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.
Tactile transducer
In the 2010s, tactile sound transducers have evolved to include higher frequencies and produce higher fidelity. The human tactile frequency range is from 1 Hz, very low frequency such as earthquakes, up to 5 kHz in some hearing impaired individuals. For most individuals 2 to 3 kHz is the upper threshold for tactile reception. These 2010s-era devices must have higher resolution than previous "shakers" to produce these frequencies. Most humans have tactile resolution to 2 Hz which is the smallest change in frequency that can be perceived. The primary use for this extended bandwidth is to reproduce the vibratory signature for musical instruments such as violins, guitars, the human voice or sound effects in movies (for example, the speeders in "Star Wars"). Also higher frequencies may be used to augment hearing through bone conduction, a consideration for people who have compromised their hearing from exposure to loud music.