Capturing Nearfield and Farfield Impulse Responses

[mmcm4]

*** This is more for fun and development of technique than capturing studio quality IRs ***


In a couple weeks finals will be over and I'll be home for christmas break. Over that time I've got a chance to play around with capturing IRs from my Bad Cat 2x12 and my AC-15 cab. I have a variety of inexpensive mics to use. I understand the process of capturing an IR through the AxeFX-II, but I've got a couple questions.

1. I understand how to capture nearfield IRs, is there any info (tips or tricks) on Far Fields (other than place your mic far away from the cab)? I do understand they are inherently more difficult to capture, but I still want to try.
2. Is there any advantage to using something like Reaper to capture an IR in a wav file (and then converting with Axe-O-Matic) instead of the AxeFX in a sysex file?

If anybody has resources they could share (other than the posted Fractal stuff, and it doesn't matter how technical the article) that would be appreciated. I'm not looking to create perfect IRs, but simply to gain a better understanding of the process.

 

[barhrecords]

From what I have learned, far field is something that folks interpret differently.

For far field impulse response capture that does not include boundary reflections, I recommend starting by a Google search of ground-plane measurement techniques.

Sound travels at roughly 1ft per millisecond. For a 41ms capture, the nearest boundary to the microphone should be approximately 41 feet of travel distance from the source speaker. This is round trip. So if the mic is 1 meter from the speaker, the ceiling and walls should be about 24 feet (41ft/2ft + 3ft(1m)) away from the microphone. The ground-plane technique eliminates the floor reflections from consideration.

I assume the IR capture builtin to the AxeFx would do some nice post processing for you. Compensating for time-of-flight to the microphone and minimum phase transformation. The latter of which is somewhat controversial to different folks too.