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.
Whistling
According to Guinness World Records, the highest pitch human whistle ever recorded was measured at 4,186 Hz, which corresponds to a C8 musical note. This was done by Michael Stuart in Richmond, Virginia, on January 11, 2016. The lowest pitch whistle ever recorded was measured at 174.6 Hz, which corresponds to a F3 musical note. This was accomplished by Jennifer Davies of Dachau, Germany on November 6, 2005. The most people whistling simultaneously was 853, which was organized at the Spring Harvest event at Minehead, UK on April 11, 2014.