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Hydrophone Handbook

      Frequently Asked Questions

 What is the difference between an HMA and HMB
 hydrophone?
 The HMB is an HMA back-filled with absorbing material. The
 backing dampens vibrations of the membrane which may occur
 during scanning. Free membrane hydrophones are susceptible
 to this issue, and it is commonly suppressed either by waiting
 for the vibrations to dampen before taking a measurement, or
 by using cross-correlation to filter them out. Because both of
 these solutions require additional time, the HMB is a good
 option for setups that use motorized scanning.
 The only other significant performance difference between the
 HMA and HMB designs is that the HMB is more sensitive. As
 mentioned in our datasheet, the HMB-0200 is about 4 dB more
 sensitive than the HMA-0200.
 How do I trigger off my ultrasound Device Under Test (DUT)?
 To synchronize the hydrophone measurement with the
 generation of the acoustic pulse from the DUT, it is best to
 trigger off an electrical signal from the transducer. Most
 ultrasound manufacturers can supply information to access this.
 An alternative method is to use an auxiliary hydrophone to
 detect the acoustic signal if an electrical trigger is not available.
 There should be some understanding of the acoustic field to
 help set up the acoustic trigger. For instance, a needle shaped
 hydrophone should be used to minimize any interference
 effects. Furthermore, the hydrophone should be positioned to
 minimize any interference effects while oriented to have
 sufficient sensitivity. It should also be noted that an acoustic
 trigger may be more suitable for certain types of transducers
 than others (e.g., curved arrays), as it is best to position it in-line
 with the firing direction.

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