The phosphorescence microphone: A device for testing oxygen sensors and films

A new concept in sound pressure detection, the phosphorescence microphone, is introduced. This device replaces the diaphragm of a microphone with a thin film of pressure sensitive paint. Dynamic changes in pressure are detected by monitoring the changes in phosphorescent intensity that are due to cyclical variations in oxygen partial pressure in the sound wave. To test this new device, the acoustic resonance spectrum of a closed cylinder is measured and compared to the spectrum obtained with a commercial electret microphone. Oscillations in phosphorescent intensity that occur at the frequency of sound are observed, and the magnitude of these oscillations is dependent on sound pressure. Several experiments are presented to confirm that the spectrum obtained with the phosphorescence microphone is due to the oxygen sensitivity of the pressure sensitive paint. In addition to detecting sound, this device has shown efficacy in testing pressure sensitive paints for high frequency, low differential pressure applications. The phosphorescence microphone was used to show that selected pressure sensitive paints could detect acoustic pressure changes as small as 6 Pa in the frequency range 150-3500 Hz on a background of 10⁵ Pa.