What kind of hearing protection do the cops need to have to keep from having their hearing damaged? Some type of noise cancelling device?
None actually - this device is extremely directional, the key to it ‘success’.
Ordinary beeswax would dampen the noise to levels that would not be ..... repelling, so to speak.
In addition to sound effects, it makes a super PA system.
More here
http://en.wikipedia.org/wiki/Long_Range_Acoustic_Device