If you tune a radio to a frequency where there is no station broadcasting, you hear noise. It's pretty loud, just as loud as the music that comes in when you find a station. And yet, when you do find a station and you have good reception, the music that comes in is crystal clear; it's not mixed with noise that's just as loud as the music.
Even more strangely, several years ago (before the advent of HD radio and digital transmission) the radio station I usually listened to was offline for a day due to technical issues. When I tuned to the station that day, I heard nothing, not even static.
Where does all the noise go when there's a station transmitting on that frequency?
Best Answer
The answers to the last question we've had about radios suggest we have some experienced radio buffs around, so regard this answer as provisional until someone more experienced replies.
Anyhow, radios generally automatically adjust the gain of a received carrier signal. This is so that weakly received stations are as loud as strongly received ones. When there is no signal at all the radio will increase the gain to the maximum to try and pull in a signal, but all that gets amplified is the noise. The noise will have about the same loudness as a station with good reception (though the ear tends to perceive the noise as louder). When a strong station is being received the noise is still there, but since the gain is much lower the volume of the noise is much lower. You could probably hear the noise during quiet moments in the transmission.
To avoid the unpleasantly loud noise, many radios will automatically mute the volume if they can't detect a signal. So whether a detuned radio produces noise or silence depends on the design of the radio rather than any fundamental physics.
I should add that in the particular case of the radio station being offline, the station may have been transmitting a carrier signal but with no modulation. This would also be almost silent, though if you turn the volume up you should be able to hear some background noise.