3. Measurement techniques
The bubble activity which originates mainly from the bulk of
the liquid is measured with a needle hydrophone with an aperture
of 500 lm (Onda, HNR-500). The hydrophone is positioned outside
the main sound beam. The sound signal is amplified with a homebuilt
amplifier based on ‘AD8066’ fastFET op-amps (approximately
50 dB amplification, 5 MHz bandwidth) prior to being captured
with an oscilloscope [18]. Each bubble in the sound field acts as
a secondary sound emitter and the hydrophone measures the average
signal over a large number of emissions from individual bub
degassing of the liquid due to the applied ultrasound. The high flow
rate requires the use of 3 degassing and 3 regassing membrane
contactors in parallel. An anechoic material (Aptflex F28, Precision
Acoustics) can be placed along the walls inside the cleaning tank.