Primary: Primary audience is the main body of viewers, which may also be known as the target audience. People who may be classed as primary audience would be Doctors, Teachers, Lawyers and others, whilst the secondary audience maybe people in careers such as Hairdressing, Sales Assistants, essentially common people who aren't in the scale of C1 TO C3 of the demographic scale would also be mainly students.
Secondary audience: secondary audience are viewers who fall outside the demographic scale of C1 to C3 but still listens to the station, however they make up a large enough percentage of the test audience to skew the statistics if not counted for. Secondary audience could also mean when someone is trying to introduce a station to another person who is unaware of it.
The difference between a pimary audience and a secondary audience is, ultimately a primay audience is the key audience the station is addressed at and the Secondary audience are people other than the intended audience who will also be listening to the station due to someone who is already an audience, this person could be a friend, daughter, and so on.
Tertiary audience: is when a person is only listening to the station because they are familiar with the guest that is on. This could also be a friend, sister, a particular celebrity and others.