Filtering background noise using two microphones

Hi,
I am new to Labview so please bear with me. I am trying to work out the sound vibration level emitted from a compressor. I have a microphone placed next to it but it is operating in a noisy laboratory and therefore I need to filter out the background noise.
I have another microphone to record the background noise. I think I might need to use an adaptive noise cancellation filter but cannot figure out how to make one on Labview. I have tried simply inputting the two signals and subtracting them but get an error saying "Resource requested by this task has already been reserved by a different task" when trying to run it.
Basically I am looking for the simplest way to get rid of the background noise in real time or in processing the signals afterwards.
Any help would be greatly appreciated. Please ask if you need more information.
Thanks a lot,
Sam

An ax (also spelled axe) is a tool for chopping down trees. I was trying to make a joke. Like some other experienced LabVIEW programmers, I do not like the Dynamic Data Type because it hides the structure of the data contained within it.
The subtraction technique is standard and simple. The biggest issue is phase shift. Suppose the two microphones are 1 meter apart. A sound arriving on the line through the microphones will arrive 3 milliseconds later at one than at the other. Sound arriving along a line at right angles to the line through the microphones and equidistant from both will arrive at the same time at each. At other angles the time will vary between those values. Compensating for that delay or phase shift is necessary to get good cancellation.
In addition the amplitude of the signals at each microphone will vary inversely as the square of the distance between the source and the microphone. For sources much closer to one microphone the amplitude difference can be significant.
Directional microphones will further complicate the amplitude variations. I think that two microphones are not enough to provide unambiguous determination of the direction of an unknown source, especially when there are multiple sources.
Lynn

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