USING LISTENING DEVICES UNDER SPECIAL CONDITIONS
Following are some conditions and equipment that can create interfering signals that overload the device and mask the signal from the victim.
Wind. Cover sensors and place them inside the structure if possible. Do not attach sensors to beams extending out of the rubble pile. Weigh down connecting cords to prevent their vibration from carrying into the sensor.
Rain, sleet, hail. Try to protect sensors from the direct impact of the raindrops. Use cloth or noncontacting wood. The water droplets will appear to be an increase in noise or a sound similar to an engine running. Decrease the sensor distance to compensate for this condition.
Running water. Sometimes similar to rain or vibrations.
Airborne sound. Usually its interference is not critical. The sound would have to get into the structure involved in the search and the sensors; in the latter case, the sensors would receive the interfering signal greatly attenuated.
Vibrations. These can come from machinery and running motors. Depending on the amplitude and location of the interfering source, they can mask the signals from the victim. If the test equipment has adjustable filtering, cutting off the low’ frequencies usually reduces the interference. {Note. Except for the very sharp line frequency notch filters, all listening should be done first in the unfiltered, wideband mode so that no signals such as scratching and crying are overlooked.)
Diamond saw. Used to cut concrete, the saw produces signals of higher frequencies. They can be filtered out by cutting out the high end of the frequency band being analyzed. Knocking by the victim still can be detected.
Jackhammer. This tool creates the most disturbing interference. If used on the same structure, it is impossible to detect sounds or knocks from the victim. An arrangement to cease the use of the hammer and similar tools must be made for the duration of the listening time.
Work on neighboring sites. W ork on nearby sites not connected to the structure being searched by solid materials such as concrete and pipes usually does not present a problem.*
