Gated Impulse Measurements with MLS - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 17 Old 12-13-16, 09:42 PM Thread Starter
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Re: Gated Impulse Measurements with MLS

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The impulse response, however derived, is the response of the whole system, including the room and its reflections. Windowing eliminates the reflections by limiting the region analysed to that containing the direct sound, before reflections arrive - the reflections have to travel further so they arrive later. That's why you move your speaker into the middle of the room - the further it is away from all surfaces the longer til the first reflection arrives and so the more of the direct response you have available to analyse. The wider that window, the lower the frequency response extends. The bit which seems to be tripping you up on the impulse response analysis is confusing the frequency domain measurements (MLS or swept sine) used to derive the impulse response with the time domain result.
Thanks John, the frequency vs time domain explanation rings a bell now. I'm kind of ashamed to admit it, but I should have remember that from my EE classes, but that was over 30 years ago.
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post #12 of 17 Old 12-19-16, 01:03 PM
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Re: Gated Impulse Measurements with MLS

The way I think about it, right or wrong, is that, with gating, REW is "looking at and analyzing the influence of" only the first "x" milliseconds of the signal immediately following any given frequency, ignoring the room's influence in that way. At low frequencies, there is not enough of a cycle of information within the x window time to allow accurate ayalysis. At high frequencies, there might be several cycles of information within the x window time, and analysis can be very accurate.
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post #13 of 17 Old 12-19-16, 02:26 PM Thread Starter
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Re: Gated Impulse Measurements with MLS

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The way I think about it, right or wrong, is that, with gating, REW is "looking at and analyzing the influence of" only the first "x" milliseconds of the signal immediately following any given frequency, ignoring the room's influence in that way. At low frequencies, there is not enough of a cycle of information within the x window time to allow accurate ayalysis. At high frequencies, there might be several cycles of information within the x window time, and analysis can be very accurate.
So in other words, the signal being picked up by the mic is passed thru a narrow swept frequency filter that is 'x' msec behind the swept frequency of the stimulus, correct? Since the value of 'x' msec must be dependent on the distance from speaker to mic, how is this value determined? Is this 'x' msec the same or different than the window used to gate room reflections?
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post #14 of 17 Old 12-19-16, 04:32 PM
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Re: Gated Impulse Measurements with MLS

No, that isn't how the sweeps are processed. Transfer function measurement using sweeps.
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post #15 of 17 Old 12-27-16, 11:02 PM
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Re: Gated Impulse Measurements with MLS

John to the rescue. Thank you, John, as always.

Although I do not know the math, and John obviiously does, my simplistic way of thinking of the analysis was not intended to imply that the actual analysis is simple or anythinig like it. If you really want to understand the analysis in detail, start with a masters degree in engineering. Otherwise, you are just going to have to trust guys like John.
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post #16 of 17 Old 12-28-16, 09:58 PM Thread Starter
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Re: Gated Impulse Measurements with MLS

Thanks for the response guys. I have one more question concerning impulse windowing. Since the lower end of the derived frequency response is limited by the duration of the window, would it be possible to edit the impulse data file and zero out all data points past the window to allow lower frequency measurements?
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post #17 of 17 Old 12-29-16, 04:09 AM
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Re: Gated Impulse Measurements with MLS

No, zero padding doesn't add any information, the low frequency content just isn't there - that goes back to AudiocRaver's comment, if the frequency you are interested in doesn't fit within the window you are out of luck. The typical way to deal with that is to make a very near field measurement for the low frequency information (within an inch or so of the LF driver) and splice that to the windowed measurement with the mid and high frequency content (using the merge function in the All SPL graph's trace arithmetic controls).
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