|February 11th, 2014
You can hear it's much quieter than a typical whistle. Comparing it with my Generation and measuring SPL  I get:
(Compare to some other methods, but note that our measured numbers are in very different ranges.)
The basic idea is to make the hole in the fipple much smaller than usual, and make the holes smaller as well. Ideally it would also be a smaller diameter, but that's the size of PVC tubing I could get.
 It's not actually a very good fit for that purpose. It requires very careful breath control to make any noise at all, so someone who didn't already play whistles would probably get frustrated and bored without figuring it out.
 SPL in dB at one meter measured with Sound Meter Lite on a Moto X, uncalibrated. Background noise registered 33dB, which means the low notes on the quiet whistle were barely loud enough to register and so are likely inaccurate.