A stereo subwoofer is an additional speaker that can be added to a music system. The advantage of using a stereo subwoofer is that it plays low frequency bass sounds between 20 and 125 hertz/Hz. First designed in 1966, the subwoofer is used for home cinema experiences as well as home and car stereo systems. Subwoofers have a crossover filter, which tends to be set at 100 Hz and plays all sounds at a frequency below 100 Hz, but not those over 100 Hz. There are a number of types of stereo subwoofer depending on the setup, amplification, size and box style.
Components of the stereo subwoofer include the basket, voice coil, spider and pole piece. The basket is the overall frame that holds the other components in place, but it is separate from the box. The voice coil is a bobbin surrounded with wire attached to the main cone. A shock absorber called the spider controls the cone’s movement. The coil’s magnetic energy is concentrated by the pole piece, a steel pole, running through it.
There are a number of box configurations for subwoofers. Boxes contain the subwoofer and help to increase sound efficiency. The size of the box is determined by box value. This means the size required to produce the best sound from the subwoofer. Some boxes are sealed, whereas others contain ports that release air for a more efficient sound. Others gain efficiency by including passive radiators, which either contain a flat diaphragm or voice coil-less speaker.
Other variations include the bandpass box and the double-subwoofer. The bandpass box is a two-chambered box where sound first passes through a sealed-chamber into a ported chamber. This sound is designed for music such as rock, techno and heavy metal. The double-subwoofer places two voice coils next to each other or facing each other to produce a different sound. There is also the free air stereo subwoofer, which has no box and the subwoofer is instead mounted on a board.
Speaker systems include different types of amplification plus systems designed for homes and those designed for cars. All systems are designed to work in conjunction with other speakers as the stereo subwoofer is designed to only play low-frequency sounds. A passive stereo subwoofer requires an amplifier to be attached to it. This can either be the stereo system’s in-built amplifier or a separate mono amplifier wired into the subwoofer. A powered subwoofer contains an in-built amplifier that ensures the speaker has enough power to do the job properly.
Stereo subwoofer sound can be adjusted via equalization or phase control. Equalization allows changes to the subwoofer settings such as changing the crossover threshold from 100 Hz to 80 Hz or by blocking frequencies that are too low. Phase control utilizes either polarity reversal switches or continuously variable circuits and allows users to control when the subwoofer sound kicks in.