So you want to be a ninja. You've got the long samurai sword and the throwing stars. You've covered yourself completely in black clothes, leaving only a dramatic opening to set off your eyes. You are within springing distance of your enemy. But it's all for naught if you come clumping loudly up to your intended murderer victim and give yourself away. You might practice to actually make yourself silent. But now, with a bit of set-up and some sound for distraction, you can conceal your sound while creeping up on your enemies - even if they're cats!
The Franssen Effect is a way of hijacking the way people perceive sounds. The victim of the ninja attack obligingly sits in the middle of a room. At forty-five degrees to the left of the victim is something that makes a noise. At forty-five degrees to the right of the victim is the ninja! The object on the left makes a sound, generally a high clear tone. Gradually, the tone gets fainter until it's barely making a noise. The ninja starts mimicking the noise, and slowly making it louder as the tone on the left gets fainter. Although the victim must hear the sound coming from the ninja on the right, they think the noise is localized on the left.
The ears are able to understand the slightest delay in the way sound gets to them, and the brain manages to understand that delay and interpret the direction of the sound. Once the sound gets locked in as coming from one direction or the other, especially through visual confirmation, it takes a lot to change the perception of direction. An anechoic chamber negates the effect. It also sounds better when the tone is high and relatively pure, rather than middle of the spectrum. But it will work on animals as well as humans. There's just something in the brain that, once it establishes to its own satisfaction where a sound comes from, isn't interested in changes.
So get a tv going to an opera program - that'll have high clear tones and provide dramatic mood music - and start creeping up on your victim. Truly now you are of the deadly order of the ninja.
Note: Apologies to Nebezial at deviantart. I briefly showed an image by this artist at the top of the article, thinking it was promotional art. Please check out their excellent art.