The output shown by the cyan curve has some significant peaks and dips and would be hard to integrate into a nice sounding system. The output shown by the yellow curve has a moderately flat response but with some dips and peaks. The output shown by the red curve has the flattest frequency response. On this graph, you can see how the enclosure volume changes the speaker's output response even though they are all ported at 30 hertz. You can see that these response curves have a lower F3 (3dB down point) than the sealed enclosure but they use a larger enclosure and the low frequency response below the tuning frequency rolls off more quickly. The respective alignments are again noted in the upper left hand corner. Below the tuning (port) frequency, it is possible to cause damage to the woofer well before power levels approach the maximum power ratings of the woofer. The ported box will provide virtually no damping below the port frequency (all of the control will be provided by the woofer's suspension). If there is insufficient damping, the woofer's cone will move significantly more than it should.
In any type of enclosure, the woofer relies (at least partly) on the enclosure for damping. When using a sealed enclosure, it is difficult to be so far off as to risk damaging the woofers. Ported enclosures are slightly more difficult to design than a sealed enclosure. Notice the frequency response curve and the relative size of the enclosures. You must decide what you need and then design the proper enclosure. Each of these designs will work well in the appropriate situation. The red curve rolls off the slowest and gives the best low frequency extension. The green curve gives a slight boost to some frequencies but rolls off earlier than the other curves. As you can see, the violet curve has the flattest response for the widest range of frequencies. The upper left hand corner shows the 'alignment' of each curve. The following curves are the predicted response from a single woofer in 3 different enclosures. Actually I'd recommend downloading some of the software from the links page on this site instead of doing the 'formula' thing.
If you have a speaker and all of the Theile/Small parameters and have decided what response that you want, you can enter them into the appropriate formulae to determine the proper enclosure volume. The sealed enclosure is the simplest of enclosures but there are still some things that you should know. Virtually all speakers will produce sound in virtually any enclosure but to get the best output from the speaker, it must be in the correct enclosure. Those completely new to audio don't realize how important the speaker enclosure is when a desired frequency response is needed.
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