Subwoofers are specially designed to deliver the bass signal from an audio source. The speakers used for subwoofers are generally larger than other speakers in a sound system because bass notes generate longer sound waves than higher sound frequencies. Accurately reproducing very low frequencies requires a large speaker capable of moving a significant amount of air. Holes are placed in subwoofer cabinets to increase the speaker's efficiency when reproducing bass from an audio source.
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A subwoofer speaker consists of a cone of specially treated paper attached to an outer stabilising ring with a metal core attached to the cone's apex. The metal core is surrounded by a voice coil that reacts to electrical impulses from the audio source. When a current is applied to the voice coil, the metal core is forced back and forth through a magnetic field. This, in turn, moves the speaker's cone and the surrounding air to produce sound.
Sound Moving Air
Speakers produce sound by moving the surrounding air. Higher-pitched sounds cause speakers, and the surrounding air, to vibrate at a higher frequency within a small range of motion. As the pitch of a sound decreases, so does the frequency of the speaker vibration necessary to reproduce the sound. The range of motion required to produce lower notes increases though as the sound frequency decreases. This is why you can actually feel the air displacement when holding your hand in front of a large subwoofer.
Air pressure inside the cabinet is usually not a significant design consideration in smaller speakers. Standard cabinets designed for optimal sound output in the high- and mid-frequency ranges can be airtight with little effect on sound reproduction. The speakers mounted inside these cabinets vibrate quickly but move relatively small amounts of air. The pressure created by a large speaker moving enough air to produce bass sounds is significant. When placed in a sealed enclosure, a subwoofer's speaker will have severely restricted movement. This restricted movement causes muffled sound reproduction and slower response to the voice coil's commands.
Porting the Cabinet
The hole in the subwoofer cabinet is referred to as a port. The size of the port is carefully calculated to allow optimal airflow while retaining the cabinet's resonant qualities. In addition to providing a secure mounting surface for speakers, cabinets act as resonating chambers to boost sound output. A port that is too small chokes the speaker and can cause an audible "chuffing" noise from air turbulence. A port that is too large causes a loss of resonance, resulting in a thinner bass sound. The acoustic result is similar to the difference between the slender strings of a classical guitar and the thicker strings of an electric guitar.
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