Plaster walls

I have installed a small wireless network in an older home which has inner walls constructed of Plaster and wood lath, rather than drywall. After passing through three of these walls, the signal from the linksys G type router was very weak at the PC. Just one bar in strength, out of five. As a result the PC's ability to get online was marginal. I installed a Linksys signal extender, which increases signal strength at the PC to 4 or sometimes even 5 bars, correcting the situation. My question is, can I attribute this low signal strength to the impermeability of these walls, or is there another likely explanation. Thanks in advance....... Dick M.