1. The link between OCR measured in Cal tank and Pistonphone OCR at 250Hz

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1 1. The link between OCR measured in Cal tank and Pistonphone OCR at 25Hz This tech-note explains the calibration approach and ties the link between OCR (Open Circuit Voltage Response) of the hydrophone and the pistonphone (25Hz PHO) OCR value on the cal plots What makes linearity of a hydrophone Linearity comes from two facts. There are no mechanical resonances in the frequency range of interest. 1) Sensor element runs like an off-resonance receiver. 2) The wavelength is much larger than sensor. At 1Hz the wavelength is (15m/sec)/ 1Hz = 1.5m. At 25Hz wavelength is approximately 6m. A hydrophone sensor tip is small for instance 1.5mm x 1.5mm on TC435/38 or 6.4mm x 6.4mm on a TC TC433-1 Impedance Summary -18 TC433 OCR [db re 1m] Ω Frequency [khz] Resistance Reactance Impedance TC433 as example resonance around 9 khz. The 9 khz resonance of the ceramics is the lowest resonance present in the hydrophone. Below 8 khz it runs completely off-resonance Frequency [khz] TC433 as example resonance around 9 khz and anti resonance at 1 khz. OCR peaks at point of anti resonance. Below 8 khz it runs off-resonance. OCV 1.2. The Acoustical Calibration Facility at The acoustical calibration tank at is 4.5 meter long, 3meter wide and 2.5 meter deep. The walls of the calibration tank are made of concrete (solid structure no absorbing materials added). Gated measurement techniques are used to eliminate effect of reflections from walls. For the gated technique to work transmit pulses can only be smaller than around 2.5ms - if transmit pulse becomes too long it will get reflected on the walls and interfere with the primary signal going out from the reference projector. Figure 1: The acoustical calibration tank at, gated measurement techniques are used to eliminate effect of reflections from walls. 1/5

2 The calibration facility can transmit CW pulses only. When calibrating hydrophones the transmitter module built into the ACS96 Cal Box is used as a transmit stage. The setup is shown below on Figure 1 and Figure 2. The trolleys of the calibration tank can handle a sonar head weight up to 13kg. Figure 2: Sketch of the calibration tank. The red pulse is 2ms long 1 cycles at 5 khz. The blue is 1 cycles at 2 khz. Pulse lengths are in scale with dimensions of the tank. Sound will propagate spreading spherically from the sound source or projector element - sketched by the black dashed lines. The sound hitting the walls will be reflected shift direction and in case of very long pulses it will interfere with the sound propagating along the direct path between the sound source and the hydrophone to be calibrated. The gate is a time window placed in the pulse where the measurement is done. To get a reliable measurement the response of both the transmitter (projector plus amplifier) and the receiving hydrophone need to be steady state. Much below 5 khz it becomes difficult to make reliable measurements in the calibration tank at RESON because of the short pulse required to eliminate interference from reflections (up to 2.5ms). The short physical pulse will contain only very few cycles and steady state response never may be obtained. Whether steady state will occur strongly depends on the characteristics of the reference projector and the hydrophone to be calibrated. RESON can obtain the following accuracy on the acoustical measurement made in the tank. Frequency interval Uncertainty of measured value 2-5 ±1.8dB 5-2 khz ±.8dB 2-5 khz ±.7dB Above 5 khz ±1.3dB Data are stored in database and accessible via unique serial numbers as identifiers for the senor elements (transducers and hydrophones) and array assemblies for Multi-beam sonar s. Data can be extracted from the cal database and exported to excel. To fill the gap below the 5 khz data point pistonphone measurements are used to validate and calibrate hydrophones. 2/5

3 1.3. Pistonphone calibration at 25Hz The principles behind piston-phone calibrations are quite simple. Pistonphone calibrations take place at a low frequency. A pistonphone setup typically uses a sound source operating at 25Hz source that is based on oscillating pistons with a fixed displacement. The hydrophone is placed in small airtight container, called a pistonphone coupler, with two out/inlets, the piston based source (B&K type 4223 or 4229, or G.R.A.S. type 42AA) attaches to the inlet and optional a calibrated microphone and sound level meter (for instance B&K type 2239) attaches to the outlet. The optional microphone is used to measure the sound pressure level inside the container accurately. By tying the sound pressure level measured with the calibrated microphone together with the voltage output of the hydrophone it is possible to calculate the OCR (receiving sensitivity) of the hydrophone at 25Hz with a very high accuracy. Typical sound pressure in TL889 (pistonphone coupler for TC432) with the TC432 volume inserted: - SPL with G.R.A.S. piston-phone type: 42AA: 138dB re 1µPa - SPL with B&K calibrator type: 4223 or 4229: 156 db re 1µPa - Accuracy: ±.3 db (relative to nominal hydrophone volume). Notice the pistonphone coupler is designed so the air cap around the sensor element is as uniform as possible. This ensures a good and efficient coupling and a response of the hydrophone that is representative for a free field response. A pistonphone calibration utilizes the fact that the coupling between air and the hydrophone sensor element is about the same as the coupling between the hydrophone sensor and water at low frequencies, 25Hz included. Thereby it is possible to make valid calibration with a much simpler setup in air. The pistonphone sensitivity is shown on the OCR plot as 25Hz value. G.R.A.S. sound source and piston-phone coupler TL889 for TC432 is shown below on Figure 3. Figure 3: G.R.A.S. piston sound-source and RESON pistonphone coupler TL889 for TC432. 3/5

4 Figure 4: Sketch of piston-phone calibration setup. Sketch above shows a TC432 mounted in the coupler. The coupler is a small sealed container (the hatched area) that mates with the sound source, the hydrophone and the reference microphone used to measure the sound-pressure inside the coupler The Cal plot with PHO Figure 5: OCR response of TC432. The receiving sensitivity at 25Hz is shown as 25Hz: dB re 1µPa, see the red circle. Customers get per default the plot above. 4/5

5 Figure 6: OCR response of TC432, 25Hz is the OCR value measured by the pistonphone setup. OCR response of the hydrophone shall centre on the PHO value at 25Hz Interpolation of data in the Hz to 5 khz range OCR at any given frequency (f [Hz]) between Hz and 5 khz is defined via the PHO value at 25Hz and the OCR measured in the test tank at 5 khz. If the 25Hz sensitivity value matches sensitivity value at 5 khz within tolerances of the calibration a linear interpolation approach can be used to estimate data points in between. The interpolation approach is possible because of the linearity given by the hydrophone design. OCR ( f ) OCR : = ( 5kHz) PHO(25Hz) 5kHz 25Hz f + y y : = PHO(25Hz) 5kHz OCR(5kHz) 25Hz 5kHz 25Hz Notice: High pass filters will alter the response below the filter point. 5/5