These LFH Series LOW FREQUENCY AUDIBLE SOUNDERS have a 520 Hz square wave, temporal 3 tone, and a 4" metal mounting bracket that utilizes a Super-Slide® feature for easy pre-wiring of the system and test for supervision. Other features include a locking mechanism that secures the signal to the bracket, without displaying any screws, and Checkmate® Instant Voltage Verification, which checks voltage drop, current draw, and match against a blue print. Wall or ceiling mount. 24 VDC. UL listed.