Calibration of ultrasonic transducers used for liquids
Hydrophones
A hydrophone is an acousto-electrical transducer. It converts the alternating sound pressure acting on it into a proportional alternating voltage. The reception sensitivity M = Uo / p must be determined by calibration of the arrangement. Here amplitude and phase are of interest.
Transmitting transducers
Calibrated ultrasonic (transmitting) transducers are required to generate defined sound fields. These transducers produce a sound field in which the sound pressure in a defined point is proportional to the supply current. The transmission sensitivity S = po / I must also be determined by calibration. Transducers of this type are required, for example, for the inspection or calibration of hydrophones and other pressure receivers.
Procedures
Primary calibration methods
The primary calibration methods, such as reciprocity calibration or laser interferometry, work on different principles. A suitable sound source and the instruments specially required for the respective method must be available. In the ultrasound range, the national institutes mainly use the two-transducer reciprocity method or laser interferometry for the calibration of discrete frequency points. A calibration at continuous frequency is ensured by the two-transducer time delay spectrometry [TDS] - reciprocity method. Other (quasi) continuous calibrations are possible through shock calibration and reciprocity calibration with maximum sequences. The two last-mentioned methods are, however, not yet used to date for precision calibrations in the ultrasound range.
Substituition calibration methods
Secondary or substitution calibration methods require a calibrated hydrophone (in general: a transducer), i.e. a reference standard. The transducer to be calibrated is then compared with the reference transducer. The time delay spectrometry [TDS] method is very well suited for calibrations at continuous frequency. An alternative method uses broadband nonlinearly distorted pulses to determine the complex-valued hydrophone transfer function.