Wireless microphones are designed with squelch circuits to mute audio when the receiver loses or cannot find the transmitter’s signal. Squelch circuits are necessary because receivers (especially analog ones) try and demodulate anything they can, including waves that make up the noise floor and interfering signals. In the absence of squelch, receivers would send a hellish, screeching static into the PA during dropouts or when the transmitters were off—possibly destroying speakers along with the tympanic membranes of everyone in the audience.
There are different methods of squelching noise, and most receivers offer the opportunity to adjust the squelch setting in some way, although for the average user no adjustment is usually necessary.
The simplest mutes the audio if the RF signal strength drops below a certain level. This works relatively well, but is far from fool-proof. Such a circuit does not consider any characteristic of the signal other than strength, and can be tricked into opening audio on noise signals that are above the threshold. This type of squelch gets more reliable the higher the threshold is set, but sacrifices operating range for reliability.
Most modern receivers determine squelch using more intelligent methods.
One is to evaluate the frequency of the audio in the demodulated signal. Since a signal arriving from a microphone will not include any high frequencies, if the receiver detects anything above 16-20 kHz, it will assume the incoming RF is noise, not signal, and squelch it out.
Another is to include an audio “tone key” in the transmitted signal that is either above or below audible range. The receiver will squelch when it cannot locate the tone key. A tone key squelch can be fooled by rogue harmonics that approximate the tone key’s frequency or by other nearby microphone transmitters (used by someone other than you) that are also using tone keys.
In practice wireless systems may use a combination of these techniques and, overall, they do a very good job at muting noise without any input from the user. Gone are the days when squelch needed constant attention and noise seemed like just another part of the job.