The physics of how the system works is very Heath Robinson. A small transducer (speaker) is directed to the reeds. These resonate in sympathy to the chimes generated by the codes. When activated, they sound the alert. This is why you get spurios SELCALs when you are tuned to a commercial station which is playing music. The music produced tones which get the reeds vibrating. If the music has a sequence of notes approximating to your aircraft's call, the SELCAL will sound. One airframe I used to fly regularly used to have it's SELCAL triggered by the tune played by the BBC world Service prior to the 'pips'.