Directional Microphone Elements

The key component of an LOAS instrument is the spectrophone. Its construction depends critically on the state of aggregation of the sample. The SP for gas samples consists of a gas cell with input and output windows for laser radiation and a microphone, fixed on an inside or outside wall of the cell. As in the case of MAS with SDLs. the sensitivity can be improved by the use of multipass SPs. Plane capacitor or electronic microphones are the most widely used. The sensitive element of these microphones is a thin (1-10 pm) elastic membrane made of Mylar. Teflon, or metallic foil. The membrane serves as one electrode of a dielectric capacitor (membranes made of dielectric materials have metallic coatings). Thus, the acoustic vibrations of a gas mixture in the cell can be directly converted to an electrical signal. Commercial microphones with sensitivities of 5-50mV/Pa are utilized in routine applications of LOAS specially constructed and optimized ones are used for ultrasensitive analyses. 

Broadband microphones (from the audible range to ultrasound), which can be achieved in a converter using the piezoelectric properties 33 in the thickness direction, have aheady been applied as transducer elements in an airborne ultrasound transducer (Yasuno et al. 2010). Application to ultrasonic probes for medical use is also expected. 

---