There's no need for ads at all though. If you don't have many listeners, then there's not much ad revenue anyway. And once you have a lot of listeners, you only need a tiny percentage to fund the entire thing.

It's totally viable to have 10% of listeners voluntarily pay while 90% of listeners freeload. And there needn't be any NPR guilt trip about it because they're not blowing millions on office space and executives.

Any successful podcast can be a very financially successful business for a small number of people.