What Is a Squelch Button Used for on a Radio?by Don Patton
Squelch controls silence the constant background noise in two-way radio receivers during periods of inactivity. Receivers used for telephony such as citizen band, police and fire, and marine radios often spend much of the time in a listening mode when there is no message traffic. During these periods the only output to their speakers is ambient noise, which the receiver’s automatic gain control may make very loud.
Some two-way radio receivers have a pushbutton switch that activates a fixed squelch level while others have a rotating knob for adjusting the squelch. Engaging or setting the squelch in the absence of a received signal causes the radio to mute its output and prevent the hissing and static from reaching the speaker. If the squelch level is set correctly, when a strong enough broadcast signal appears at the input the receiver will turn on and send the audio output to the speaker or headphones. The squelch feature keeps the receiver quiet during periods of inactivity but active when messages arrive without the operator continually having to adjust the volume.
Receivers with automatic squelch controls contain circuitry to distinguish noise from useful signals and automatically enable the output only when they detect incoming signals. Some radio bands use another feature called tone-key squelching where a set of unique tones outside the audible range accompanies the transmitted signal. The receiver detects these tones when a signal is present and only then enables output.