How much does sound dissipate in plywood over distance as measured by a contact mic?

I put a contact mic on a sheet of 22mm plywood. I tapped on the plywood with a pencil eraser first right next to the mic, and then at the far corner 1.5 meters (5 feet) away. The peak amplitude was about 12 dB lower in the latter case as opposed to the former.

[@makeperceive]

You have a very interesting niche. These experiments are useful. Planning on building some instruments with these mics. Does the frequency response change over distance?