A method of anodizing an aluminum substrate comprising heating the substrate to a first temperature of 200 C. The interdigital transducer satisfies 0. The key has a body having one end inserted in an opening formed in the side face and the other end exposed to the outside of the housing, and an extension extending along the side face from the other end of the body to the outside farther than an end edge of the opening. The flexible membrane is less than 3 microns thick to allow the use of a high yield strength membrane material. At least one of the resonance circuits has a resonance point that differs from the remaining resonance circuits.