Why use ultrasound instead of regular sound?
The primary reason is because a highly directional receiver can be built in a size that is easy to carry. Think of the large parabolic dish microphones on the sidelines at football games which are used to pick up the sounds from the playing teams. One could design a system that would home in on a specific audible sound, but such a device would have to be several feet in diameter and would not be easy to carry at a fire scene. In order to make the Tracker receiver about 3 inches in diameter, the sound frequency needs to be around 40 kHz. A second reason for the use of ultrasound is that a fire can put out a lot of lower frequency audible sounds. However, there is not much sound output from a fire near the ultrasonic frequencies used by the Pathfinder system. Consider how high-end stereo systems are designed. A typical surround-sound system will have five small speakers for the high frequency sounds, located around a room. There will be only one sub-woofer for the low frequency sounds, w
