spatial - Designing a sound that localises well

I am trying to create or find a sound that localises well.

I read that emergency vehicles are changing their sirens; people found it difficult to localise the sound of the old siren (a sinusoidal sweep), and apparently this led to a lot of accidents. Apparently a single frequency is difficult to localise.

I've experimented using harmonic tones — I found that my brain can resolve higher frequencies, say > 600Hz, much better than lower frequencies, say < 150Hz.

Unfortunately this is no good to me — I'm building a musical trainer game, and it's important that I communicate a direction without suggesting a pitch, so I need something else, maybe something like the sound of glass breaking or a clap -- anything that doesn't suggest a pitch.

I've experimented with static bursts, and found they don't localise nearly as well as the tones.

Could anyone suggest a good sample sound source?

I wonder whether something like a twig snapping would trigger some sort of primal reflex :)