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Author Topic: Standard for smoothing acoustical responses  (Read 3296 times)

Goran Milosevic

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Standard for smoothing acoustical responses
« on: December 19, 2010, 11:41:29 am »

Greetings everyone!
I'm interested in whether there is a standard for smoothing acoustical responses, because I have noticed that 1/3 octave smoothing differs from program to program. Attached is a chart from RoomEQ and ARTA. 1/3 octave smoothing and 5dB/div are in both cases. The microphone was not moved, and, in both cases, measurement was obtained by employing the slow sweep method.


Kind regards
Goran

index.php/fa/16020/0/
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JohnM

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Re: Standard for smoothing acoustical responses
« Reply #1 on: December 19, 2010, 12:06:14 pm »

The appearance of the plot will be affected by the window types and durations used to generate it as much as the smoothing applied, so you need to make sure the windowing is the same before looking at smoothing comparisons.

After getting the windowing the same the difficulty is that "1/3 octave" really only specifies the frequency span encompassed by the smoothing applied but does not define the smoothing kernel. A simple moving average that has a 1/3 octave span will appear much less smoothed than a Gaussian kernel used over the same span (in fact a Gaussian is well approximated by multiple passes of a moving average). REW's smoothing is approximately Gaussian.

ARTA uses a 6th order Butterworth filter on data that has been converted to logarithmic spacing, from the manual:

Quote:

Note: Preceding examples show 1/3-octave smoothed curves. The smoothing of frequency response curve is done in different way than smoothing of spectrum magnitude. The ARTA uses approach to first interpolate and average frequency response on log-frequency axis, then makes smoothing by convolving that response with response of bandpass 6-pole Butterworth filter.


With the same windowing you should find the two approaches look pretty similar.
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Ethan Winer

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Re: Standard for smoothing acoustical responses
« Reply #2 on: December 20, 2010, 03:10:43 pm »

Goran Milosevic wrote on Sun, 19 December 2010 11:41

I'm interested in whether there is a standard for smoothing acoustical responses, because I have noticed that 1/3 octave smoothing differs from program to program.


This doesn't directly answer your question about differences between programs, but I can tell you how I do it using John's fabulous Room EQ Wizard program. Below 300 Hz I like to see very high resolution, to assess the true extent of bass peaks and nulls. Above 300 Hz 1/6 octave smoothing makes more sense because it lets you see the overall response better, without being distracted by the typical closely spaced peaks and nulls.

--Ethan

DanDan

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Re: Standard for smoothing acoustical responses
« Reply #3 on: January 03, 2011, 05:04:09 pm »

I believe the third octave filter is pretty well defined internationally.
Is there a way to directly relate smoothing to ISO filter definitions?

Similar approach here. Hi Res for modal work below 300. 1/3 to see the tonal trend.
Many noobs are misdirected by what appear to be 'rules' e.g. 'Don't use smoothing'
IMHO smoothing and zooming and range control are all similar, they can all make it easier to see particular aspects.

DD
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