LISTENING DEVICES IN HEAVY SEARCH AND RESCUE
Seismic/acoustic “listening” devices, which are relatively new tools for the fire service, provide visual and audible indication of signals from live victims trapped in the rubble of collapsed buildings, trenches, and shafts. Some versions allow four or more sensors to be used simultaneously and are capable of picking up signals over a frequency range from subsonic/seismic frequencies up to the audible range, covering the total frequency range (1 Hz to 3 kHz) with one type of sensor. These devices can he useful for collapse rescues when employed in conjunction with proper operating procedures and incident command strategy.
When setting up communications at an incident where listening devices are being used, provisions must be made to ensure that the listening device operator will be able to hear others around him or her. If soundattenuating headsets arc used, they should not cover both of the operator’s ears. Frequencies and procedures for using the radio must be agreed on in advance, and a liaison with the incident command post must be appointed.
The effectiveness of listening devices varies with the situation and depends on numerous variables, including the materials surrounding the victims.
SOME GENERAL GUIDELINES
The following points should be considered when engaged in operations involving sound/search techniques:
- Low frequencies travel farther and are less attenuated than high frequencies.
- The more energy put into the signal, the farther it will travel. Knocking with a heavy rock carries farther than scratching. Knocking contains many frequencies. If detected close to its source, it still retains many of its higher frequencies and yields a “bright” sound. The higher frequencies become attenuated as they get farther away, and the sound will be
- muffled and more difficult to hear.
- Scratching contains mostly higher frequencies; knocking with anything, even the fist, probably will carry farther.
AIRBORNE VS. STRUCTUREBORNE SOUNDS
Sound outside of a collapsed structure travels much easier into the rubble structure than sound emanating from the inside and traveling out, probably because of the way the sound migrates in the air spaces between the rubble pieces. The sameholds true in cases of avalanches: The victim under the snow hears the rescuers quite well, but the rescuers cannot hear the victim. The sound created inside quickly encounters obstacles in its path; the sound from outside the avalanche or collapsed structure can spread first and then find ways to get into the structure. It therefore is quite probable that a victim will hear the verbal instructions shouted from the outside or amplified through a bullhorn and will respond.
Since structurcborne sound travels much farther and better than airborne sound, victims should be asked to knock rather than yell, liven w hen the airborne sound is not capable of getting into the structure and reaching the victim, knocking three times with a heavy rock against the outside of the structure or parts of it —pipes for instance—usually stimulates the victim to knock three times.
Sound carried by air and able to come out from the collapsed structure will be heard by the rescuers. Because of the surrounding interference from equipment, traffic, wind, etc., directional microphones and straight sound amplifying devices are of limited help (see sidebar opposite). Small microphones (available with some systems), especially when combined with a small speaker, can be used effectively to access voids and basement areas. They are especially useful for establishing voice communications with the victim who has been located through other means to determine information such as identity, address, and whether other victims are present. The microphones should be small so that they can be lowered into holes drilled into layers of a collapsed structure. Voice contact is most valuable in situations involving young victims or panic.
In large structural collapses of solid buildings where no airborne sound usually is picked up, listening devices can help detect knocks—which are carried for long distances in a solid structure —that cannot be heard on the surface. These instruments are capable of picking up the subsonic low frequencies as well as frequencies in the audible range.
MATERIALS’ ROLES IN CONDUCTING SOUND
Assume that the more solid the material, the better it will carry the sound and the less attenuation the sound will suffer. The sound propagates through differing modes of oscillation. Hie surrounding material also influences how a signal travels. A beam buried in loose sand will not carry structureborne sound as far as one in air or gravel. Following is a rough ranking of materials according to their sound-carrying abilities:
Excellent: steel, unbroken concrete, solid brick, glass.
Good, loose concrete, rubble brick, gravel, wood.
Fair: wet soil, compacted soil.
Poor, dry sand, snow, acoustic tile, carpet, fiberglass.
SENSORS
Device sensors rely on receiving the mechanical vibrations of the structure. This means the better the mechanical contact with the structure, the larger the signal. Mechanical contact can be made in various ways. Sensors can be attached to iron structures with magnetic holders. Spikes attached to the sensors are very effective in soil or gravel and also in crevices or holes, where the spike can be pushed or wedged in and the sensors attached in this way. Spikes can hold a sensor solidly against the structure through the weight of the sensor alone.
Good sensors are omnidirectional and not position-sensitive. This means they can be placed in any position — horizontally, vertically, or even upside down—and will pick up signals regardless of their sources. Alternately, the sensor can be held with clamps, wedged into a crevice, or covered w ith a piece of rock or concrete.
The distance between sensors depends on the construction material of the structure or rubble pile involved and the sections of material through which the structureborne sound is expected to travel. Interfering signals could lead to a further reduction in sensor spacing. Spacing between sensors should not exceed 25 feet in any case. Usually 15-foot space intervals cover the area adequately, even under difficult circumstances.
Trying to locate a victim with onlyone sensor is difficult because the rescuers would have to remember the amplitude and clarity of the signal from sensor location to sensor location. Being able to compare several sensors and to switch from one to another allows the operator to quickly identify the sensor with the largest and/or clearest signal. If a signal is detected, it is generally advised that the sensor emitting the signal he left in position and that the sensors surrounding it be repositioned for more accurate determination of the location. The more sensors available, the larger the area that can be searched and the quicker a victim can be located.
The patterns used in searches are determined by the number of sensors and helpers available; the type of collapse and structure to be searched; and the accessibility, safety, and number of interfering signals present.
The majority of collapse sites will be made out of inhomogeneous materials. requiring that the larger structural parts be accessed or that the sensor be placed on similar materials instead of working with theoretical search patterns and assuming equal sound distribution and attenuation. The “stereo effect” used in some listening devices is effective only if homogeneous materials are present.
Practical experience in the use of listening devices is limited, since they are not widely available to rescue crews. However, they have gained acceptance as a tool for urban search and rescue and are being considered by a number of departments.
(Photos by author.)
