Logo of the Physikalisch-Technische Bundesanstalt

Measurement of sound field in cavitating media by an optical fibre-tip hydrophone

01.11.2006

A fibre-optic technique was applied to measure the sound field in an ultrasonic cleaning vessel under practical conditions.

High-power ultrasound is used in many technical processes of industrial manufacturing, service procedures or medical applications. Most of these applications work in fluids and the mechanism behind is cavitation as the generation, oscillation and collapse of small bubbles in liquids. Cavitation and all dynamic processes involved strongly depend on environmental parameters and there is the need for a quantitative description of the cavitation processes for a more effective use. The application in manufacturing requires cost-effective methods, and the reduction of the cleaning time or the economical use of surfactants may reduce the costs. In a longer running project methods for the description of the sound field and the cavitation effects in ultrasound cleaners and sonochemical reactors are developed in the sound department.

A fibre-optic technique was applied to measure the sound field in an ultrasonic cleaning vessel under practical conditions for higher spatial and temporal resolution than in measurements with common hydrophones. A metal-coated fibre tip, embedded in different materials (Fig. 1), is used as a sensor and a heterodyne interferometer detects the change in the optical path resulting from of the movement of the fibre tip in the sound field. Spectrally resolved sound field parameters such as the fundamental, the subharmonic or cavitation noise are extracted from the measurements and compared with results obtained by a piezo-electric hydrophone. It was found that the fibre sensor provides a signal related to the velocity in the sound field, but the information about cavitation-related parameters is similar to the information for pressure sensing techniques. The fibre-optic sensors have a uniquely high spatial resolution and the sound detection process is strongly influenced by single cavitation events close to the small fibre tip.

Embedded in polyurethane fibre sensor thus protected against cavitation damage Embedded in a holow stainless steel tube fibre sensor thus protected against cavitation damage

Figure 1: Fibre sensors protected against cavitation damage: left embedded in polyurethane, rigth embedded in a holow stainless steel tube

This work showed that fibre-tip sensors are an alternative to common hydrophone techniques. They can open up new possibilities for measurement problems for which so far no solution exists, in particular when a high spatial resolution is required or when the measurement site is small.

Contact person:

Christian Koch, FB 1.6, AG 1.63, ultrasonics@ptb.de