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Re: Please explain: Transmission line bass loading.

166.82.238.13

Posted by Clayton Oxendine on March 30, 1999 at 19:59:39

In Reply to: Please explain: Transmission line bass loading. posted by mikem on March 30, 1999 at 17:06:31:

T/Ls work well for midrange and bass. There are tradeoffs with all enclosure types, and the one for T/Ls is size. The benefits of the design are unparalleled neutrality, uncolored sound,moderate efficiency, and powerful extended low frequency support.
As the name implies, the enclosure is a transmission line. The line can be either 1/4 or 3/4 wavelength long at some critical frequency. The shorter lines are more popular, although, they are a bit more critical to stuff. Stuffing the line makes it possible to make the physical size of the enclosure smaller because the speed of sound through the line is reduced. The reduced speed of sound within the line makes the overall line length shorter than the equivalent undamped wavelength by a significant percentage. The overall size of the enclosure can be reduced even further by folding the line back upon itself, hence the name: "folded T/L".
The "critical frequency" is that frequency at which you want to have a 180 degree phase reversal. The phase of the rear wave is reversed, exits the terminus and provides bass augmentation for the sound emanating from the front of the driver's cone. Broadband support is provided by the frequencies close to and on either side of the 180 degree phase shift.
Transmission Lines are not resonant enclosures in the same sense that ported or reflex enclosures are. The opening at the end of the line is properly referred to as a "Terminus" rather than a "Port". Because the air pressure within the line is nearly that of ambient, box colorations are essentially non-existent. System resonances can be eliminated by tapering one wall of the line. Doing so creates an infinite number of very minute resonances that are then absorbed by the stuffing. The stuffing also contributes to the driver's exceptional damping. There is a book from Old Colony that is the only book I know of on T/L boxes. They are difficult to get right and take lots of fine tuning.

Clayton

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