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Main Speaker Design Theory compared to Transmission Line

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Here is an article from University of Michigan Engineering site that I thought would be an interesting addition to this site.


Transimission Line (TL): Theory

Standard Subwoofer Enclosure Types

Because an Electrostatic Loudspeaker is incapable of producing low frequencies, a tradeoff must be made. Therefore, for a standard dynamic loudspeaker is used. It is then of paramount importance to design the enclosure for the dynamic subwoofer to be as accurate as possible to match the ESL. There are three main speaker enclosure types for the general consumer: acoustic suspension (sealed), bass reflex (vented), and compound (bandpass). Each one of these designs has variations but are similar enough to be ignored for our purposes. Each type of enclosure makes tradeoffs between efficiency, frequency response and accuracy.

By far, an acoustic suspension yields the very best accuracy. The air in the box acts like a spring, and counteracts the motion of the diaphragm. This helps to decrease the amount of over and under shoot giving the sealed enclosure very tight and defined bass. It also tends to protect the speaker from extreme excursion that would cause the cone or voice coil to "bottom out" or hit the frame, damaging the speaker. However, like any spring system, there is a resonance, and below this resonance, the output falls away at about 6db per octave. Also, due to the speaker having to fight the air spring or compliance of the box, the efficiency is low.

The bass reflex or vented enclosure has a greater efficiency. The back wave (sound from the back of the speaker diaphragm) that would normally be dissipated as heat in the enclosure is channeled out through a tuned vent to add with the front wave. In this way, the system is more efficient, but does not have the protection of the air spring. The frequency response tends to be not as flat as sealed. Furthermore, the tuned resonant system has very poor response below the resonance point and can be easily damaged, as the diaphragm decouples from the air and can bottom out with no air spring protection.

The compound or bandpass enclosure is composed of mounting the loudspeaker in a combination of vented and or sealed enclosures in front and behind. The most used is the 4th order bandpass enclosure, where one side of the speaker is in a sealed box and one side is in a vented box. This allows for a double tuned system that has very high efficiency in its pass-band range. However, like the vented box, anything outside of the range is severely attenuated. Also it is difficult to design properly and easy to abuse. The bandpass enclosure is not for audiophiles and tends to give the "one-note-thumper" sound of teenage car audio systems.

For more information on speaker enclosure types, please refer to The Subwoofer DIY Page v1.1

Transmission Line Basics

A Transmission Line (TL) is not like any of these three enclosure designs. The Transmission line is an old design, but as it turns out, is almost a perfect system. For example, the Bose Wave Radio uses a TL enclosure to produce its award winning sound. As the name implies, a transmission line is a long tube that expends from the back of the loudspeaker. By tapering the line, there is NO possible way in which sound can reflect back and forth and therefore standing waves and resonances common to standard speaker enclosures are eliminated. By eliminating back-wave reflections, the driver is also protected from having the back-wave re-radiate through the diaphragm, causing distortion and diaphragm breakup. The purpose of the transmission line is to eliminate the phase cancellation that would occur if the driver was in free air. Because of the length of the line, there is not enough time for air to travel through the line and cancel the front-wave. The magic of the system is what happens to the back-wave. The length of the line creates a tuned chamber much like an open ended pipe from a pipe organ. This causes a phase shift depending on the frequency and the length of the line. Through proper design, this causes the wave from the end of the TL to reinforce the front-wave at the frequencies where the front-wave begins to decrease due to increased air resistance at lower frequencies. Also, the tuned aspect of the TL strongly effects the fundamental resonance of the loudspeaker. It causes a very heavy dampening effect, which also helps to eliminate the overshoot and undershoot of the massive bass driver diaphragm. But this dampening is unlike the air spring of a sealed box and the diaphragm does not have to fight for motion. As a result, the efficiency is better than bass-reflex enclosures, the accuracy is better than acoustic suspension, and the frequency response and linearity is better than all systems.

The line length is usually tuned to be 1/4 of the fundamental resonance of the driver loudspeaker. The TL can be folded, or in other words curved. If stuffed properly with damping material such as wool, the actual length can be decreased due to the resistive effect of the material on the air. The construction and design of a TL bass system may be more difficult, but the result is a dynamic driver than can truly complement an electrostatic loudspeaker.


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