Harman Kardon
In 1959, Harman Kardon marketed the Citation II, an early ultra wideband stereophonic tube amplifier. Designed by Stewart Hegeman, it featured 60 watts/channel output with a frequency response of 18-60,000 Hz at 20 watt output. The company promoted their philosophy of designing high fidelity sound using amplifiers that provided widest possible audio bandwidth. Although the human ear highest audible range is around 20,000 Hz, the full range of sound goes beyond that with harmonics and overtones that may be beyond the hearing range of the human ear. These harmonics interact with other frequencies to produce audible secondary sounds or interference. Harman Kardon promoted the design in audio magazines and product brochures.
Ultrasound
Dogs and cats' hearing range extends into the ultrasound; the top end of a dog's hearing range is about 45 kHz, while a cat's is 64 kHz. The wild ancestors of cats and dogs evolved this higher hearing range to hear high-frequency sounds made by their preferred prey, small rodents. A dog whistle is a whistle that emits ultrasound, used for training and calling dogs. The frequency of most dog whistles is within the range of 23 to 54 kHz.