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Transducer characteristics and calibration methods:
A hydroacoustic transducer is a device that converts acoustic energy and electrical energy each other. The position in sonar is similar to the antenna in a radio, an acoustic system that emits and receives sound waves in seawater. A piezoceramic ceramic cylinder transducer in which acoustic energy is converted into electrical energy is called a receiver or a hydrophone; a transducer that converts electrical energy into acoustic energy is called a transmitter. Some sonars use the same transducer to transmit and receive sound; others use separate emitters and hydrophones.
Hydrophone performance mainly refers to sensitivity and directionality. The receiving response of the hydrophone is the receiving sensitivity.
A hydrophone usually is an acoustic/electrical linear conversion device that has a scaling factor between the voltage it produces and the sound pressure of the sound field. This scale factor is called the response of the transducer. The receiving response ( receiving sensitivity) of the hydrophone is the terminal voltage of the hydrophone generated by the plane wave of the unit sound pressure (before the hydrophone is placed in the field). Conventionally, the receiving response of Pzt piezo ceramic is represented by an open circuit response in which the hydrophone is not connected to the load. Usually the receive response is recorded in decibels, the reference level is 1 dynes/cm2, and the sound pressure is 1V, and is written in dB. The transmitter's emission-current response indicates the axial distance in the beam pattern when the unit current is injected into the emitter. Sound pressure generated at 1 m. The emission response is usually expressed in decibels. The reference level is the sound pressure generated at the reference distance when is injecting the current of the emitter, and is written in dB. Conventionally, the reference distance refers to 1 m from the sound source; if the reference distance is 1 code, Plus 20log39.4/36, ie +0.87dB correction, transform the transmit response.It is the sound source level.
Hydrophones almost always use hydrophone arrays in their applications. Hydrophone arrays have a higher signal-to-noise ratio than individual hydrophone components because they can be used from isotropic or several identical noises. The signal coming from the direction pointed by the array is extracted, that is, The directivity index is a receiving acoustic wave parameter used to measure the ability of a hydrophone array to extract signals from noise with its beam pattern. It is the number of decibels in which the signal-to-noise ratio at the output of the directional hydrophone is higher than the signal-to-noise ratio at the output of the non-directional hydrophone.
In order to ensure the performance of the cylindrical underwater acoustic transducer and the need for the transmission of the values, the hydrophone and transmitter must be calibrated. Calibrating a transducer is a function of the frequency and directivity of the transducer response. There are now a large number of methods for measuring transducer response. a summary of the calibration methods compiled by the Naval Ordnance Laboratory T.F. Johnston based on the literature. These methods have their own advantages and disadvantages for different transducer sizes and frequency ranges.