To get some insight on the situation, I consulted Ron Henselman, our resident DXer and RF expert, for some technical info about music broadcasting on AM. Here's what Ron had to say:
AM stations have certain limitations which make them inferior to FM for broadcasting music. They have a limited audio frequency response of up to about 5000 Hz although the current FCC legal limit is 10,200 Hz. Most of the audio program is below 5000 Hz.
(To give you some perspective, all musical tones have a frequency. For instance, middle "A" is 440 Hertz (vibrations per second), and Middle "C" is 262 Hertz. A modern piano might have frequencies as high as 4186 Hertz. In what follows, remember that "KHz" means "times 1000 Hz", for example, 5000 Hz means the same as 5 KHz. So, a piano plays notes in the range from about 100 Hz to 5 KHz.)
There is a technical reason for limiting AM frequency transmission to a max. of 5000 Hz: AM modulation is a non-linear frequency mixing process. If we transmit a 5000 Hz tone on 1090 KHz, the two frequencies mix together. We end up with the original 1090 KHz, but we also end up with a signal at 1085 KHz and another at 1095 KHz. If we transmit any tones higher than 5000 Hz, we are occupying bandwidth of the stations right above and below our station.
For instance, KMOX in St Louis has a center frequency of 1100 KHz. KMOX's channel is actually from 1095 to 1105 KHz --- it's exactly adjacent to KAAY's. Now, say that KAAY transmits a bass note or tone at 200 Hz. At the instant that tone is transmitted, the occupied bandwidth of KAAY would only be 1089.800 KHz to 1090.200 KHz. You just subtract and add the audio frequency from the RF carrier frequency. AM channels in the USA are spaced 10 KHz apart for that reason. The AM stations in many other parts of the world are spaced 9 KHz apart.
On the other hand, one of the good things about AM is the radio frequency range can travel long distances at night. The signals which normally are lost into outer space during the day are reflected back down by the ionosphere during hours of darkness.
There is another reason AM is no longer popular for transmission of music. The frequencies used for AM broadcast suffer from static crashes or noises when storms are present. This is worst during the summer months. Any type of electrical pulse noise is also picked up by an AM receiver. I hated it when somebody in my household ran an electric mixer or electric razor when I was trying to listen to KAAY. Light dimmers also are notorious for interfering with AM. It is even a bigger problem with spark plug noise (ignition noise) in automobiles. It is sort of obvious why most of the music broadcasters moved over to FM.
FM is somewhat immune to pulse noises due to something called a limiter stage in the receiver. Storm static is much reduced on the higher RF frequencies, and the limiter stage pretty much reduces what is there. The method of modulation for FM is not nearly as simple as the simple mixing method of AM. FM also occupies more bandwidth when the audio frequencies become higher, but this was engineered right into the band plan from the FCC. FM stations are spaced 200 KHz apart, so one signal can occupy quite a bit more bandwidth than an AM station. The highest tones transmitted on FM are quite a bit higher than the 5000 Hz limitation for a typical AM station.
FM antennas can be smaller and still work fairly well. AM antennas are always a compromise with short lengths.
There usually aren't any sky waves present on the FM frequencies, so interference from other stations is not as likely unless you are located some distance from the station. Even then, there is something called capture effect on FM which basically means you pretty much only hear the stronger of two or more stations.
So why would anybody try to broadcast music on the AM band these days? Well, most major markets don't have any spectrum available for new stations in the FM band. Many people consider the AM band to be a poor investment due to its waning popularity with listeners; consequently, it is easier to take over an existing AM station.
As Ron makes clear, there are technical reason why FM is better than AM for music broadcast. On the other hand, as Ron notes in his last paragraph, music on AM might make a comeback. The last few summers, my wife and I have been taking road trips into New Mexico and Arizona, and the best radio we find is on the AM side --- the Navajo and Hopi Indian Reservations have really good AM music stations, and there is a lot of great Mexican and World music elsewhere on the band. In contrast, the FM side sounds like computer-programmed "Jack FM.'' Maybe innovation is slowly happening in music radio...on the AM side....
---Dave S. ( firstname.lastname@example.org )