It is based on peak frequency, you can visually see it here: https://speakerdeck.com/hdm/derbycon-2011-acoustic-intrusions?slide=24 https://speakerdeck.com/hdm/

It is based on peak frequency, you can visually see it here: https://speakerdeck.com/hdm/derbycon-2011-acoustic-intrusions?slide=24 https://speakerdeck.com/hdm/derbycon-2011-acoustic-intrusion... IIRC (it has been a bit), there was a specific frequency only used by modem negotiation but not fax machines (3150?).