I'll be honest from the beginning; designing and building a subwoofer needs a little more effort than simply buying a woofer and mounting it in a wooden box. But unlike designing a full range loudspeaker, which needs careful adjusting of the individual loudspeakers, it is easier to design and construct a good subwoofer. This is because a subwoofer works in the lower regions of the sound spectrum where the human ear is least sensitive. In essence a subwoofer design is a piston design, much like the pistons in your car engine, moving back and forth in a very limited frequency range. Obviously the basic principle of a subwoofer is to extend the bass response of your loudspeakers, or add some extra push to the lower regions of the sound spectrum for home theatre use. Due to the limited frequency range a subwoofer operates in, usually between 20 to 100Hz, all that is usually required is a large woofer able to move a lot of air in a properly calculated enclosure.
However there is a big difference between the boom boxes sold by many retail outlets and true audiophile bass extension. The boom you often hear from these subwoofers is caused by a peak in their frequency range that has nothing to do with an accurate reproduction of the recording. It however is what a lot of, often uninformed, people identify with good bass reproduction, and far easier to accomplish than a subwoofer that has uniform output throughout its frequency range. When put to the test you will see that none of these boom boxes are able to offer the same sound pressure level throughout their frequency range which is what a properly engineered subwoofer aspires to do.
Fortunately the workings of a (sub)woofer are extensively documented, both mathematical as well as practical, which gives us plenty of background information to calculate and evaluate performance and frequency range prior to actual construction. The starting point of these calculations is the specifications of the woofer, which usually are supplied by the manufacturer. All that we have to do is to set a few realistic design requirements based on which we will then select a suitable woofer and calculate the enclosure and filter accordingly. In the following pages we will discuss how to properly select a suitable woofer, enclosure and filter as well as how to go about constructing an audiophile subwoofer that is both accurate and precise, but also has a high, linear sound pressure level throughout its frequency range.
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