Real-time measurement of acoustic field displacements using ultrasonic interferometry

A measurement technique using ultrasonic interferometry is described for non-contact, quantitative measurement of acoustic vibrations. A pair of low-cost ultrasonic transducers was used in a transmit–receive mode to direct a 40 kHz carrier wave to the surface of a solid target vibrating acoustically, such as a speaker cone. Demodulation of return signals consisted of a phase sensitive detection circuit to assess interference between the carrier wave and return signal. Recovered signals were enhanced using real-time digital signal processing hardware to improve signal-to-noise ratios for audio vibration measurements. Phase resolution of at least 3.4 mrads has been demonstrated in the audio frequency range. This represented an equivalent displacement of 2.3 µm in air.