However, unanticipated pitfalls can become apparent very quickly that can render even the best system useless. Professional systems are available, some at very high cost, and can be very effective. Code-division multiple-access (CDMA) has been used for many years by the military for covert and secure communications, since it is highly immune to interference, carries a high level of inherent security and can be made to appear indistinguishable from ordinary broad-band noise. The detection of an obvious RF source may be undesirable and encryption or frequency-hopping may be required to avoid interception. A brief description of an inexpensive and effective hydrophone is given near the end of this page.Ī radio-link can be useful but can suffer from disadvantages arising from weak signals, break-through, etc, especially if the source is mobile or the working environment is unknown or untested. Sub-aqua applications have been seen where an (expensive) microphone is sealed within one or two condoms and then thrown into the sea. Ideally, a complete system can be pulled out of a river, the water shaken off and be used immediately. Rugged water/weather-proofing will be vital for external use.
At the same time, monitoring of an output may be required. A choice of power source will be necessary, eg mains, battery or both. Electret or FET capsules, although offering higher noise, are far more sensitive (a chart comparing typical replacement capsules can be found at the bottom of the page). A magnetic dynamic type may suit most applications offering a low internal noise figure. An omnidirectional type will pick up sound from all directions, whereas an unidirectional or cardioid type will exhibit a directional characteristic. The choice of microphone itself can be important. When designing a sound pickup system a number of elements must be decided from the outset. Proprietary systems can cost hundreds or even thousands, but surprising results can be obtained at a fraction of such costs. In such circumstances, hi-fi quality though possible may not be as important as proximity or placement in respect to the required source. However, situations can arise for which other solutions are required, say for example, if a microphone is to be worn, or if a miniature, high sensitivity or discrete approach is required. The dynamic microphone, very similar in form and function to the conventional loudspeaker, is the industry standard for most applications and is seen most frequently in the hands of a singer or presenter.
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