ABSTRACT
When we use any kind of waves, we need to know what will reflect them, what will transmit them and what will absorb them. Remember https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315193892/2143c097-43f6-474f-b507-c12feffdfd35/content/pg139_1.tif"/>
