Question

Normal telephones do not transmit pure tones with frequencies below about 300 Hz. But a person whose speaking voice has a frequency of 100 Hz can be heard and understood over the phone. Why is that?

Answer 1

This is possible because telephones do not transmit your voice via a string under tension. They transmit a modulated signal over the line: your frequency adds to another frequency called carrier frequency. 100 Hz in the condition is the frequency of your voice (mechanical frequency), while 300 Hz is the frequency of electric signals.