The squelch used on HF SSB is a "voice activated" or "syllabic" squelch. It is all but impervious to noise, static crashes, tones, etc. It operates on the principle of the 3.5 to Hz syllabic rate of the human voice.
The usual squelch operates on an AGC voltage change, or, in the case of FM, an amplification and detection of noise, usually 7-8Khz or above, and application of that to silence the audio until an incoming signal overcomes the noise, reducing the applied detected voltage and opening the squelch, thus passing the audio.
The syllabic squelch was developed for the transoceanic aircraft industry and maritime industry; both of whose users had better things to do than listen to white noise constantly. It interfered with their more important duties of navigation and safety.