The whistle turned out to be very useful, both to get student's attention, and to mark out who the instructor was. In the following two weeks the whistle has been used at several events, including the Canberra Innovation Network's First Wednesday pitches and ACT Government energy workshop.
The only scholarly work I could find addressing the use of a whistle in the modern classroom was for law students in the SCALE-UP classroom: Student-Centred Active Learning Environment with Upside-down Pedagogies (Burke, p. 200, 2015):
"If the room is large, a wireless microphone is desirable because this type of learning space can become noisy. A low-tech option that this professor has used is the classic whistle."However, I found in practice, even with a good quality public address system it is difficult to get the attention of 300 students. Playing tune from smart phone via the PA system did not work either: the echo noise cancellation function of the PA system filtered the music out.
Debra D. Burke (2015) Scale-Up! Classroom design and use can facilitate learning, The Law Teacher, 49:2, 189-205, URL https://doi.org/10.1080/03069400.2015.1014180