The acoustic window is the surface through which the sound waves travel. It occupies the space between the piezoceramic assembly and the water. Any material used to create the acoustic window will absorb some of the sound waves that pass through it. Therefore, engineers carefully choose the least absorbent materials. The acoustic window is sometimes referred to as the acoustic face.
Epoxy, plastic, and urethane are the three materials Airmar uses most often for our acoustic windows. These materials have sound wave carrying capabilities or acoustic properties between those of the piezoceramic element and water.
Acoustic window materials fall into two categories. Soft, rubbery, elastic materials like urethane carry sound waves in almost the same manner as water. So, water and urethane are said to have similar acoustic properties. Because of this close match, the thickness of acoustic windows made from urethane does not need to be tightly controlled in our product designs.
Hard materials like plastic and epoxy have acoustic properties somewhere between those of piezoceramic elements and water. In other words, the plastic or epoxy acts like an intermediate acoustic step between the fluid water and the rigid piezoceramic element. A plastic or epoxy acoustic window is called a matching layer. Layer thicknesses are carefully calculated and produced to match the sound wavelength at the operating frequency.