Hydrophones (Process Analysis)
A process is an activity which occurs over time and has clear goal relating to the outcome to be achieved. It's hierarchical because may consist of sub-processes (Muller 1). Indentifying boundaries is important when understanding a process i.e. its beginning and end. The beginning of a process is when a trigger causes some action to be taken by a group, person or another process. On the other hand, ending takes places place when the outcomes are passed on to a group, process or another person. Hydrophones they are low frequency piezoelectric underwater acoustics transducers. In other words they are microphones that operate under water (Dolphinear.com 1). They are designed from special materials called piezoelectric because sound transmission through water is different from transmission through the air (Caruthers 16). Piezoelectric materials generate small voltages when agitated or deformed.
Hydrophones work on the principle that when a sound wave from a particular source hits the piezoelectric material, it builds-up stresses on the material which are afterward transformed into a signal (electrical). The produced signal is then amplified ready to convey into a set of earphones, external speaker or a tape recorder. One thing to note about the hydrophones is they have to give the same frequency response which means they produce equal voltage amounts per the quantity of acoustic agitation despite the frequency. To achieve this, piezoelectric material is mounted on a backing with the same or higher acoustic impedance and the end it with high electrical impedance. With these boundary conditions, the piezo outputs the voltage which is proportional to pressure input. This technique results in the desired flat frequency response. Low frequency performance of the hydrophones tends to be limited by the electrical load impedance leading to a simple RC time constant. Hydrophones are normally used below their resonance frequency and a wide frequency band where uniform output levels are produced.
For proper functioning of the hydrophones, they must be able to tolerate shipboard abuse; they should also be capable of resisting corrosion, bio-fouling and also high hydrostatic pressure. Reliability is so essential with hydrophones because frequent inspections cannot be achieved. It is a good practice to shield hydrophones from stray electrical fields which is done through using a screen-like metal grid which is made to surround the sensing element and shield the preamplifier. Modern hydrophones use independent internal shielding for the sensor element and the auxiliary circuits. The external shield is electrically insulated from the metal housing which is water-exposed. In essence, preamplifier drivers are preferred to the cheap transformer-coupled systems because the transformer-coupled systems have more limited frequency responses which is a desirable element when designing a reliable and a functional hydrophone. For a well designed hydrophone, a precision of 10-ohm resistor in series with a sensor for remote calibration purposes is provided (Caruthers 52).
Caruthers, J. Fundamentals of marine acoustics. Elsevier, 1977.
Dolphinear.com. Underwater microphones - hydrophones, 2000. Retrieved February 27, 2010, from http://www.dolphinear.com/data/underwater_microphones.htm
Muller, G. What is a Process? , n.d. Retrieved February 27, 2010, from http://www.gaudisite.nl/WhatIsAProcessPaper.pdf
