or RT60.

2. RT30 is still about half of the RT60 value. Indeed in most cases it is difficult to measure the reverberation time down to -60 dB as the environmental noise is already higher. Thus usually RT60 is calculated from RT30 by doubling the according value.

ISo IMO it is not ok to calculate the FDW values from the reverberation. BUT: indeed there has been also some discussion about using the reverberation curve as a guide to design the target curve. I remember that I have seen some good argumentation for this but I do not remember where. There is too much web information around.

[whitesilo]

Thanks Uli for the explanation about the need to sometimes use an RT30 figure to calculate RT60 because the noise floor is too high. That makes sense.

I re-ran STransform in top view, changing chart scales and label resolution to obtain -30dB times for 20Hz and 20kHz of 88ms and 0.140ms for left and 83ms and 0.145ms for right. They are nicely consistent. I chose 20Hz and 20kHz because the Acourate wiki entry for macro1 https://www.audiovero.de/acourate-wiki/ ... om_macro_1 indicates these are the two frequencies to which the FDW values are applied (with presumably a straight line slope between them).

Dividing these by the cycles times for 20Hz and 20kHz of 50ms and 0.05ms I get FDW values of 1.8/2.8. These don't seem right so, unless I am doing something wrong, I'm not sure this is going to be a helpful way to determine amplitude FDW values, at least for my setup. An interesting technique to try out though.

I might do some investigation into the idea of using the reverberation curve to help shape the target curve as you have suggested.