One of the biggest contributions of science is that the existence of "electromagnetic waves" was confirmed in 1888. The electromagnetic wave unit is known as "Hertz" (often abbreviated as: Hz), which is named by Heinrich Rudolf Hertz's surname, to commemorate his contribution to the world.
Calculated in Hz / kHz on the earphone is the Frequency Response, which refers to the number of periodic vibrations per second when playing music. When the Hz is higher, the more the number of vibrations, the higher the pitch; The lower the Hz, the less the number of vibrations will be, and the lower the pitch. When the frequency response range of the earphone is wider, the earphone can restore more high and low frequency details, and the human ear can hear the standard range of 20Hz - 20kHz.