But if it 's varying at all , which is gonna be the case for almost any real situation , you have to do it on - line , uh , with some forgetting factor or something.
So do you  is there some long window that extends into the past over which you calculate the average ?
Well , there 's a lot of different ways of computing the noise spectrum. So one of the things that , uh , Hans - Guenter Hirsch did , uh  and pas and other people  actually , he 's  he wasn't the only one I guess , was to , uh , take some period of  of  of speech and in each band , uh , develop a histogram. So , to get a decent histogram of these energies takes at least a few seconds really. But , uh  I mean you can do it with a smaller amount but it 's pretty rough. And , um , in fact I think the NIST standard method of determining signal - to - noise ratio is based on this.
A couple seconds ?