NEMA published NEMA SBP 4-2015 Low Frequency Audible Signals, which addresses need for and development of low-frequency audible signal used in fire alarms, CO alarms, and fire or CO alarm systems. Developed by the Fire, Life Safety, Security, and Emergency Communication Section, document addresses why low-frequency signal is needed, difference between low-frequency signal and standard audible fire alarm signal, and when and where low frequency signal is required.