Ultrasonic devices do not require government licenses as do some RF devices. The modulated signal used in the PPi PAS system nearly eliminates interference from all possible sources. Ultrasonic signals will not penetrate walls. All the transmitters and all the receivers work on the same frequencies. The monitoring center knows exactly where to direct the response.
Ultrasonic signals tend to reflect off hard surfaces, such as concrete or dry wall.
Some ultrasonic systems use a simple penlike transmitter in which a small hammer strikes a tuning fork. Pens make better writing instruments than they do emergency signaling devices. These pens generate a short duration single pure tone. Unfortunately many things in nature create a single pure tone - hair clippers, dental drills, bells, squeaking cart wheels, keys and clanging metal doors. A modulated ultrasonic transmission scheme can solve this problem.