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Membranes

It has been seen that some balance types use a thin membrane in their design. Typically, this is used to seal the measurement system or to entrain the propagation medium. However, the presence of the membrane will affect accuracy of the power measurement and to an extent that depends on several factors. The influence of membranes on measurements of power made on continuously driven transducers has been analysed and related specifically to the use of thin membranes placed between the transducer and the target to act as streaming shields.

When aligned parallel to the face of the transducer (which would normally be the case when using a membrane-equipped balance to carry out measurements of ultrasonic power), coherent reflections travelling back to the face of the transducer can alter the output of the transducer to an extent dependent on the properties of the membrane, the acoustic frequency and the distance between the transducer and the membrane. By varying the separation between transducer and membrane, maxima and minima in the generated power were defined by the relationship between the separation and the acoustic wavelength. The effect was removable by tilting the membrane, and hence destroying the coherence of the reflection. The effect is strongest for highly resonant (high Q-factor) transducers - physiotherapy treatment heads are a prime example. A similar effect would be anticipated from the direct loading of the treatment head onto a membrane using coupling gel which effectively means that the measurements of output power using a device of this type are not being made under the standard free-field conditions specified by IEC 61161.

In vertical configurations where a water stand-off exists between the treatment head and the entrance membrane to the power balance, it may be possible to tilt the treatment head slightly for power measurements. This will destroy the coherence between the wave reflected back to the face and the treatment head drive. The validity of this approach would need to be assessed for the particular treatment head using ancillary measurements, however, such as investigating any dependence of the measured power when the stand-off distance is varied. The influence of reflections will depend strongly on the characteristics of the ultrasound transducer itself as well as the membrane. Attempting to correct for the effect is complex. It is likely that for very thin membranes (< 20 mm thick) made of a material such as polythene, the effect of back reflections on a treatment head will be minimal (probably < 3%). Generally, if it is possible to make power measurements without a membrane in place then this should be the preferred route because it allows measurements to be made in a standardised way according to IEC 61161.

References

• Beissner K. The influence of membrane reflections on ultrasonic power Acustica 1982; 50: 194-200