The song was a world-wide hit, going top 10 in most countries and as high as #2 in the U.S. It was the last single from Perry's huge Teenage Dream album. Here's what to listen for.
1. The vocal begins with an interesting delay effect on the doubled background vocals (which are slightly spread left and right), with a med delay that has a long regeneration and long half-note delay with a slight ping-pong. This delay continues even on the lead vocal.
2. The vocal is very compressed and you can especially hear it work on the attacks in the B section and choruses.
3. There's also a lot of auto-tuning going, sometimes more obvious than others.
4. Check out the cool stereo delay effect at 2:45 that's spread hard left and right.
5. The harmony background vocals in the choruses sound like they're Katy vocal samples triggered with a keyboard.