Audio frequency is an audible periodic vibration measured in hertz. The higher the frequency, the higher-pitched the sound. On average, humans have the ability to hear sounds within the range of 20 to 20,000 Hz. Subwoofers are loudspeakers built to reproduce the lowest audible frequencies. Capabilities vary between subwoofers, but generally, they are able to produce frequencies from 20 to 200 Hz. As a general recommendation, it is wise to limit a subwoofer's range to 100 Hz and below through the use of a crossover.
Limits of Human Hearing
Some subwoofers can produce tones below 20 Hz. However, as frequencies drop below 20 Hz, fewer people are able to discern the sound. Under optimal conditions, where a listener has exceptional hearing and the environment is free of competing sounds, the human ear is capable of picking up frequencies as low as 12 Hz. Typically though, frequencies within the range of 4 to 16 Hz are felt but not heard. A subwoofer capable of producing such pitches is contributing physical vibrations, rather than audible sound, to the experience.
Lower Limits of Subwoofers
The efficiency of subwoofers drops off as frequencies approach zero. Larger diameter subs are better suited to reproduce the lowest tones. High excursion and ample wattage are also contributors to low frequency production. Quality of materials and enclosure design play important roles as well. Some subs are capable of reaching down to 1 Hz, though that achievement can only be verified with scientific equipment; 1 Hz is well out of the range of human sensation.
Upper Limits of Subwoofers
Subwoofers are typically limited on the upper end at around 200 Hz. Loudspeakers are capable of producing frequencies in excess of the limits of human hearing, but subwoofers are by definition built to handle the lowest frequencies only. Expanding a sub's upper frequency limit is possible, but the design modifications needed to do this would mean its lower frequency limit would suffer. The work of producing frequencies of roughly 200 Hz and above is best left for other speakers in an audio system.
Crossovers limit the frequency response of a speaker. Subwoofers typically employ a crossover called a "low-pass filter." A low-pass filter only permits source frequencies of a certain hertz and below to reach the subwoofer. For example, a subwoofer might be capable of producing sound in the range of 20 to 200 Hz, but a crossover could limit the sub to producing only frequencies of 100 Hz and below. This could be desirable if other speakers in the system are capable of handling frequencies of 100 Hz and above, and the system builder wants the subwoofer to contribute only the very lowest of tones.