[ICOM] Icom 9100/7410 roofing filter question

[John Geiger]

I had a chance to play with an Icom 9100 briefly at my QTH, where I have
some consumer economics crud noise on 40 meters.  When I switched in the
3khz roofing filter on the 9100, it seemed to lower the noise by 1 to 2 S
units.  I don't remember the 6khz roofing filter having that big of an
effect.

Is what I saw due to filter insertion loss, where any signals would be
reduced by the same amount?  I am guessing not,since I didn't see the 6khz
filter having such an effect, but it is possible that the 3khz filter has
greater loss.

Or is it likely that the roofing filter actually does reduce the noise,
improving the signal to noise ratio?  Has anyone else studied this
phenomenon on their 9100 or 7410?

73 John AF5CC