National Physical Laboratory

Hydrophone Mounting

Hydrophones 

There are a number of important factors that can influence the performance of a hydrophone under measurement conditions. One of these factors is the method by which the hydrophone is mounted.

The effect that a mount or rigging can have on hydrophone performance can be split into three main categories:

  1. Perturbing effect on the acoustic field: If the mount significantly perturbs the acoustic field, acoustic reflections from the mount may arrive at the hydrophone element during your signal measurement window, compromising acoustic free-field conditions. Typically hydrophones with small device elements and short bodies are likely to be more susceptible to such influences. Ideally, the mount should be designed to be as non-perturbing as possible.

  2. Mechanical influences: In addition to perturbing the acoustic field, the mount may modify the performance of the hydrophone in other ways. For example, excessive clamping forces can exert stresses and strains on the hydrophone body, altering the hydrophone performance. If a non free-flooding tube is used, the air trapped in the tube above the hydrophone body may scatter the sound and influence the performance. Also, some mounts may exhibit resonances of their own in the frequency range of interest, or may couple vibrations to the hydrophone from the support structure around the test tank. All of these effects are undesirable and should be avoided wherever possible.

  3. Alignment/positioning: Poor alignment will affect the response of a hydrophone, the degree of influence depending on its directional response at the measurement frequency. Misalignment may be caused by the use of a mounting pole that is not straight, leading to an error in hydrophone position and potentially an error in hydrophone orientation.

Properties of a good hydrophone mount

A hydrophone A number of mounting methods are in use for hydrophones, ranging from free-flooding tubular poles made from carbon-fibre, metal or plastic, to miniature clamping brackets locating around the hydrophone body, sometimes custom designed to fit a particular hydrophone model. In general, the mount should possess enough rigidity and strength to enable accurate positioning and orientation of the hydrophone, but should perturb the acoustic field as little as possible and introduce as few artefacts into the hydrophone response as possible. Some of these requirements may conflict, and inevitably a compromise must be reached. It is generally desirable to make the hydrophone mount free-flooding so that the possibility of trapping air is minimised.

Reporting of calibration results

Since some hydrophones may be sensitive to the mounting method employed, it is recommended that the mount be viewed as an integral component of the hydrophone such that any calibration is regarded as representative of the hydrophone-mount combination. Permanent alignment markings should be made on the hydrophone body to ensure consistency in alignment between measurements, and the hydrophone mount should have corresponding markings to enable the hydrophone and mount to be correctly aligned.

Examples of the effect of different mounts

To highlight the adverse effect an inappropriate mount can have on hydrophone sensitivity, a calibration was performed of a B&K 8104 hydrophone. Two mounts were used, a free-flooding carbon fibre pole and a more cumbersome steel clamping bracket. The graph below illustrates the influence a poor mount can have on the sensitivity with variations in sensitivity of up to 2 dB observed. In this case, it is likely to be caused by reflections from the large steel bracket.

Underwater Acoustics Tech Guides Hydrophone Mounting Chart 01 - The influence a poor mount can have on the sensitivity

The plot below shows the results of an additional experiment where a Reson TC4023 hydrophone was calibrated using mounts constructed from different material. Five mounts were used in total, these being: carbon fibre tube, aluminium tube, nylon tube, 4 mm steel rod and 16 mm steel rod. The results of this experiment are shown in the graph below, each curve being the mean of four independent measurements with the hydrophone re-mounted between repeats. Typically, the standard deviations of the measured data were 0.1-0.2 dB. All data has been normalised to that of the carbon fibre mount.

Underwater Acoustics Tech Guides Hydrophone Mounting Chart 01 - Using mounts constructed from different material

For further information, contact Justin Ablitt

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Last Updated: 30 Mar 2012
Created: 28 Oct 2010

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