A subwoofer is a loudspeaker designed to reproduce low-pitched audio frequencies, typically below 200 Hz for consumer subwoofers, below 100 Hz for professional subwoofers, and below 20 Hz for subwoofers used in THX-certified systems. Subwoofers are used to augment the low-frequency response of loudspeakers that are unable to reproduce the entire audible frequency range, such as small bookshelf speakers or computer speakers.
18-inch subwoofers are some of the largest and most powerful subwoofers available, and they are capable of producing extremely low frequencies with high levels of output. This makes them ideal for use in large home theater systems or professional sound reinforcement systems. However, 18-inch subwoofers are also very expensive and require a lot of power to operate, so they are not suitable for all applications.