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windowing
PostPosted: Wed Sep 08, 2010 6:40 pm Reply with quote
Jack
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Joined: 18 May 2008
Posts: 9




I want to measure automated polar response of loudspeaker
for exporting in speakerlab and would like to know the method
for windowing impulse response without loosing phase data
and without cutting amplitude of the impulse.For example
if I have delay time of 10ms in my audio card and another 10ms
of time running from the loudspeaker to the mike,and the first
reflexion at 10ms(at about 20ms relative to 0 time) after reception of the impulse,what will be the best place for the marker?I must care of the beginning of the window because when rotating,the impulse will change
in time.Is it ok to place one marker at say 18ms and the other at 19ms
to windowed out all data after 19ms as I saw in easera tutorial?
If I place the left cursor to far from the impulse,the window cut some of
the impulse.
thanks.
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PostPosted: Thu Sep 09, 2010 4:14 pm Reply with quote
Charlie
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Joined: 28 Apr 2005
Posts: 51
Location: Charlotte, NC USA




Hi Jack,
I recommend that you use the dual channel FFT setup for this measurement. The second channel will be your reference channel. Do a loop back from the output of your audio card to the input of the audio card for the reference channel. This will eliminate the latency (delay) in the audio card from the measurement.

I also recommend that the front baffle of the loudspeaker be aligned with the Point of Rotation (POR) for your measurement. This should help to minimize the range over which the arrival time of the IR occurs as the loudspeaker is rotated.

Placing the right marker just before the first reflection is a good choice. However, the arrival time of this reflection may occur earlier as the loudspeaker is rotated if it gets closer to the reflecting surface. Check your setup to see just how much earlier before this first reflection you need to place the right marker.

I generally place the left marker about half-way between the on-axis IR arrival and the right marker. This gives a more gradual window than one which spans only 1 ms in your example. The trade-off with this is a slight loss of frequency resolution.

Hope this helps.

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Charlie Hughes
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windowed follower
PostPosted: Thu Sep 09, 2010 7:31 pm Reply with quote
Jack
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Joined: 18 May 2008
Posts: 9




Yes, this helps me and I will proceed as you explain but it remains
a question about when I will save the automated measurement,
If I use dual channel FFT,can I save the windowed impulse as
if I measured without the reference channel?In this way,I just
have to window the impulse in the post process,no other post
process added.Am I right?
thank very much for your quick answer.
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PostPosted: Thu Sep 09, 2010 9:04 pm Reply with quote
Charlie
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Joined: 28 Apr 2005
Posts: 51
Location: Charlotte, NC USA




Hi Jack,
If I understand your question correctly, yes you save the windowed IR the same way regardless of whether you are using single-channel FFT or dual-channel FFT. The windowing process can be applied at the time the measurement is taken, before the IR is saved to your hard drive or the window can be applied with post-processing after the IR has been saved. I prefer to apply the window during the measurements as this fully automates the process and requires less work for me.

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Charlie Hughes
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windowing ok
PostPosted: Thu Sep 09, 2010 9:14 pm Reply with quote
Jack
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Joined: 18 May 2008
Posts: 9




OK it's clear for me now and will apply the window during the measurements too.
Now I will get some measurements And notice you if anything goes wrong with
the procedures.
Thanks very much Charlie.
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windowing
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