Goniometers, Mic Position, and Phase

While chasing the wily whomp I started recording with microphones widely spaced, 'A-B' position. With close mic'ing and wide spacing I was able to capture an interesting guitar sound while minimizing the 91 hertz output from the sound hole, which unfortunately was the frequency the open 5th course was sounding.



Recently I've gone back to a Jecklin array, changed pitch, and added a lot of acoustic fiberglass. I started playing with another Steve Harris application, meterbridge, and the pictures here are a tune recorded with the AB setup and a more recent tune, with the Jecklin array. This first image is the Jecklin array piece. The meterbridge, in its 'jellyfish' or goniometer setting, is the window entitled jf-bridge 9493. In the other window are the jack connections that hook the meter into the sound card and music file. Here's what Steve says this application will show you:

A jellyfish meter is a stereo scope display, it can be used to spot phase and mono compatibility problems.
Some charateristic problems can be seen, eg. a signal with a stronger left channel is seen as a shape sloping to the left [\], more right slopes to the right [/], and dead mono is a vertical line [|].
The meter below the scope display indicates the degree of stereo correlation between the channels, a value of +1 means that it is perfectly mono compatible, and -1 and that the channels are in anti-phase (so they will cancel out if mixed).

This recording looks good. The horizontal green bar at the bottom seems to hover between +0.4 and +1.


But this next recording, done with wide spaced and close mic'd AB configuration, could have problems if played mono. The horizontal bar at the bottom sometimes flips over to the red zone (zero to minus one). There could also be problems for people with a woofer connected that sums the left and right channel. I guess the bass would drop off. This meter doesn't show the frequency of the sound that is causing the problem. I could probably feed it through a filter and see, but for now I'm simply sticking with the Jecklin.