Hi Jeff,

 

I have been having the same problem.

It can be difficult to interpret empty room

recordings. We make one every day, after

tuning the machine, and we have a Matlab

script that will give noise, spectral plots and

covariance matrixes for all channels, but it

is not easy to get useful information from

those.

 

Basically you have two main sources of noise:

external noise and noise that's coming from

the sensors and the electronics themselves.

The internal noise is minimised by tuning,

but we have less control over external noise.

I wouldn't know how to tell them apart in

the recording, except that perhaps external

noise should be more affected by SSS, so

should be removed to a greater extend by

Maxfilter.

 

As a test I double grounded our MSR a few

weeks ago. This should seriously increase

external noise, as the MSR is then much less

effective. But I really couldn't see much

difference between the normal empty room

recordings and the ones made with double

grounding. This surprised me, I have to admit,

but there might be more analyses I can still

do on the data. Any suggestions are very

welcome.

 

Maarten

 

 

I should restate, for simplicity, that my question really is: