Many hydrophones are best calibrated at a single low frequency or broadband at low frequencies. In some instances, though, where the frequency response of a hydrophone must be characterized over a wide frequency range, the calibrations must be performed in water using relatively complex hardware and software. Based on its decades of experience, GeoSpectrum developed its own portable system, the Portable Acoustic Calibration System, or PACS.
Key features of this system are:
Portable: GeoSpectrum’s in-water calibrator is contained within one shock-mounted, splash-proof Pelican case. Ancillary equipment, such as sound projectors, reference hydrophones, cables and supplies are contained in a second Pelican case. The system is small enough and light enough that it can be carried by hand and used on a small boat, thereby enabling you to perform acoustic calibrations when you wish, where you wish, and at low cost.
Powerful: Projectors can be driven with up to 600 V and 1 kVA (pulsed). The system is powered by 120/220 VAC single phase. You can either plug this system into the mains or use a high-power, pure-sine DC-AC converter.
High frequency: The DAQ simultaneously samples all channels at 2 MHz so the upper limit on frequency is almost always the reference hydrophone. No anti-aliasing filters are required.
Low frequency: The custom-built, variable-ratio step-up transformer for the power amplifier uses a high-performance core that enables powerful operation even at 20 Hz or lower.
Key software features: GeoSpectrum has used dozens of different calibration systems. Our software combines the best we have seen of other systems and many features that were lacking.
Multiple frequency ranges: The frequency range can be sub-divided into as many sub-regions as you desire and all calibration parameters can be selected for that sub-region. As an example, you can choose different measurement windows, projector drive voltages, frequency decrements, and number of averages for each frequency range. Such control results in higher quality calibrations.
On-the-fly plotting: The plots of TVR, SPL, Admittance, and Receive Response are displayed during the ping sequence. You do not have to wait until the end of a calibration to find out that you have made a mistake. You can catch mistakes as they appear and abort the calibration, thereby saving time.
Near-field corrections for directional hydrophones: The software makes the appropriate near-field corrections for directional hydrophones. Without these corrections the phase and amplitude responses are seriously in error, particularly at low frequencies.
Secure: The software can be instructed to write data only to a portable hard drive or memory stick (intermediate results are in memory only). Thus this software minimizes security concerns when doing classified or commercially-sensitive calibrations.
Ease of use: The user-interface to the software is straight forward and minimizes the opportunity to make errors.
Comprehensive manual: The system comes with a comprehensive manual which will be sufficient for those already experienced and skilled in calibrations. For all others, GeoSpectrum offers training.
GeoSpectrum has used this system since the summer of 2010 to calibrate hydrophone and sound projectors from a variety of platforms. Contact us to find out how the GTI PACS can help you.