This is a sonic device that creates a standing wave field in any type of semi reflective space. It can be instrumented to turn into an intrusion detection system. Working prototypes have been built.
When a tone is played in a room, it fills up with standing waves. If a person, sitting in the room, moves their head or a sound meter around. They hear or measure variations in the sound level in the standing wave pattern. If someone else moves in the standing wave field, the pattern of the field changes and the stationary listener or sound meter will again detect changes in loudness or phase.
A typical low frequency wavelength for human detection is 4 feet, 282 Hz. The standing wave pattern a pressure zones are every 2 feet and objects 1/5th wavelength, around 10” or larger become part of the set of surfaces which support the standing wave pattern. Move any part of the standing wave pattern support surface and the whole standing wave pattern changes. This means an intrusion detector can be located almost anywhere in the standing wave field.
The long wave object detection method is very different from line of sight systems. It fills the volume of the room with a pattern, including around corners, behind cabinets, under desks and even inside closets if the door is left open. The present embodiment is intended to support police, military or rescue people as they move through and clear a house or building. Also, because it is a long wavelength detector, false alarms due to small critter movements are minimized.
The current embodiment is a ½ wavelength (2 feet) long tube that has a1/30 wavelength diameter (about 2”). A small loudspeaker is located at the halfway point inside the tube, along with batteries, digital signal generator, amplifier, detector and alarm. This version includes a large ball wind ball at each of the two ends of the tube. They provide a cushion effect when the tube is tossed and it ensures that the acoustic opening of the tube does not get plugged by being too close to some surface.
The detector is a microphone located at the halfway point along toe tube but it is only open the exterior of the Tube. Attached to the mic is a preamp and a loudness or phase change detection circuit and alarm system. The two ends of the tube are always acoustically exactly out of phase with each other. This means there is a sound cancel zone at the middle of the tube, outside. The detector microphone is located in this sound cancel zone so it cannot hear the direct sound generated by the tube. However at this position it can easily hear the sound of the standing wave pattern in the room, including whenever it changes. This type of detection system is not sensitive to noise unless the noise has exactly the same frequency, which then causes a field pattern shift.