Real-time measurement of acoustic field displacements using ultrasonic interferometry

Abstract

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.

Design and Development of Ultrasonic Process Tomography

“…Resolution and penetrating power of an ultrasonic wave depends on the resulting wavelength of excitation inside the material in question. Greater wavelengths or lower frequencies generally penetrate much further into a material (Kannath & Dewhurst, 2004). Higher frequency ultrasonic excitations with smaller wavelengths generally decay more rapidly inside a material, but resolution capability is improved.…”

The Front-End Hardware Design Issue in Ultrasonic Tomography

“…Resolution and penetrating power of an ultrasonic wave depends on the resulting wavelength of excitation inside the material. Greater wavelengths or lower frequencies generally penetrate much further in to a material [10]. Higher frequency ultrasonic excitations with smaller wavelengths generally decay more rapidly inside a material, but resolution capability is improved.…”

“…Normally, a liquid (wet coupling) is used to allow easy application and conformity to the void between the transducer and the surface. It also should be a very good conductor of sound energy to allow maximum transfer to the structure [10]. Other characteristics of coupling materials are generally dependent on particular applications.…”

Model for continuously scanning ultrasound vibrometer sensing displacements of randomly rough vibrating surfaces

“…Ultrasound vibrometers may also use an incoherent approach of directly measuring the Doppler shift spectra. 12 Lasers have a narrow collimated beam and the angle incident on and reflected from the surface is often approximated as each traveling in one planar direction. 11 The resulting phase shift of the reflected field is then assumed to be linearly related to ground motion.…”