The systems of sirens and whistles convert the kinetic energy of
a fluid into an acoustic wave. In sirens, the fluid is forced to pass
across a hole, thus generating turbulence that constitutes a
mechanical wave. In whistles, the fluid is forced across a thin blade
which causes the blade to vibrate. For each vibrational movement,
the leading face of the blade produces a pressure wave (Mason,
1998). In liquid application, the whistle constitutes a powerful tool
for mixing and homogenization (Mason and Lorimer, 2002). Da
Mota and Palau (1999) used a siren system to improve onion drying.
A low frequency (1.6 and 3.2 kHz) was used in these experiments experiments
to partially avoid the acoustic energy attenuation; this
action, however, may involve an intense noise that could be an
obstacle to its use.