Re: FIR filters
I agree, they work.
Remember phase and time are related. Take, say a 90x60 horn, take a bunch of IR measurements within the coverage pattern, then time-align and average the measured IR's. In theory the averaged IR will represent the direction-invariant characteristics of the horn. These characteristics include the internal resonances/reflections or time domain effects of the horn. Now take the averaged IR and generate a filter which flattens the HF phase (and optionally magnitude) from say 3kHz and up. Put the FIR filter inline in a processor and remeasure. The result is an improved or sharper time domain response and the FIR filter need only be a few milliseconds long. I've done this with the FIR design tool for just a few measurement points, and it works. (I haven't tried this for the whole coverage pattern of a horn, nor at many levels.)
As I understand it, the real tricks/challenges are in getting accurate, useful measurements at the right drive levels, and in the time alignment and averaging. One could also model the IR of the early part of the horn. From what I've read and heard, Dave has developed methods for doing both to "fix" the time domain response.
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