Low frequency sigaling tone on radio
 

When listening to a talk radio program (syndicated, piped in?), when they go to commercial, there is a short duration, low frequency sine wave signal I hear. Seems to be in the mid 30s Hz range.

Apparently it is some standard developed to signal the station to run the commercials, ect. Just wondered about the standard, and info about it.
Thanks.

this is used on cart machines to signal the end of the loop for cuing. the machine senses the tone and stops. often times when they start the cart a bit of the tone will be left. most radio stations haven't used carts in years however! so that may be what you are hearing. or maybe if the program is a nationally broadcast program they send the tone to signal local commercials to play??