Hydrophone Calibration Research Articles

Reciprocity and the calibration of microphones; its history and new problems

A reciprocal theorem is the basis of the reciprocity method for the absolute calibration of microphones and other electroacoustical instruments. Such theorems are commonly found in the mechanics and acoustics of gases, liquids. and solids, in electroacoustical systems, in electric circuits, and in the propagation of electromagnetic waves. The earlier invention of the condenser microphone facilitated the invention of the reciprocity method in 1940. It was then developed and eventually standardized as a very accurate and convenient method for measuring the response of a microphone in both amplitude and phase. The first applications were to the sound pressure calibration of microphones used at audio frequencies in air. Earlier calibration methods were based essentially on standard sources such as the thermophone, pistonphone, and electrostatic actuator, and on the use of a Rayleigh disk. Systematic differences between these led to a search for a more accurate method, and discovery of the reciprocity method. During the war years (1940–45) it was applied to the calibration of underwater hydrophones, and shortly thereafter to the calibration of accelerometers. The extension to freefield calibrations led to problems still present today, and for which we offer a solution.

Complex hydrophone calibrations using pseudo‐Gaussian noise signals

A computer‐based hydrophone calibration system has been developed for the measurement of complex receiving sensitivity over a wide frequency band in an open water acoustic tank. A pseudo‐Gaussian number sequence is generated for transmission into the tank via a digital‐to‐analog coverter, filter, power amplifier and wide‐band projector. A cross spectral analysis is performed between the sampled hydrophone output and the corresponding number sequence, averaging over several independent noise records. This process is carried out both for the “unknown” hydrophone and a “standard” hydrophone using the same pseudo‐Gaussian noise records and the same hydrophone position in the noise field. The amplitude and phase response of the unknown is calculated relative to the standard, which may be one hydrophone of a group to be calibrated or may be a calibrated reference standard. The system is usable over the frequency range 20–9000 Hz.

Avoiding the effect of nonuniformity of field during low-frequency calibration of hydrophones

Avoiding the effect of nonuniformity of field during low-frequency calibration of hydrophones

Noise isolation in an infrasonic hydrophone calibrator

Suspending a heavy hydrophone calibrator from pneumatic springs produced an order of magnitude improvment in isolation of the calibrator from strong sources of noise in the 5‐ to 10‐Hz frequency range over that provided by a cork‐nested foundation.

Hydrophone calibration in a chamber with active-impedance termination

Hydrophone calibration in a chamber with active-impedance termination

A Shipboard Hydrophone Calibrator

A hydrophone calibrator has been developed by the Naval Research Laboratory to calibrate hydrophones as large as 12 cm in diameter by the comparison method in the frequency range 25–1000 Hz at sound pressure levels from 130 to 190 dB re 1 µPa. Thus, it is possible to calibrate a hydrophone immediately before deployment and immediately after recovery. The calibration can include the complete electronic measuring system as well as the hydrophone. Vertical stabilization of the calibrator for ship roll and pitch of 10° and isolation from ship's noise are provided.

Calibration of hydrophones : Levin, P.A., Bruel & kjaer technical review1 (1973) 3–17

Calibration of hydrophones : Levin, P.A., Bruel & kjaer technical review1 (1973) 3–17

Steady-State Transducer Calibration in a Reverberant Water Tank

A nonanechoic water tank can be used for transducer calibration in either a pulsed “free-field” or steady-state “diffuse-field” mode. The diffuse-field mode may be preferred if the desired output is a random incidence calibration (e.g., for a hydrophone to measure noise in a diffuse environment). The diffuse-field mode may also be a desirable alternate technique for nondirective transducers. We have evaluated some of the sources of calibration variance for the BBN reverberant water tank (dimensions 14×22×33 ft) both for small, nondirective transducers and for transducers with DIs of approximately 20 dB. The degree of diffusivity at high frequencies has been measured using a directive hydrophone. Both secondary (comparison) and primary (reciprocity) hydrophone calibrations will be discussed.

Plane-Wave Simulator in a Mechanically Driven Standing Wave Hydrophone Calibrator

A method has been developed to provide plane-wave operation in the Mark III pressure gradient hydrophone calibrator [Wide Range Calibration System for Pressure Gradient Hydrophones, B. B. Bauer, L. A. Abbagnaro, and J. Schumann, J. Acoust. Soc. Amer. 51, 1717 (1972)]. Since this calibrator uses a mechanical drive rather than a transducer drive to establish the standing wave field, conventional time delay techniques could not be employed. Rather, two loudspeakers were used in conjunction with the calibrator drive to achieve a plane wave at the center of the tank. A simple operating procedure is described for using the plane-wave mode, and some experimental results are presented.

Investigation of the Parametric Radiator as a Wide-Band Calibration Source

The parametric difference frequency radiator appears to offer advantages over conventional sources used for hydrophone calibration, i.e., wide bandwidth with a single transducer, narrow beamwidths, and short pulselengths. The combination of these characteristics promises simplification of calibration procedures and elimination or reduction of boundary reflection problems. Experiments are described which measure the performance of the parametric source for calibration purposes and which point out some of the special problems inherent in this new technique.

Multipath Interference Smoothing in Hydrophone Calibrations

Development of a computer-controlled calibration system established the need for a simple digital technique for smoothing the fluctuations produced in hydrophone calibration data by interfering signals. The source of the interference is signals arriving at the hydrophone by paths other than the direct one when low-frequency sound is transmitted between a projector and hydrophone closely spaced in shallow water. The simple method of a running mean is remarkably effective, within certain limits, if the span or “window” of the mean is chosen carefully. Fourier analysis of typical data samples confirms the method of choosing the smoothing windows.

Wide-Range Calibration System for Pressure-Gradient Hydrophones

A gradient hydrophone calibrator consists of a precisely made water-filled tank, with ports to receive the hydrophones to be calibrated and the reference pressure hydrophones, which is set into axial oscillation with suitable drivers to develop a standing wave of sound in the water. An improved calibrator is described, which, by using a very rigid tank, covers a range of 3–2500 Hz. Measurements are simplified through use of electrical feedback, which maintains a constant pressure-gradient versus frequency function. The applications and limitations of this system to the measurement of sensitivity, frequency response, and phase response of pressure-gradient and pressure hydrophones, and for obtaining the polar patterns of gradient hydrophones, are described.

Calibration System for Pressure and Pressure Gradient Hydrophones at Infrasonic and Audio Frequencies

A complete measurement system for the calibration of pressure and pressure gradient hydrophones over the frequency range 5–3000 Hz with static pressures to 1000 psi is described. Measurement of hydrophone amplitude, phase, and directional response to a plane progressive wave is made within a stiff-walled cylindrical sound channel terminated by active transducers. Sound-pressure levels to 140 dB above 1 Pa may be employed. Adequate plane-wave operation in the infrasonic region is assured by the separation and individual alignment of the even and odd pressure distributions that compose a plane wave. Each wave component is monitored by a symmetric distribution of pressure responsive probes in an amplitude sensitive alignment technique. An analysis of required pressure field tolerances and subsequent termination transducer drive tolerances is discussed. The low frequency limit of the system is shown to be a direct function of the field tolerances and sound channel length. Two complete systems have been fabricated, and data have been collected on a large number of moving coil gradient hydrophones.

Vertical Hydrophone Calibrator for Laboratory Use

A vertically oriented calibrator has been developed for use with gradient and pressure hydrophones. The calibrator is comprised of a water-filled tank suspended to permit motion in the vertical direction. A mechanical drive generates oscillatory motion in the vertical axis and produces pressure and velocity standing waves suited for the calibration of pressure and vertically directed gradient hydrophones. A description of the calibrator and the measuring techniques utilized are discussed.

A Portable Hydrophone Calibrator

A portable hydrophone calibrator with associated electronic circuitry is described for calibration of a wide variety of hydrophones. The calibrator covers the frequency range from 1 Hz to 3.5 kHz and simulates operating depth by means of hydrostatic pressure to 1000 psi and temperature variation from 4°C to room temperature. The calibrator is a closed chamber in which a sinusoidal pressure is produced by a piezoceramic tube that forms half of the cylindrical wall. A second piezoceramic tube forms the other half and measures the pressure in the region of the hydrophone. A step pressure establishes the absolute calibration of the system.

Velocity Hydrophone Calibration

Measurements show that the standard 10-dB maximum correction factor for particle velocity augmentation in a spherically divergent sound field can safely be increased to 25 dB.

Interference Filtering in Hydrophone Calibrations

A method is suggested for filtering the interference signal that is superimposed upon the direct signal when low-frequency sound is transmitted between a projector and a hydrophone closely spaced in shallow water. The simple digital technique of a running mean is remarkably effective, within certain limitations, if the span or “window” of the mean is chosen carefully. The method lends itself to machine processing of data from automated calibration systems. Fourier analysis of typical data samples confirms the method of choosing the smoothing window.

Hydrophone Calibrator

The USA Standard procedure for calibrating a hydrophone at low frequencies is the two-projector null method in which the two projectors are electrodynamic. A. N. Golenkov, USSR, developed a calibrator on the same principle using two piezoelectric projectors. A new calibrator, using piezoceramic cylinders for projector and receiver, has been developed that eliminates the null balance requirement and thus simplifies the calibration procedure. The reference is again a step change in water head achieved by a two-way valve that connects one manometer and then a second. An absolute calibration of a hydrophone can be made up to 4 kHz and for hydrostatic pressures from 0 to 2000 psi.

Generating and Calibrating a Uniform Aquatic Sonic Field for Behavioral Bioacoustics

The use of two matched projector faces as the ends of a small tank enabled the generation of a uniform aquatic sonic field. The projectors were operated 180° out of phase to maintain a push-pull system. Shock electrodes and a dividing barrier of rubber, acoustically transparent in water, made the tank an aquatic shuttle box for avoidance conditioning. In addition to applications for behavioral bioacoustics, the system can be employed for calibration of small hydrophones. [Research supported by NINDB and ONR.]

Equipment for reciprocity calibration of hydrophones in a free field

Equipment for reciprocity calibration of hydrophones in a free field