If I connect this little mechanical buzzer to a 3V supply, then hold my AM radio close to it with the radio tuned so I can't hear any stations, I can hear the buzzer on the radio. If I turn up the radio volume, the buzzer is amplified much louder over the radio than what I can hear just from the buzzer. I tried connecting a little piezo buzzer instead of the mechanical buzzer but I couldn't hear the piezo buzzer on the radio.
Why is the mechanical buzzer sound transmitted to the AM radio?
A mechanical buzzer works by powering an electromagnet which moves some contact arm to disconnect power from the electromagnet and thus the contact arm moves back to connect power to the electromagnet. The arm movement may also vibrate a membrane of some sorts to better couple the arm movement to sound waves in air.
As the electromagnet is a coil and coils have inductance, and disconnecting inductive load makes the magnetic field to collapse rapidly and it creates back-EMF or inductive kick which basically tries to keep the current flowing, it will raise the voltage over the contacts and it may spark and sparks emit electromagnetic radiation. Even if it does not spark, it will consume current while the magnet operates and does not consume current when the magnet does not operate so the buzzer supply wires are like a small current loop antenna that transmits magnetic pulses at the buzzing frequency.
Basically, mechanical buzzers are just small devices that radiate bursts of broadband electromagnetic interference, and the radio will also receive the electromagnetic interference pulses as it would receive any other radio signal so you can hear it on the radio speakers.
A piezo buzzer just has an electronic oscillator which drives voltage to a piezoelectric disc which will deform under voltage to transmit acoustic energy so there is no mechanism which would transmit electromagnetic interference out.
A magnetic buzzer diaphragm moves like a relay then breaks the series current with moving contacts and the voltage gets clamped with an internal diode. This protects the contacts from arc energy as well as allows the current to continue yet decay slowly then repeat in a cycle that determines the audio frequency.
As the current breaks from the power supply, it switches over to the diode with some back EMF with a sufficiently fast risetime to reach the AM band. aka an EMI noise maker. The diode path is slightly different from the power supply path so both become voltage transient antennae over a range depending on the area of the loop and R/L = T Risetime .
A bigger battery +/- wire separated loop might reach a greater distance.
If you had a relay doing the same buzzer effect without the diode but with an arc ,it might be able to interrupt cell phone reception nearby. But wouldn't last very long from burning contacts.
