Very interesting project. Via acoustic detection, the product aims to detect illegal usage of drones.

The usage cited are:

1. Smuggling of illegal contraband into prison

2. Illegal taping / paparazi usage

I am very surprized that no paparazi has used drones extensively. Surely you get much better angles than if you were hiding in the bushes (e.g. if you wanted to get an exclusive shot of Prince Willaims)

People are already using drones to photograph the new Star Wars movie outdoor sets.

The production company tried to get these devices but they are filming in another country and they are illegal for export.

It's probably too overt of a thing to use around powerful people if you are trying to exploit a grey area.

Helium balloons + smaller motors = "stealth drone"

I guess using a linear output stage instead of a PWM would help avoid detection. Or use a plane instead of a quadcopter, to limit the noise.

I've found it's the propellers themselves that cause the noise, not the PWM junk on the motors.

It's the same reason they can make helicopters only so quiet, moving that much air with sharp edges is loud.

They poured a fortune into making "stealth" or even just quieter helicopters without much success.

Or glide/dive from a high altitude?