Hydrophones

General information about depth, temperature, frequency range, and sound pressure level is requisite to selecting a hydrophone standard best suited to a particular need. Generally, hydrophone standards were developed to provide measurements independent of temperature and pressure. Each hydrophone is issued with a formal calibration curve showing free-field voltage sensitivity at the end of the electrical cable. Typical directivity patterns at specific frequencies are provided with user instructions.

Projectors

Characteristics such as source level, operating frequency, directivity, and impedance are paramount in selecting the type of acoustic projector to be used. Projectors are issued with only typical transmitting current and voltage response data shown in the user instructions. Because the acoustic environment may affect the sound pressure produced at a given location, using a hydrophone standard in the test configuration to determine sound pressure is strongly encouraged. Typically, transmitting response data is used as a baseline to determine whether the projector is operational.

Reciprocal Transducers

Transducers in this category have the versatility to function either as a hydrophone or a projector. However, their frequency range of operation may be different depending on whether they are used as a projector or a hydrophone. The above discussions on projectors and hydrophones apply to this category.

Calibrators

Two types of portable calibrators are available to provide rapid, economical, and reliable measurement of the free field voltage sensitivity of hydrophones. The G19 bench top calibrator is used in the frequency range from 100 Hz to 1000 Hz for hydrophones up to about 2.5 centimeters in diameter. The type G40 calibrator was designed for shipboard use and can be used in the frequency range from 100 Hz to 1000 Hz for hydrophones up to about 10 centimeters in diameter. Either type is supplied with a calibrated standard hydrophone to provide accuracy within ± 1.0 dB.

Specialized Transducers

In addition to the transducer standards loan program, USRD maintains in-house expertise in transducer engineering and design to develop specialized measurement transducers. The Transducer Standards Laboratory at USRD has a complete state-of-the-art facility dedicated to underwater transducer design, prototyping, fabrication, testing, repair, maintenance, and failure analysis.