measurements of the same source from different positions

[giacomus]

Hello everyone. Sorry if this has been already discussed in the forum, but I could not find a detailed and precise how to about how to deal with multiple measurements taken in the main listening position and around it, for example MLP + 4 other positions around.
So,
1. Lc and Rc are the main listening position measurements and L1, R1, L2, R2, L3, R3, L4, R4 the other four;
2. sweeps taken with acoustic timing reference, all of them

The guide says, in two points:

a) Cross corr align, which time aligns the currently selected measurements by cross correlation of their windowed impulse responses, using the measurement which appears first in the list of those selected as the reference (the order of measurements can be changed by dragging them up or down the list). This may be useful to align measurements from a mic array or measurements of the same source from different positions prior to vector averaging.
Further down in the guide I read "Vector average, which averages the currently selected traces taking into account both magnitude and phase. It can only be applied to measurements that have an impulse response and is most appropriate for multiple measurements taken from the same position, or measurements which have been time and level aligned before averaging them." So this does not contradict the first scenario (measurements for multi point mic positions) as long as SPL aligned and measured with time reference. OK. compared to b) this averages and preserves phase. Useful mostly for.. ?

b) RMS average, which calculates an rms average of the SPL values of those traces which are selected when the button is pressed. This does the same as the Average The Responses button, the dB values are converted to linear magnitudes, those magnitudes are then squared, summed and divided by the number of measurements, the square root of the result is taken, then the value is converted back to dB. Phase is not taken into account, measurements are treated as incoherent. If only a single trace is selected the result has the magnitude data from the source measurement and no phase data. If the measurements were made at different positions (spatial averaging) it is usually best to first use the Align SPL... feature to remove overall level differences due to different source distances

First question: when to use a) when to use b) ? if the target is EQ'ing with inversion. I would say b)

Second question: b) If I understand correctly, take all LEFT measurements, Align SPL and then RMS average, and work with L-average ever since (phase information is gone, but ok for designing minimum phase filters/ EQ-ing), then take all R measurements, Align SPL and then RMS average.

Bonus explanation, would be much appreciated: why aligning SPL before RMS averaging? to adjust (separately L and R) levels due to different mic pos before averaging?

With kindest regards

 

[John Mulcahy]

Use cross correlation alignment and vector averaging if you need to with with an impulse response, for EQ RMS or dB averages are often more suitable. Aligning SPL is important if the measurements are from significantly different distances from the mic. Cross correlation alignment is unlikely to work well if the sources are different, e.g. if you were to try to align measurements of both left and right speakers.

 

[giacomus]

@John Mulcahy thanks for your reply. I am optimizing for one main listening position and I was thinking to take one MLP +4 measurements at 20cm to the left, right, back and front. I understand the expected differences are small. I am also going to apply FDW (12 to 8 cycles to avoid small bin corrections).
In this scenario, cross correlation + RMS? or cross correlation + Spl align + RMS?
Concerning your observation about cross correlation on different sources, I see that it is done when aligning sub woofers. Now wondering whether this is appropriate or not.
With kindest regards

 

[John Mulcahy]

RMS averaging or moving mic method probably best for that.

 

[giacomus]

RMS averaging or moving mic method probably best for tha

 

[giacomus]

thanks.
I noticed that fdw solves timing issues by smoothing out narrow peaks in the region < transition. I understand that multi positioning contributes but given the small 20cm delta I will still apply fdw 12 down to 8. apparently I obtained much better results timing wise (with one measurement only as per the first experiments)