Looking for IR Help - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 2 Old 09-04-16, 09:10 PM Thread Starter
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Looking for IR Help

I'm trying to understand impulse charts and what they are telling me. I'm trying to integrate and time align 3 subs using REW that are connected to a MiniDSP 2x4 HD. (3 Rythmik F15HPs) Attached is the overlay graph for Impluse on the 3 subs.

Config is 2 subs up front next to mains about 14 ft from MLP, one near field 2 feet behind MLP. Impluse graphs of Sub Left and Sub Rear look roughly good (Sub Rear running 10.6 ms delay), but Sub Right looks really strange to me. I swapped Sub Right and Sub Rear and ran without 2x4 HD and same graphs. Looking for any thought on why these don't align.

I would have expect the left and right subs to have identical graphs as they are equidistant from the MLP. Am I thinking about this incorrectly?
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post #2 of 2 Old 09-05-16, 05:04 AM
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Re: Looking for IR Help

Your expectation is Correct. The Sub L and Sub R Impulse Response (IR) shapes are pretty similar, but they are offset in time by roughly 17 ms.
The time difference here is due to REW automatically shifting the IR locations on the time axis to align the largest peak at 0 ms. This is done to remove time-of-flight and thus allow some charts (phase, group delay and others) to be plotted more correctly. In this case the largest peak of Sub R is negative and the largest peak of the Sub L is positive. Thus the difference in IR positions on the chart. This occasionally happens when the positive and negative peaks are nearly the same size. The 'Controls' panel on the Impulse Graph allows us to manually shift the IR locations as needed for analysis.

For the purpose of aligning the timing/delays of the SWs using IR locations we do not want REW to automatically shift the IR to near 0 ms. We want to use one of the features in REW to provide the actual relative IR locations. These are; the 'Loopback Timing Reference' (used for XLR Mics with analog IO) or 'Acoustic Timing Reference' (used with USB mics and other digital IO situations). The relative IR offsets will then agree with the physical measured relative offsets of the SW positions. REW has basic information on all the features mentioned here in the 'Help' information. That is a good first reference source. We can answer resulting questions.

There are several ways to setup SW timing. Physical measurements is probably the easiest. Using IR is one of the more complicated to employ, but it can help provide clues in the cases where room issues arise.
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