Author Topic: Time Windowing with SATLive  (Read 2234 times)

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Jeffsco

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Time Windowing with SATLive
« on: September 17, 2011, 17:33 »
I was reading a blog on another Forum where the author was explaining how to use dual FFT measuring systems to window out the rooms response so as to be able to take a more accurate measurement of Speaker's frequency response. here is the original text:

quote "The most obvious benefit of the transfer function is this ability to show the changes a system makes to a signal, but the advantages don’t stop there. Because the transfer function knows what’s going into the system and what’s coming out, it can measure the difference in time between the two measurements and align them… this time difference can be due to anything from signal processing delay to the distance between the speaker and the reference microphone. Once this delay is compensated for and the two measurements aligned, advanced transfer function measurement tools then apply what are called “time windows” in order to separate the system’s response from the room’s interactions with it. To go back to my original example of the room that is excessively reverberant at 500hz, a properly set up transfer function would see the 500hz segment of the signal going to the mains and know based on this time alignment when to expect it at the microphone. Once that time has passed, it ignores unrelated 500hz content that may arrive at the microphone, “windowing out” a large number of room reflections and reverberance. The measurement engineer can then properly equalize the system without having their measurement affected by the room or other noise sources unrelated to the PA. " unquote.

Question 1: How do I achieve this with SATLive? What do functions / buttons do i need  set this up?

Question 2: I assume that the reverberant frequency is time dependant? What I mean is...a reverberant frequency at 500hz will arrive at the Mic slower than a 2.5k frequency? Or do they all arrive the same as per the speed of sound?
Question 3: Is the time window based on the distance path of reflected sound? In otherwords....do we calculate the window based on the time the sound will take to travel past the mic, bounce off a reflective back wall....and return to the measuremetn mike? Do we window out for the nearest reflective surface?



TomyN

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Re: Time Windowing with SATLive
« Reply #1 on: September 20, 2011, 10:28 »
Hi,

thanks for your posting. The whole subject is not as easy as it seems.

The first question is: What do we want to measure? Is this really the pure frequency response of the speaker?
Or do we prefer to get a result which is close to what our ears will hear?

SATlive, as the other dual FFT measurement programms, tries to get a result close to the 'subjective' perception of the human ear.
Therefore there is no 'global' windowing applied, but the size of the window used varies depending on the frequency range calculated. In SATlive the size of the window is less than two milli-seconds at high frequencies and more than 600 milli-seconds at low frequencies. More details can be found at the 'SmoothFFT' section of the SATlive help.

The human ear distingues between 'good' and 'bad' sound also by windowing. You might have heard about the so called 'Haas Effect'. The ear treats the first wave-front and the part of the sound arriving shortly after as 'good' and uses it to extract information. The later arriving part of the sound is treated as 'bad' and not used. The length of the interval changes by frequency (as higher as shorter). This is why a the result of a DualFFT is closer to your perception as the result created by looking onto pink noise using an RTA, especial in reverbant areas.

In SATlive you can set a maximium window size, which will shorten the window used at lower frequencies. This setting is located in Tools -> Apply Window, and is available in the transfer-function measurement only.

Furthermore there is a value called coherence, which is a calculated value indicating the affinity of the signal measured to the reference signal. This value is shown as the thin red line in the amplitude display area, and can be used to exclude values spoiled by signals not related to the source signal. This is helpful when dealing with external noise.

If you want to do windowing by hand, then in SATlive the impulse-response module is the way to go. You can window the impulse response in any way you want.

For dual-FFT the window should start at the arrival time of the direct sound. Under normal circumstances you achieve this by using the delay-finder. In the IR module you should use the same time to start the window, but for special tasks you might use an other time to start (you might want to look on the reverbant sound field only, or see the sectrum of the first reflection or..).

The length of the window depends, as stated above, from what you want to examine. If you want to see the frequency response of your speakers (which by the way should stay the same all the time), you need to exclude as much room as possible, which means you should use a window that ends before the reflections of the room occur. Because there is a relation between the length of the window and the lowest frequency you can calculate, this will work for higher frequencies only. As a rule of thumb at least one full wave must fit into the window to get a result that makes sense. So a window size of 10 ms yields a lowest frequency of 100 Hz. In SATlive there is not frequency response displayed below this limit.

Tomy

« Last Edit: September 20, 2011, 10:31 by TomyN »
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mfunke

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Re: Time Windowing with SATLive
« Reply #2 on: September 20, 2011, 21:29 »
In some situations it is necessary to have the option to measure only the pure frequency response of the speaker. Would be nice to implement this in the MAT module as well.