AFMG Network Forum

JS · Member

Hi there,

I'm going through the tutorial and just tried to reproduce the results that are shown on page 63 and 64 for the example file.

I found that with the standard options for left and right window (Tukey 50%, 1ms length before peak) and a window length of about 30ms as stated in the tutorial it is impossible for me to achieve a phase plot as shown on page 64.

The closest I can get is placing the "From" marker at 79,88 ms ("To" marker is at 109 ms). The result is similar except for the spike at 475 Hz.
(A screenshot can be found at: http://s4b.directupload.net/file/d/1617/tf4gwlg2_png.htm)

To get a graph like in the tutorial I have to change the right window to "Hann" type (fyi: the TFC didn't show reasonable results in this expample) and I achieve the best compromise between accuracy in both phase and magnitude if the left window is a Blackman tpye with 2 ms length before peak ("From" and "To" marker set as above).

Questions:
Which window type (or combinatin) do you suggest for sound system measurements? The Blackman as stated on p.95?

It seems to me that the phase display responds very sensitive to the placement of the "From" marker. In the example above the marker has to be placed exactly on the positive peak of the impulse in order to get a good phase plot. Is this always the case? Can you provide guidelines for placing the window?

Thanks a lot in advance,
Jochen

Charlie · Member

Hi Jochen,
I don't have access to the tutoiral right now but the screen shot you posted looks like the "peak" in the phase response at 475 Hz is actually a wrapping of the phase from -180 to +180 degrees. This phase response also increases above about 1 kHz. This indicates the left marker (From) is probably set a bit too late in time.

To help understand how to better place the markers you might enjoy an article I wrote on this. It can be found at http://www.excelsior-audio.com/Publications.html. Scroll down the page and click on "Phase Response & Propagation Delay". Hope this helps.

JS · Member

Hi Charlie,

tried moving the left marker back and forth but won't get rid of the "peak" or wrapping at 475 Hz with the default Tukey window.

Anyway, thanks a lot for the link. Seems like there's a lot of useful information on your page and I'm looking forward to read the article.

Cheers,
Jochen