# Impulse response window functions



## nugat (Dec 24, 2016)

Hello, it's my first post here.
In the IR Windows the top two choices are windowing functions: Tukey, Blackman, Flat-Top etc. What are their
uses and for what types of measurements? Is there a default one good for all situations?
TIA


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## nugat (Dec 24, 2016)

I will add some details to my original post. I am fairly new to REW and DIY audio in general. 
I am building 2-way dipole mains speakers to be supplemented by separate subs.
My measurements are done outdoors (I live in the countryside). The speaker is elevated in the field, 
mic 1 meter away at the height of +2 meters (7 feet). The dipole peak is there at ca 300Hz, as expected
for the baffle size. When I apply windowing from -1ms to 9ms (just before the ground bounce at ca 10ms)
the dipole peak shape depends on the chosen window function from the "IR windows" pop-up (Tukey, Blackman, Flat-top etc).
The peak height can vary by 4-5 dB depending on the chosen function. Eg, with flat-top it looks like no EQ is required.
Which window function is proper for the kind of outdoors dipole speaker measurements I'm doing? TIA.


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## jtalden (Mar 12, 2009)

I'm no expert on various window choices vs analysis types, but will make some basic comments in case they are helpful. If you were looking for the ins and outs of all the various windows choices for various charts, I don't have any link to any overall summary.

> The default window Tukey 0.25 for SPL charts works very well for me.
> Any left window change is not significant for the SPL chart so long as it is positioned far enough away so it does not encroach on the impulse. 
> A right window change can sometimes be helpful depending on the chart type. I previously used it to clean up the phase chart slightly. Now I find the 'frequency dependent window' function works much better for that purpose so I now just use the default window.
> Window encroachment on the impulse will reduce the SPL level by the level of encroachment. The 'Window' shape shown shows the range of that impact.
> Some of the charts default to other window settings as shown in REW preferences. I have not changed those defaults either.

I don't know what you mean by 'dipole peak shape' change so more info would be helpful. Is it the SPL chart you are looking at? An apparent change to the impulse chart itself can be very misleading. It is not necessarily a concern. It is only important to the extent that there is an impact to the other charts as SPL, Phase, etc. which have direct relationships to sound quality.

You can load the same measurement twice and adjust the window settings on them differently to show the difference you see and then save the file in order to post it here. We can then better understand the concern.


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## nugat (Dec 24, 2016)

Please find attached the full mdat file of the outdoors measurement.
When the IR is gated , say -1ms to 9ms, and windowing functions are selected in the "IR Windows" pop-up, 
the SPL graph will change it shape depending on the function selected. The dipole peak (ca 300Hz) becomes either more pronounced or flatter.
With Flat-Top or Blackman-Harris 7 the peak looks like it won't need any EQ. Not so with Tukey or others.
The intended crossover from the dipole to separate subs will be at ca 120hz. In this measurement the mid woofer is running full range with no filters/XO.
I am currently preoccupied with the peak and what to do about it, one thing at a time...


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## jtalden (Mar 12, 2009)

See charts below.
1. The SPL comparison you cited. Tukey (red) Vs B-H7 (green).
[some SPL reduction at lower freq with B-H7.]

2. Impulse Tukey
[Window shows data impact starting at ~+8.5 ms.]

3. Impulse Tukey
[Window shows data impact starting at ~+0.5 ms. Almost all the data above +5 ms appears to be almost completely suppressed with that setting.] 

The low freq SPL is reduced with the B-H7 setting due to the shape of that window. To get nearly the same SPL at low freqs with that window the time setting would need to be set much greater, maybe +20 ms. That will result in a small portion of the ground reflection data to be included, but it is likely to be of no practical consequence based on the window shape.


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## nugat (Dec 24, 2016)

Thank you for this analysis, that's really helpful.
In "3." did you mean "impulse tukey" or "impulse B-H7"?
In practical terms, does the "B-H7" window function analysis show that
no further EQ for the dipole peak is necessary? Or I'm misinterpreting something?
In fact my in-room measurements (multispot listening window averages) show that the 300Hz peak is attenuated by
the two dipole speakers+room behaviour to within +/- 3dB of the house curve. And that's even before applying
Dirac, my preferred DRC. Obviously I am looking for a justification to leave it like that :wink2:


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## jtalden (Mar 12, 2009)

nugat said:


> In "3." did you mean "impulse tukey" or "impulse B-H7"?


Yes, for 3 I meant; Impulse B-H7 



> In practical terms, does the "B-H7" window function analysis show that no further EQ for the dipole peak is necessary? Or I'm misinterpreting something? In fact my in-room measurements (multispot listening window averages) show that the 300Hz peak is attenuated by the two dipole speakers+room behaviour to within +/- 3dB of the house curve. And that's even before applying Dirac, my preferred DRC. Obviously I am looking for a justification to leave it like that :wink2:


It is interesting and educational to confirm the anechoic SPL response of a DIY design. Since you are relying on Dirac to perform the EQ there is no need be concerned with the that response however. I would expect Dirac will end up with a similar LP response no matter the initial anechoic response. Speaker and LP placement in the room along with other room acoustic characteristics will have a much bigger impact on the sound quality than any reasonable difference in a speakers anechoic response.


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