Hydrophones acoustic pressure measurements

Fig. 8.6 Acoustic pressure measurements carried out with a hydrophone in 800 ml of saturated brine and a sucrose solution (30°Brix). Ultrasound was applied with a...

The response of hydrophones or microphones can be calibrated in terms of either sound level (dB) or acoustic pressure either of which are related to ultrasound intensity. The Fourier transform of these give a frequency-dependent signal, which is also related to sound intensity. It is possible therefore to obtain a plot of sound level (and thus theoretically ultrasound intensity) against frequency. However, as already mentioned, ultrasound power measurements using acoustic probes are not straightforward and require preliminary calibration of the probes with another... 

The acoustic pressure can be measured through the effects produced in the propagation liquid, namely (1) heating, measured calorimetrically and (2) the action of radiation pressure, determined by measuring the force exerted on a surface (the radiometric method). While the former allows the measurement of the mechanical power of the vibrating source, the latter is associated with the acoustic power transmitted by the acoustic radiation. The acoustic pressure can also be determined using calibrated hydrophones (the acoustic method). 

The acoustic pressure amplitude determines the growth of a cavitation bubble and consequently the chemical effects upon collapse. The amplitude of the pressure wave can be measured with a hydrophone or can he calculated using a calorimetric method (9,10), in which it is possible to determine the ultrasoimd power (Qus) that is transferred to the liquid. With the ultrasound power and the surface area of the ultrasound source (Aus), the acoustic amplitude can he calculated according to equation (2), for which the ultrasoimd intensity is the power input divided by the surface area of the source (11). 

Currently, quartz is often utilized in accelerometers. Due to their high piezoelectric voltage coefficient lithium sulfate and tourmaline are often applied in commercial hydrophones especially to measure shock and pressure waves. Rochelle salt can be found in acoustic pickups and special devices to measure acoustic pressure. Due to their long-term stable piezoelectric properties natural crystals are in particular perfect for sensor applications where the monitoring of a quantity has to be made over long periods [85]. 

Ultrasound-based sensors for metal-coated fiber optic measurements based on interferometric determination of the displacement using a Michelson interferometer have also been designed. The input acoustic field can be detected by using two reference methods, namely (a) time-delay spectroscopy with a calibrated hydrophone (a hydrophone with known frequency response determining the sound pressure, the input displacement being obtained by simple algebra) and (b) the interferometric foil technique (the displacement of a metallized foil situated at the surface of the fluid measured by the interferometer used for fibre tip measurements). The frequency dependence of the transfer function compared well with the theoretical models [51]. 

Berthelot method in water inclusions in quartz [45]. The inclusions appear to give a much more negative Pcav But one has to remember that the liquid density only is known (assuming that the inclusions keep a constant volume and remain sealed), and that the pressure is deduced with an extrapolated EoS. Therefore, a direct measurement of the liquid density at the nucleation threshold has been performed in the acoustic experiment [52], thanks to a fiber optic probe hydrophone [51], Figure 8b presents a comparison of the acoustic and inclusion measurements in terms of density. The solid bullets are direct measurements acquired with the hydrophone and they compare well in trend and magnitude with pressure estimates that were converted to density with an EoS, but the major discrepancy with the inclusions is persistent. 

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