The pitch or frequency of audible sounds is defined by either frequency in cycles per second, or wavelength. The International System of Units (SI) designation for audible sound frequencies expressed as the number of cycles per second is "Hertz," abbreviated Hz. This SI unit was named after Heinrich Hertz, a noted German physicist who lived during the late 1800s.
Most people in their early 20s can hear most sounds in the frequency range between 20 and 20,000 Hz or cycles per second, with the lowest pitched sounds (rumbles) being 20 Hz and the highest 20,000. It is in actually both hearing and feeling the lowest frequency sounds in the range of 20 to about 45 Hz that our brains perceive the depth of low frequency realism. So we feel the movie "Earthquake" as well as see and hear it.
Since humans can't pinpoint the source of low frequencies less than 100 Hz, subwoofers are monaural and can be located anywhere within the listening space. Subwoofer amplifier electronics combine the bass sounds from both stereo channels into a single channel which is then amplified and played through a low-frequency speaker (subwoofer driver) in a specialized enclosure (subwoofer box). Their wooden enclosures are carefully designed to have a resonant frequency range that provides the desired sound impact and realism in the difficult low frequencies.
Using amplified subwoofers to segregate and produce the lowest frequencies in a sound system frees up power and dramatically reduces distortion in the higher frequency registers delivered by the other speakers, such as mid/bass and tweeters. Musical instruments, such as the piano, violin, flute, and vocals, sound cleaner, clearer and far more pleasing and natural.
About 90 percent of the space occupied by classic speaker systems is to produce only the lowest frequencies under 100 Hz. Using a subwoofer can allow you to use quite small speakers for the remainder of the audio range, providing many more aesthetic options for your room space.