For this question to make sense, you need to distinguish between the average audience and the cumulative audience (or reach). Also, the meaning of "audience" must be defined precisely.
The average audience is the average number of people who listen to the station, read the publication, etc. For a print publication, this is called Average Issue Readership.
The reach (or cumulative audience) is the number of different people who listen to or view your station, read your publication, attend your concerts, visit your web site, etc. This concept only makes sense when it is related to a time period. The longer the time period, the larger the reach. For example, the number of different people who read a daily newspaper at least once a week will be larger than the number of people who read the same newspaper on the average day - because not everybody reads it every day.
As the time period grows, the reach also grows - but more and more slowly. The number of different people who read the newspaper at least once a year will not be much larger than the number who read it at least once every six months. The difference between these two figures is the number of people who read it less than once a year, but more than once in six months.
"Audience" can be defined in a large number of ways, ranging from very loose to very tight.
A very loose definition, for a radio or TV station, would be all the people who have seen or heard the station at least once in their lives, even if they did not personally tune in, and even if they did not realize which station it was. For many radio and TV stations, "audience" defined in this way would include almost the entire population of the area covered.
At the other extreme, a very tight definition is people who listen to or view the station for more than half the time it broadcasts, every day. With this definition, the audience would be almost zero.
In practice, the definitions most often used are: