A subwoofer is a type of loudspeaker. Subwoofers reproduce the lowest frequencies or tones audible to human ears. We use a subwoofer to reproduce tones your main speakers cannot reproduce.
In this guide, we're going to learn why we need a subwoofer and then dissect the parts of a subwoofer. What makes a subwoofer tick?
Why Do I Need a Subwoofer?
You need a subwoofer because you have great speakers. Those speakers are straining to reproduce low frequencies with any presence. Drums, bass musical instruments, and sound effects lack authority. You may hear the main speakers strain when watching a typical Hollywood action film. The subwoofer will help your beloved main speakers sound their best! Subwoofers help by relieving them of the task of reproducing low bass. The mains speakers will not have sub-bass signals crowding out the power. They are free to reproduce vocals, winds, synths, cymbals, brass, and strings.
No single musical instrument can play tones throughout the spectrum of human hearing. In a similar vein, no single driver can reproduce all the tones that humans are capable of hearing. A typical floor-standing speaker consists of a woofer and a tweeter. Some of them will include a midrange for more vocal clarity. But few floor-standing speakers will play below 65hz, or the “deep C” on a piano (much less “pedal C”).
Adding a subwoofer to an existing system can improve your listening experience.
What Are the Parts of a Subwoofer?
A subwoofer system consists of a subwoofer driver and an enclosure. Some “powered” variants will include an onboard amplifier.
The Subwoofer Driver
A subwoofer’s driver is usually larger than a typical home theater or stereo speaker main driver. The subwoofer driver will be 20cm (8 inches) or larger in diameter. Some engineers have designed smaller drivers, too. Engineers choose a cone, voice coil, and suspension to handle more power and play deep.
The Subwoofer Enclosure
A subwoofer’s enclosure helps shape the sound. These enclosures can be a simple “box” around the back of the woofer. They can also be well-engineered physical air manipulation devices.
Sealed Subwoofer Enclosure
The simple sealed enclosure isolates the rear wave from the front wave. This prevents any cancellation between the front and back of the speaker. It also serves as both a spring and a mass component for the cone. This property allows the engineer to adjust the system’s resonance for a given driver.
Ported Subwoofer Enclosure
The ported enclosure is more complicated than a sealed enclosure. You may see this listed as a "bass reflex enclosure". Ported subwoofers get their name from the port that allows the rearward wave to escape. But escape is only one part of the story. The port shifts the phase of the wave at a specific tuning frequency. Engineers can bake in a bit of distance to change when the rear wave exits the box. They adjust this to increase Sound Pressure Level (SPL) or to increase the depth. Changing the enclosure's net volume can do both!
A ported enclosure uses a simple round tube most of the time. Some ported subwoofer designs use a slot port. The engineers use the outer walls of the box as part of the port. The internal port walls also work as internal bracing for the subwoofer box!
Other Subwoofer Enclosure Alignments
Other alignments increase a subwoofer system’s complexity.
Bandpass subwoofers use more enclosure air spaces on either side of the driver cone. These spaces may or may not include ports. You may have seen one of these at an electronics store in the 90s. They generally had a plexiglass window. Engineers can tune these systems for an incredible low-frequency extension.
A quarter-wave pipe uses a column of air equal to the quarter-wave of a specific tuning frequency. This accomplishes one of two goals. 1) increase the low-frequency extension of the subwoofer; 2) or increase SPL at a specific band. The column of air's cross-section area is usually equal to the loudspeaker's cone area. These enclosures can get very large.
A horn-loaded subwoofer uses an air mass that feeds into a small “throat”, and then flares out. Think of how a tuba works. It is more efficient than other alignments, at the expense of space. These are generally used in pro audio and sound reinforcement. You may have seen a horn-loaded subwoofer at an outdoor concert.
The Subwoofer Amplifier
Marketing companies call home theater subwoofers either active subwoofers or powered subwoofers. This means the subwoofer system has an on-board mono plate amplifier. This provides your receiver some relief from attempting to play low frequencies. This helps like the large subwoofer driver relieves your smaller speakers. This further cleans up the sound reproduced by your main speakers. The amplifier is reproducing a smaller bandwidth for your speakers.
Powered subwoofers are also starting to appear as an option for car audio systems. These serve as an easy upgrade for people who want to add a little boom to their existing factory stereo.
Final Thoughts About Subwoofers
Most stereo systems will sound better with a subwoofer. The type of subwoofer you choose will depend on your listening habits. You'll need to look at how much space you have. You'll need to think about the kind of listening you do. Music or home theater? You should arrive at a solution that improves your listening experience!
Further Reading: How To Design a Custom Ported Subwoofer
