Easy blowing, High tone dog whistle, fitted with solid, waterproof pearl, to give a trill for those who want a distinctive and different sound. With a frequency that fluctuates by 750 Hertz. 210 Small ultra high pitch whistle has a natural cork ball which produces the familiar 'trilling' roll of the pea whistle. 210.5 Same as the 210 but with out the cork ball, produces a single extremely high frequency.