Transmission Line White Paper


What is a Transmission Line (T/L) enclosure?  Why use them? To answer   these  questions, it  will be  helpful  to  understand the  problems  caused  by conventional closed   box  or  vented enclosures  as  are  used  in  most  other  speaker  systems.  Closed  box   enclosures (also  known as sealed,  infinite baffle, or  acoustic suspension  enclosures)  ruin the  transient  response  of the  woofer, fail  to support the deep bass, and add extraneous vibrations  that "color" the sound.

The transient response is ruined because the woofer compresses the air in  the box as it is driven into the   enclosure.  The compressed  air  acts  like  a spring and  it forcibly  pushes the woofer  back out of  the enclosure when  the  musical signal reverses phase or the music stops.  The woofer cone needs to  stop when  it gets  to its  neutral position  to accurately  follow the  music and  have  good transient  response. The relatively high mass of the woofer   cone has a lot of inertia and it tends to sail   far past ("overshoot") the desired stop point. As if the woofer’s mass wasn't trouble enough, the compressed air in the box also pushes the woofer past the stop point. The result is a great deal of “overshoot and ringing." This ringing causes the bass to sound muddy, slow, and ponderous.

Woofers   have essentially   no bass   output below    their fundamental   resonance frequency.  A closed box has a relatively high resonant frequency so fails to support the deep bass.   To try  to  improve   the  deep  bass,  designers   tend  to  use  very   massive   cones  with  soft suspensions to  try  to lower  the fundamental resonance. Although this does make it possible for the system to have deeper bass, the increased mass degrades the transient response even more.

Vented enclosures are    deliberately designed to    have strong resonances    to help support   the deep bass.  Although this  does  give more  output  to a  very limited range  of bass  frequencies, the  response is  highly non-linear  and  unnatural sounding.  Also, resonance is the opposite of transient   response. For good transient response, you want ZERO resonance.

The radiation from the back of the woofer is just as intense as that from the front. This   rear energy is directed into the enclosure - but what happens to it? Most of this energy is dissipated forcing the walls of the enclosure to flex and vibrate. This causes unwanted sound to come from the enclosure sides, "coloring” the sound.  Energy that   is not dissipated in   the enclosure is radiated back through the woofer cone, again causing sounds to be heard that are not part of the music.

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A Transmission   Line solves   all these   problems. A   T/L is    essentially a long, tapered tube with the woofer mounted in one end. The tube is filled with a soft, absorbent “damping material" much like pillow   stuffing.   Obviously a long straight   tube would be impractical   in your living room, so   T/Ls are best rectangular in shape and "folded" into a more or less conventional box.

A T/L causes the woofer to have good transient response because it “damps" the woofer’s motion. Unlike a closed box, the woofer cannot compress the air   in a T/L because the air can “leak" out the end of the line. When the woofer moves back toward its neutral spot, instead of compressed air pushing it beyond that point, the woofer has to "suck" air back into the line. This causes the woofer to stop instead of flying past its neutral point.

Most of the rear energy of the woofer (BLUE arrows) is absorbed in the   damping material by being converted to heat as it pushes its way through the millions of fibers in the line. Therefore there are virtually no unwanted sounds coming from the enclosure sides or back through the woofer cone.

The T/L    cannot stop    the very    long wave-lengths    of low    frequency sound   (RED arrows). These very low frequencies escape from the end of the line. However,  the damping  material will have slowed  these waves  considerably, and  between that  and the  long length  of the  line, these frequencies will  be significantly delayed before being released  from the enclosure. The result is that the low frequencies come out in-phase with the front radiation of the woofer.  When they are in -phase, they support the deep bass - and they do so over a very broad frequency   range. This is why T/Ls are noted    for having such excellent deep bass.      Finally, all enclosures have resonances that color the sound.  Properly  tapering  transmission  lines   will ensure  that instead of  having  two  or   three  large resonances  like  conventional enclosures, the  T/L  will have an  infinite  number  of very  tiny  resonance's  that are  completely absorbed  by the damping material.  The result is an enclosure that is resonance-free.

The bass  from  a  properly built   T/L  is  utterly  clean,  has excellent   deep bass   extension, no   overshoot  or   ringing,  no   resonances,  and  superb transient response. It is no surprise that the transmission line woofer system blends perfectly with an electrostatic element.

T/Ls normally are large, complex, and expensive to build. They are still difficult and expensive to build, but there is no other way to build a flawless hybrid electrostatic system.


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