one additional factor - this has to work outside, in widely ranging weather conditions.
@jremington - yes i thought the same thing, i am going to experiment with a few. if i needed 12 (at 30° fov) per unit that is still no problem when they're that cheap. and i can handle the delay from sequential firing, i think. but worried about waterproofing.
commonly used waterproof sensors such as maxbotix WR series (MB7040 I2CXL-MaxSonar-WR – MaxBotix) would cost $1,200 per unit (12 sensors per unit) so that's no bueno.
@crossroads - i don't want to use motorized scanning since the sound and motion will be distracting. also that is more parts that can potentially fail (the sculpture will up for one year).
the thermal detector suggestion is interesting, and might fix the PIR flaw. are you talking about something like http://au.mouser.com/new/omronelectronics/omron-d6t/? they're a bit pricey - 8 sensors for 360° = >$500 per unit. but intriguing.