#### Keith_W

##### Major Contributor

- Thread Starter
- #81

**Method 2: Reverse engineering the measured room response**

I can't remember who suggested this method to me, maybe it was @OCA. The guy seems to be a treasure trove of brilliant suggestions. Me, I am only a follower.

1. As always, we start by determining the delay. Do a frequency sweep from MLP. Then zoom in and study where the peaks and dips in your response are:

**25Hz**is the first peak. 25Hz has a wavelength of 13.72m, or slightly less than twice the length of my room (7m). So far the model is holding up. This means that this is a first order mode, therefore there should be a dip at the next half wavelength, or

**37.5Hz.**Sure enough there is a dip there. We predict a peak at

**50Hz**and the measurement kindly obliges. Above 50Hz, the model falls apart and the peaks and dips no longer follow predictions. Nonetheless, it is a good place to start with the VBA.

So we do some maths to convert this into 48kHz samples. We take a half wavelength, so (13.72/2) = 6.86m. Then convert it to time (6.86/343 * 1000) = 20ms transit time. And 20ms is (20 * 48) = 960 samples. Now we subtract the delays of the subwoofer, which I have previously determined to be 200 and 220 samples for left and right subs respectively, and we have a calculated delay of 760 samples for the left sub, and 740 for the right.

At this point, does it disturb me that the delays calculated by this method (Left sub: 760 samples) are different to the delays calculated by the previous method (Left sub: 564 samples)? Yes it does. It disturbed me very much. Maybe I should have taken a 1/4 wavelength, but that produced a realization that the MLP is not exactly in the center of the room, and additional modelling would have to be made for the non-centered MLP position. That made my small brain hurt. But never mind, let's put doubts aside for the moment and go on with the method. We will take measurements later and see which model holds up.

Now we have calculated the delays, we go through steps 1 - 8 in the Acourate procedure described above to generate the filters. Again, we compare the VBA to the measurement:

It seems to be a much better fit, indicating the delays calculated by this method is probably correct!

Now, onto the measurements.

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