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| By: cplkittle on Fri, 3 Oct 16:54:25 -0500
| | Transmission lines
A transmission line enclosure is for the serious audiophile looking for superior performance. The basic theory is having a labyrinth of a port which is 25, 50, or 75% of the wavelength equal to the free air resonance of the driver. Any of these lengths act as a low pass filter of around 75Hz. 25% of the wavelength will give you a 90 degree phase shift with the most accurate response of the three. 50% will give you a 180 degree phase shift with a lesser front wave reinforcement. 75% will give you the least amount of front wave reinforcement, and the longest of the three in length. 25% is the most commonly used due to the fact that it is the shortest in length of the three. The size of the port is equal to 25-50% of the driver radiating area.
The tuned aspect of the transmission line greatly effects the fundamental resonance of the subwoofer. The dampening effect of the transmission line is greater than that of a ported or extended bass shelf enclosure, but less than that of a sealed enclosure. Therefore the enclosure retains some air suspension, while at the same time it does not have to fight for motion. The efficiency is therefore better than that of the bass reflex enclosures. The accuracy better than sealed or acoustical suspension, and therefore the frequency response is better than all other systems
Most transmission lines are tapered to prevent standing waves. The taper is usually going to a smaller size at the end of the port. Filler may be used to additionally slow the speed of the sound and to further reduce backwaves or standing waves within the box or line. I could not find an equation that links filler to reduced line length, so this is where the experimentation comes in to play.
Good luck
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