Level wrote on Thu, 09 September 2004 13:14 |
A ported box has a 12dB/Octave slope below the fe3 (lowest frequency at 3db down in a free field environment) but usually have a higher sensitivity within its range.
A sealed enclosure will have a 6dB/Octave slope below its fe3
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Very close. A sealed enclosure will roll off at 12dB/octave, and a vented box at 18-36dB/octave. A driver in air free air will also roll off at 12dB/octave below its FS. This is a consequence of the behavior of radiation impedance.
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Sealed can ultimately go lower but sealed systems are generally lower in sensitivity. (db/1w/1M) The internal "air spring" keeps the woofer in a controlled environment.
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It should be mentioned that even though the sealed box generally has more extension, the vented box has more output, because of the excursion protection afforded the driver by the resonant air mass in the port.
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TL enclosures can be described as variable VAS in the acoustic sense and variable QTS and QMS in another. (acoustical/electric) The added advantage is the ability to reach the lowest octaves but the enclosures are quite large. I do not like to use a TL in a monitor environment unless the crossover is so very low that it only gets signal below 35hz. D Collins mentioned how "slow" a TL design sounds.
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TL designs are fractional wave pipes which use (typically) the 1/4 wave eigen mode to support the driver. The systems are then stuffed w/ lossy material to quell the q of the resonance, and are often overdamped in the process. This, and the lack of harmonic content being reradiated through the cone out of the cabinet is likely where the "slow" reputation comes from. In a sense the TL may be thought of as very bad horn, which only had a good impedance match the air at one or two eigenmodes, whereas a real horn has a pretty good impedance match over a reasonably extended bandpass.
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One of the most promising ideas is that of the ported transmission line. In a TL, the end of the line is not a port but a pressure release. In the Ported TL, an actual tuned port is used part way down the line. Wierd, but they can go very low with a reasonable sized enclosure.
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I am not familiar with this design but it sounds like they are trying to moderate the above issues w/ the TL. You make it shorter and increase it's modal density, and then use a helmholtz resonator to give you some extension.
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All loudspeakers are a controlled set of trade offs and compromises.
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Agreed heartily. The sealed box and horn, however, emerge as the designs with the best overall phase response. Also, the horn works as a series lowpass to the driver, attenuating the extensive out of passband harmonics all moving coil speaker drivers produce. Almost all the deigns have their place, though, and the inherent flaws in certain designs can actually be euphonious for certain types of music.