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FAQ: Ultrasound Questions
7. What is ultrasound?
“Ultra” means “high.” Ultrasound is simply sound at a frequency that is above the normal human hearing range. A person can hear sound up to about 20,000 cycles per second (20 kilo hertz, or 20 kHz). As people get older, the top of their hearing range decreases to about 10 - 15 kHz. The Pathfinder™ system uses ultrasound at about 40 kHz.

8. How is ultrasound like regular sound?
Ultrasound behaves very much like audible sound. It travels through air as a pressure wave and is reflected off walls and other solid surfaces. Ultrasound will penetrate porous materials like cloth because of the air spaces between the cloth fibers. For example, ultrasound will go right through a thick terrycloth towel. A good way to determine if ultrasound will go through a material is to cover your ears with the material and see if you can hear sound.

9. Can dogs hear the ultrasound from the Pathfinder™ system?
No. Dogs hearing stops at about 30 kHz, well below the 40 kHz frequencies used by the Pathfinder system.

10. How is ultrasound different from regular sound?
The only difference is that the frequency is above the normal hearing range.

11. Does smoke affect the ultrasound?
No. Unlike light, which is scattered by the smoke particles, ultrasound in the 20-100 kHz range is unaffected by smoke.

12. Does water spray affect the ultrasound?
The ultrasound wave is passed by the air molecules between the droplets of water. As a result, ultrasound is unaffected by water spray.

13. How high a frequency could you use?
For location and navigation systems up to a distance of 200 feet, the ultrasonic frequency should be below 100 kHz. The Pathfinder system uses several frequencies around 40 kHz.

14. 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, which can be put in almost any part of the room. The reason for this is that a person cannot distinguish the direction of low frequency sounds. The direction of only the higher frequencies can be sensed. By using very high frequencies (that is, ultrasound), the Tracker uses this principle to have a small-sized receiver be very directional, so that the direction of the Beacon can be sensed.

15. Is ultrasound similar to sonar?
Sonar, which stands for “sound navigation and ranging,” uses sound or ultrasound in water. The Pathfinder system is very much like a passive sonar system (one submarine listens for the sounds of a second submarine). The Tracker listens for the sound being transmitted by the Beacon.

16. Is ultrasound dangerous?
No, ultrasound is very safe. Ultrasound is used by medical imaging systems to make pictures of internal organs and even unborn babies.

17. Is ultrasound a new technology?
No. Ultrasound has been used for more than a hundred years in sonar and other underwater applications. Furthermore, ultrasound has been used for almost fifty years in medical imaging systems.

18. Do any fire sounds interfere with the operation of the Pathfinder system?
No. Fires can produce a significant amount of acoustic noise. Much of this noise is in the audio range, and only a small amount reaches the ultrasonic frequencies used in the Pathfinder system. Since a Beacon in alarm mode transmits a continuous ultrasonic signal at a very specific frequency, the Tracker looks for this continuous signal and ignores all signals that are not at the correct frequency. As a result, a fire may occasionally give a small “ping” of ultrasound – which can cause the LED bar graph to momentarily turn on one or two LEDs – but will not give a continuous reading on the bar graph.

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Updated 05/10/05
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