Wireless System. The SM31 headset features moisture-resistant technology to prevent possible corrosion within humid environments, whilst the wireframe fits securely and comfortably. The wireless system offers analogue wireless transmission and includes the BLX1 bodypack transmitter, BLX4RE wireless receiver, and the SM31 Fitness Headset microphone. The BLX4RE receiver comes with a rack mount kit and removable antennas for quick and intuitive operation. The BLX system range prides itself on professional sound that is easy and efficient to setup, giving you the freedom to perform with quality. Frequency: 518-542 MHz