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Selecting a Subwoofer Alignment

 There are three basic subwoofer enclosure alignments, sealed, vented, and bandpass. There are however, a number of variations of each of these styles. Most subwoofers are designed to function optimally in a specific type of box. Contrary to popular opinion, you don't just select a subwoofer and then decide you want to put it in a sixth-order bandpass enclosure because you heard this type of subwoofer box means the loudest or lowest bass.

 An inexpensive subwoofer loaded in the proper enclosure will sound much better than an expensive one in the wrong box. If you choose the wrong type of box for your speaker, the speaker may sound bad or suffer mechanical damage. There are some general rules to help you decide which alignment is the right one for your subwoofers. You need to compare the Thiele-Small parameters of your driver to the guidelines below.

 Sealed box (and sealed bandpass) woofers should have the following characteristics:

  • Qts => 0.40
  • Fs <= 35 Hz
  • Xmax => 4 mm

 Vented or ported box (and ported bandpass) woofers should have the following characteristics:

  • Qts <= 0.40
  • Fs <= 45 Hz
  • Vas should be relatively low (less than 4 cubic feet unless enclosure size is not a factor).

 An additional calculation to help you determine the proper enclosure is the efficiency bandwidth product. This is figured by dividing the Fs value by the Qes value. If the result is close to one hundred, a vented enclosure is the right choice. Closer to fifty means this driver will function best in a sealed alignment.

 The best results (widest passband) for bandpass subwoofers are obtained when the Fs value divided by the Qts parameter is ninety or higher.

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