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.
Hearing range
As dogs hear higher frequency sounds than humans, they have a different acoustic perception of the world. Sounds that seem loud to humans often emit high frequency tones that can scare away dogs. Whistles which emit ultrasonic sound, called dog whistles, are used in dog training, as a dog will respond much better to such levels. In the wild, dogs use their hearing capabilities to hunt and locate food. Domestic breeds are often used to guard property due to their increased hearing ability. So-called "Nelson" dog whistles generate sounds at frequencies higher than those audible to humans but well within the range of a dog's hearing.