High Tech Pet Sound Barrier Extra Receiver SBR-1 Expands your Sound Barrier Indoor Sonic Dog and Cat Fence SB-1 to protect multiple areas Uses sound, not shock, to deter pets Alarm is triggered only by your pet's individual electronic transmitting collar, allowing people to come and go without triggering the alarm Requires transmitter collar model MS-2 (included with Sound Barrier Indoor Sonic Dog and Cat Fence SB-1), MS-2A or RX-10 Requires one 9-volt alkaline battery (not included)