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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: Re: More on T-Lines posted by Jon Risch on December 12, 1999 at 15:03:45:
It seems funny to me that the original intent of using a "transmission line" enclosure is never mentioned. In the early days of audio much work was done to find a way to get rid of the energy from the rear of the driver. The idea at the time was that the perfect "loudspeaker" was a driver in an infinite baffle. From this infinite baffle it was much easier to work with the driver specifications without complications from the box, resonances, acoustic compliance, etc. It greatly simplified the calculations thus making it easier to design a speaker system that operated closer to theory.The acoustic suspension box was a different approach in many ways. Certainly the driver was now in effect an infinite baffle (no way for the rear wave to get out), but there was also the added effect of the compliance of the air inside the box. In electrodynamic systems, the mass of the air and the mass of the driver combine to create a compliant effect, which may be modeled as a capacitor in an electrical circuit. In other words, it can store up energy.
Offsetting this is the reactive inductance of the voice coil. In order to make the drivers work with more "snap" and move larger distances while in a linear voice coil range, designers had to make larger and more powerful voice coils. The problem now was that the inductive part of the circuit was outgrowing the capacitive. The only way around this was to make a heavier cone, but no one wanted that (other than to make it stronger and reduce cone breakup) because it just furthered the chicken-and-egg problem with inductance and capacitance.
The acoustic suspension box let the designers use the air inside the box as a spring, albeit a somewhat nonlinear one. That aside, if they could make the air volume inside the box a certain size, they could add capacitance (in effect) to the driver without increasing its mass. Voila.
The t-line approach was simililar in that it was originally designed to completely absorb the rear wave of the driver but was thought to be a much more effective and linear approach. Linear because the t-line could be constructed to maintain a continual compliance throughout it's length as the driver back-wave was absorbed by the stuffing, thus maintaining a more linear compliance range with respect to frequency and excursion. Effective because with the complete absorbtion of the rear wave you had no worries about internal resonances, standing waves, etc. as you did with the acoustic suspension box.
The theory of course comes from AC transmission line design, where a constant impedance is seen from any point on the transmission line, looking forward or backward.
The last type, the bass-reflex, was a half Helmholtz radiator, half acoustic suspension idea. The boxed air acts as a compliant factor down to resonance and the air in the box could be used to actually do something with the rear wave of the driver instead of wasting it.
Today t-line speakers are more of a half-bass-reflex, half-t-line type of design. In effect they are a bass-reflex enclosure that uses the t-line idea of linear compliance and other attributes as well as using the rear wave in a Helmholtz radiator.
Today the t-line is used but not as frequency as a simple bass-reflex box mostly (I think) because of construction costs vs. benefits and the fact that drivers are becoming so much better.
I got most of this information from articles published in the AES way back in the 30's and 40's by the way. These are great articles for beginners because they start "at the beginning", so to speak.
Hope this was elucidating.
Shawn Harvey
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Follow Ups
- Re: More on T-Lines - Shawn Harvey 19:43:25 12/12/99 (1)
- Re: More on T-Lines - Jon Risch 11:09:04 12/13/99 (0)
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