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A passive subwoofer is a non-powered speaker that is responsible for strictly reproducing low frequencies. These speakers are powered through an external amplifier and generally reproduce bass frequencies in the 20 - 200 hertz (Hz) range in home audio equipment, and below 100 Hz in professional sound systems. Subwoofers are an integral part of any sound system as they are specially designed to transmit bass frequency with greater clarity than full range speakers.
There are two types of subwoofers: active and passive subwoofers. Active subwoofers have a power source built into the speaker enclosure and can be directly plugged into an external supply, such as a wall outlet. A passive subwoofer needs an external power source and is typically powered from a receiver in a home audio system or a power amplifier in a professional sound system.
Subwoofers can be installed in a variety of loudspeaker enclosures, depending upon the purpose of the sound system. The enclosure for a passive subwoofer in a car audio sound system might be drastically different from the enclosure used to house subwoofers for a sports stadium. Subwoofer enclosures can also be constructed from a wide variety of materials including wood or plastic. The building materials used to construct the subwoofer enclosure also play an important role in how the bass frequencies resonate out to the listening audience.
When using a passive subwoofer, a crossover will be needed in order to ensure that the sound system is operating at peak performance. A crossover takes the full range of an audio signal and sends just the low frequencies to the subwoofer, while sending the high and medium frequencies to the loudspeakers. Sound systems employ crossovers because subwoofers are designed to only transmit low frequencies, so it is therefore inefficient to send high and medium frequencies to the subwoofer as it can not transmit these frequencies. The crossover ensures that the subwoofer receives only frequencies that it can transmit to the listening audience.
In a home audio environment, a passive subwoofer allows smaller speakers to transmit the high and mid-range frequencies, while the subwoofer transmits the bass frequencies. This allows the audio system to have a smaller footprint without sacrificing the transmission quality of the lower frequencies. In professional applications, such as in a dance club or church, a passive subwoofer allows a venue to transmit bass frequencies to a larger audience at extremely high volumes.