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.
Perception of infrasound
Infrasound is sound at frequencies lower than the low frequency end of human hearing threshold at 20 Hz. It is known, however, that humans can perceive sounds below this frequency at very high pressure levels. Infrasound can come from many natural as well as man-made sources, including weather patterns, topographic features, ocean wave activity, thunderstorms, geomagnetic storms, earthquakes, jet streams, mountain ranges, and rocket launchings. Infrasounds are also present in the vocalizations of some animals. Low frequency sounds can travel for long distances with very little attenuation and can be detected hundreds of miles away from their sources.