A Measure of sound

We only had time to do the "Winding up" section of the Sound lab. We found that the more surface area of the nail the coils covered the lower the reading on the detector was. I have no Idea why this was. Maybe it was producing a of of negative energy (electrons?) and that is what was making the reading lower. I think the lowest result we got was about -1.7. I wonder why sometimes we were able to get a positive reading with the same number of coils? What did this exercise have to do with sound?