Streaming music radio service Pandora has just announced that it will be ending free unlimited streaming for mobile users. Before you grab your pitchforks, Pandora is assuring users that this will only affect a very small portion of its mobile listeners.

Starting this week, free mobile listening will be limited to 40 hours per month. According to Pandora, most users won’t have to worry about this change because the average listener only streams about 20 hours worth of music per month. They say that less than 4% of the company’s total monthly active listeners will even be affected by this.

It appears to be all about costs. Apparently, Pandora’s per-track royalty rates have gone up over 25% in the last 3 years, and are only going to go up over the next few years.

Pandora knows that this isn’t something users want to hear, but the costs must be mitigated.

“Limiting listening is a very unusual thing to do, and very contrary to our mission,” says the company in a blog post.

Pandora promises that they will alert anyone who is fast approaching the 40-hour limit. If you’re about to cross the barrier, you have three options. You can simply stop listening, or you can pay $0.99 for unlimited access for the remainder of the month. There’s also the option to purchase Pandora’s premium subscription, Pandora One, which will keep your listening unlimited and knock out all advertising.

It’s important to note that this limit only affects mobile listeners. You can still stream as many hours per month as you want at home.

Last month, Pandora announced that users had listened to over 13 billion hours of music and created over 1.6 billion stations in 2012.