A New Method for Airborne Sound Detection Using Total Internal Reflection and Its Application to Microphone

A new method for detecting the sound pressure in air, which uses the total internal reflection at the curved interface between glass and air, is proposed, and its application to microphone is discussed. The critical angle for total reflection changes by the refractive index of air, which depends on the air density. The density changes by the sound pressure. Therefore, the sound pressure is measurable by detecting the intensity of the reflected light from the total reflection area. The sound pressure sensitivity of the proposed method is investigated theoretically and experimentally. Experimental results show that the microphone using the method is feasible though its sensitivity is low in the present stage. When the sensitivity is improved dramatically for practical use, the microphone becomes very sensitive to the surrounding conditions. A method to compensate the fluctuation of atmospheric pressure or temperature is presented. 1. Introduction Ordinary microphones have a limitation in frequency range and a difficulty for the measurement of great volume of sound as the mechanical vibration is used when the sound pressure is transformed to the electrical signal. Therefore, the development of new method for electroacoustic conversion without a diaphragm has recently been expected, especially in the field of next-generation audio, impulsive noise measurement, or airborne ultrasound. Using light is an effective way to meet the requirement. Some studies on the detection methods for airborne sound without diaphragm using optical measurement techniques have been reported [1–5]. The change in the refractive index of air due to the sound pressure is detected in those reports. However, there is no method which can measure the sound pressure at a point in the sound field with high sensitivity. We have proposed a method, which can detect the sound pressure at a point in the sound field using the total internal reflection at a curved interface between glass and air, and demonstrated the feasibility of a microphone by use of the method theoretically and experimentally [6, 7]. The sound pressure changes the refractive index of air, which causes the change in the optical reflectance. Therefore, the sound pressure is measurable by detecting the change in the intensity of reflected light. The drawback of the ways to detect the sound pressure using the change in the optical reflectance is that the sensitivity is low because the pressure change in air due to sound is extremely small compared to the atmospheric pressure. However, the problem can be overcome by using the