Shepherd's whistle
They are also produced commercially. Simple whistles may cost less than 1 USD in bulk; expensive ones may be quite ornate. They are commonly made of plastic, aluminum, stainless steel, silver, brass, titanium, Corian, jade, buffalo horn and other materials.
Whistler (radio)
The first spectrogram through which people heard the whistlers was taken from a 48 second long nightside plasmaspheric pass on March 26, 1996. These whistlers were seen at a frequency below 1.5 kHz.