An Attempt To Understand Spectral Content From An ETC Response. An Article from Nyan Mellor.

[Kvalsvoll]

Thanks. Why not use a shorter rise time and shorter right side window so the data in each slice doesn't overlap?

Shorter rise-time works, but the graph will look different because it includes less from what happened earlier in time.

Shorter window does not work so well, because you loose low frequency resolution.

 

[Kvalsvoll]

Having access to the MDAT

This is the most important, so you can use different tools to visualize and adjust scaling.

 

[Tom Danley]

The ETC (Energy Time Curve as it's originator Dick Heyser coined it) or Envelope Time Curve as popularly used now shows the total energy reaching the microphone relative to the the signal's origination.
This is related to Impulse Response in that the impulse response is the Resistivity portion of a complex signal while the Doublet Response is the reactive component.

None of these are very informative for broad band signals as the Time domain makes lower and lower frequencies harder and harder to see even if at the same levels. In other words an impulse at 20Hz takes 1000 times longer on a display than a 20KHz impulse and having the same "volume under the curve", is also much lower in level.


The ETC shows significant delayed energy when present and when one considers the speed of sound, one finds that each .883 ms represents one foot of travel and so the distance to the source and a delayed reflection signal at 8.83ms later represents a 10 foot path total length for that delayed signal. One early variation on the ETC was a 3D or polar ETC which could identify the vertical and horizontal angles of reflections in rooms.

In commercial / installed sound this was a handy tool as generally the bigger the room, the more difficult acoustic problems are.

The Time Delay Spectometry system Heyser conceived also used the ETC to locate exactly how far away the acoustic signal was from the microphone (subtracting all the fixed delay from the phase) and then the Time Delay Spectometry can measure the amplitude and accurate acoustic phase response.

This is an old one that might be interesting, re-printed in 2013 but deals with time, magnitude and phase and written by one of the folks who changed the industry about one of the others who changed the industry and my path in audio.

https://www.prosoundweb.com/a-matter-of-frequency-the-nyquist-plot-explained/

Tom

 

[Adhoc]

The good old discussion about why "RT60 is meaningless in acoustically small rooms" Let's not go there.

I agree with that, a diffuse field is impossible in any normal sized room, which is the reason I didn't write RT60. Decay time varies with frequency and low frequncies cannot be diffuse in any normal sized room. I don't agree with your remark in the posted diagram though, about "noise in the measurement" is the reason for the Schröder Integral curve starting to rise above the straight line. Move the microphone from a place where (low) modes are less prominent to one where they are more prominent, and the Schröder Integral curve will start to rise sooner in time above the straight line.

 

[ernestcarl]

I agree with that, a diffuse field is impossible in any normal sized room, which is the reason I didn't write RT60. Decay time varies with frequency and low frequncies cannot be diffuse in any normal sized room. I don't agree with your remark in the posted diagram though, about "noise in the measurement" is the reason for the Schröder Integral curve starting to rise above the straight line. Move the microphone from a place where (low) modes are less prominent to one where they are more prominent, and the Schröder Integral curve will start to rise sooner in time above the straight line.

Ah... I noticed that mine starts to rise consistently at right around 60ms -- corresponds only when I have the subwoofer turned on. Turn it off and it goes back to flat.

 

[markus]

I agree with that, a diffuse field is impossible in any normal sized room, which is the reason I didn't write RT60. Decay time varies with frequency and low frequncies cannot be diffuse in any normal sized room. I don't agree with your remark in the posted diagram though, about "noise in the measurement" is the reason for the Schröder Integral curve starting to rise above the straight line. Move the microphone from a place where (low) modes are less prominent to one where they are more prominent, and the Schröder Integral curve will start to rise sooner in time above the straight line.

OK let's go there From the perspective of RT60 as it is strictly defined (homogeneous and isotropic), low frequency modal behavior within an acoustically small room can (must?) be considered "noise". Amir wrote a more detailed text at https://www.audiosciencereview.com/...large-small-rooms-a-matter-of-statistics.569/