Recently we ran a Tech Tip about playing material on the radio (inSync 3/3/00) in which we made reference to the bandwidth limitations for audio to be transmitted over FM. We mentioned the upper limit of 15 kHz, but did not discuss the lower limit, which of course prompted many readers to write us inquiring about that.
The upper limit of FM broadcast frequency response is dictated by the FCC and technical limitations in the way stations are set up across the FM band. There aren’t any restrictions on the lower limit so radio stations may differ, but it’s safe to assume they aren’t going to transmit anything below 20 Hz. Obviously any equipment in the signal path may limit the response even more. The important point to remember, however, is that just because it’s possible to transmit 20 Hz doesn’t mean we need to. Putting a lot of low frequency energy in our mixes eats up headroom and sometimes triggers the limiters all radio stations have before they would ideally be triggered, and the benefits of this low frequency content are lost on 90% of the listeners. Think about it. What good is 30 Hz audio in your car (especially when it’s moving), or your Walkman, or at the volleyball party? Most projects optimized for radio play, either in mixing or mastering (or both), have almost no content at very high and low frequencies. This allows the frequencies that matter (midrange) to be optimized and as loud as possible. And in radio loud is important because you are competing against what comes before and after you. It’s a sad state of affairs for us audiophiles, but that’s the name of the game.