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Subwoofers are designed to reproduce the lowest frequencies of our audible hearing range. These sound waves from the subwoofer can exceed 30 feet in length, and interact with a room's walls, ceilings, and floors. Depending on room placement, a sub creates cancellations, or reinforcements of the sub-bass. You can even prove this for yourself.
Compare the performance of your subwoofer based on room placement. Try it in the middle of the room, against a wall, and then in a corner. You should notice a marked difference in the level of bass from one location to another. Additionally, the subwoofer will sound different based on your position in the room in relation to the subwoofer.
For home theater, placement in the corner generally gives the smoothest response. For music and music videos, the subwoofer MUST be placed in between the main speakers (or very close) to avoid loosing some notes in some listening locations. Some hardcore audiophiles will use SPL meters, and advanced test equipment like RTA's to optimize subwoofer placement.
There are three basic types of subwoofer enclosures sold commercially today:
• Band Pass
As with most things, each type of subwoofer has its pros and cons. Sealed Boxes are the simplest design, being just a speaker mounted in an airtight box. A benefit to this design is that they tend to roll off slower in the deep bass than their Ported and Band pass cousins. Their main downfall is efficiency. All else being constant (amplifier power, speaker size, etc.), a sealed enclosure sub will require more than twice the amplifier wattage (and a speaker that can move more than twice as much air) to reach the loudness levels of a ported or band pass design. This type of subwoofer may need some type of crossover to block out the high frequency sounds.
Band pass subwoofer enclosures are a different animal. Where sealed and ported subs have a speaker directly firing into the area of a room, a band pass design is like two boxes in one. A speaker is mounted in an airtight box, which may or may not have a port. This speaker then fires directly into another airtight box which has one or more tuned ports firing into outside world.
Many computer speakers use a subwoofer of the band pass design. The advantage of a band pass design is improved efficiency over a sealed system no need for a crossover. The front chamber of the enclosure acts as a crossover, effectively filtering out the higher frequencies (except for a midrange port resonance that is oven higher than the main output). Their downfalls are complexity and the tendency to have 'muddier' bass than their sealed or ported brethren.
Speakers are designed to move air. When they move air, they create sound. When speakers excite the air and create sound around them these sound waves are measured in wavelengths. Subwoofers are designed to reproduce the lowest two octaves of audible sound, about 20 Hertz to 80 Hz.
To reproduce big sound waves you need a BIG speaker or a BIG amplifier and a very long excursion speaker. Modern subwoofers range in size from 8 inches to over 18 inches. For the same low frequency response, the efficiency is linearly related to the box size. So, for twice the efficicency, you need twice as big box.
The easiest way to add a subwoofer to your music system is with a powered subwoofer. The advantage of a powered subwoofer is it's simplicity. You need only plug it in, run speaker cables to it and place it in between your main speakers and your in business.
A well written instruction book will allow you to set the controls of the subwoofer which would include volume level, phase, crossover frequency, or bass equalizer. All of these settings will tailor the output of the subwoofer to your listening environment.
Most modern recordings, whether they are music or movies, contain a lot of program material between 50 Hz and 100 Hz. Many movie effects are in the 20 Hz to 50 Hz range (and a few organ notes). Although most modern speakers can reach below 100 Hz, it sometimes makes more sense to have a dedicated subwoofer to cover the bottom end (especially if you're planning a home theater). Once your main speaker's woofers are freed from covering the sub-bass material, they'll play with less distortion and handle more power.
Of the three basic types of subwoofer enclosures, Ported enclosures have been around much longer than the Sealed or Band pass designs. A ported subwoofer enclosure is basically a speaker mounted in an airtight box that has a 'tuned' hole or port on the face of it.
A tuned port is designed to interact with the speaker and boost the level of sound at the deepest bass frequency. A ported enclosure is about twice as efficient as a Sealed enclosure, but below the tuned frequency of the port, bass output drops off much quicker than a sealed enclosure. This type of subwoofer may need some type of crossover to block out the high frequency sounds.