Real Expert or Just Self Proclaimed

[gedlee]

Originally Posted by villastrangiato

"Any idea of "impedance matching" is simply not the case."

The above quote is so utterly incorrect, I'm having a hard time believing you just said that.

Are you talking about the electrical impedance? Certainly not the acoustical impedance. And just what "impedances" are being matched?

[villastrangiato]

Originally Posted by gedlee

Are you talking about the electrical impedance? Certainly not the acoustical impedance. And just what "impedances" are being matched?

Both - the conversion of electrical to acoustic energy involves BOTH. If a resonant cavity that's coupled to a driver resonates at the driver's peak resonance distribution frequency, the electrical impedance is reduced substantially at the resonant frequencies in question and the resistance to the transfer of acoustical energy both in front of and behind said transducer is also reduced. This is what is meant by achieving critical damping. I suggest you read Augspurger or Shutz's published papers for evidence of this phenomenon. I'm not the one who invented or discovered this - many others have documented this most basic and fundamental characteristic of TL's. Improved deep bass response is only one part of the equation. Critical damping allows increased output with less distortion and more linear response to applied signal input. Again, I cannot believe someone with your background is either not familiar with this phenomenon or not willing to acknowledge its existence.

[smokinghot]

I witnessed this trainwreck at the last site the OP barraged.

Any lurkers should pull up a chair, stay tuned, and grab a ...

I forsee bin time.

[PB2]

Originally Posted by villastrangiato

I invite you to stipulate why you believe the statement is incorrect. Transmission lines whether they be electrical or acoustical are by definition impedance matching structures intended to maximize conveyance of either electrical or acoustical energy. The true transmission line conveys this energy away from the exciting element (transducer) and additionally re radiates useful low frequency output in phase with the primary front wave while absorbing unwanted, out of phase upper frequencies with stuffing along the length of the line. In the process of acoustical impedance matching - the driver's resonance or energy storing characteristic is maximally suppressed. Is there something about this concept that you are having trouble with?

OK, I notice that Geddes told you that you were wrong, and now I'm going to tell you that you are wrong. I derived a T&S style model for transmission line speakers as a senior (MQP) project (1979-1980) at WPI and I was a teaching assistant for microwave design, including transmission lines while in graduate school. Take the simple case of a 75 ohm video cable. They are source match terminated by being driven with a 75 ohm source and they are far end match terminated also by having a 75 ohm load, both matched to the 75 ohm characteristic impedance of the cable. There are no reflections on the line in this situation and no impedance matching of any kind going on as a result of the transmission line. There is also no resonance when the line is match terminated.

Now, a TL speaker is a completely different animal, the far end is not match terminated and therefore there are reflections and resonance on the line. The behavior around l/4 is much like that of a lumped vented system.

What makes a transmission line is the fact that it is not a lumped parameter system, there are both capacitance and inductance distributed down the line and partial differential equations are required to model such a system.

A horn _IS_ a special case of a transmission line where the characteristic impedance varies down the line due to the taper and it does therefore act as a matching impedance transformer within its passband. But this is not the TL that you are talking about.

You talk like you are an engineer but I have to wonder where the basis for your understanding comes from.

I suggest that you build a TL speaker and probe with a mic down the line. Read up on VSWR in electrical lines that is what you can measure by probing.

The first, and one of the best papers on TL speakers was published in the 1930s, 1936 IIRC, covering the Stromberg Carlson labyrinth speaker. There is also a patent from that time:
http://www.google.com/patents/about?...lson+labyrinth

They knew that it was a TL and probed down the line - they were probably RF engineers and knew what they were doing.
http://en.wikipedia.org/wiki/Stromberg-Carlson

[planet10]

Originally Posted by godfrey

Hi Dave

Would you mind clarifying your terminology?

e.g. For a 40Hz fundamental:
In electronics-land, 80Hz is referred to as the 2'nd harmonic and 120hz is referred to as the 3'rd harmonic, whereas in musician-land, 80Hz is the 1'st overtone and 120Hz is the 2'nd overtone.

I'm a little confused by your term "1'st harmonic", unless you're referring to the fundamental.

By 1st harmonic i mean the 1st undesirable line harmonic. Fundemental (1/4 wl), then 1 (3/4), 2 (5/4), 3 (7/4)...

dave

[villastrangiato]

Originally Posted by Loren42

Well, I have some interest in experimenting to see if that is true. It does seem to have a slightly slower roll off at the bottom end at the cost of three things:

1: Ripple of the frequency response which must be dampened and even at that, it is not perfect.

2: Enclosure size is large, particularly when working with larger drivers like 15".

3: I have noticed that (at least with simulation software) that the efficiency of the TL cabinet is a good bit less than that of a ported design. In my world efficiency is important because I use a tube amp to drive my speakers. I am limited to 50 to 60 WPC, so having efficient speakers goes a long way.

Nevertheless, I look forward to all the remarks on this post.

What your modeling is telling you may in fact be very true - if you are modeling an improperly designed transmission line which seems to be the case more often than not these days.

If you read Augspurger's extensive testing results and published papers, and if you build a proper TL with either a 4:1 taper OR an offset design of significantly greater length with a non tapered tube, you will find improved bass response overall. Unfortunately, most of the current TL designs combine offsets with tapers - like the poorly performing Thor design. Augspurger clearly states that this results in designs that are overdamped - worse performing than many bass reflex designs. This is one of the problems I have with King's designs - offset is combined with taper with very little science and engineering to account for negative effects of mixing the taper with the 5th to 9th resonant multiple cancellation that the offset produces. And by science or engineering - I'm not talking about equations with numerous fudge factors applied - I'm talking about putting desensitized microphones in resonant cavities and measuring the results - directly correlating them with changes in structure and configuration and modeling the actual results in equations.

With most of the "new" TL designs, you effectively have an overdamped bass reflex. If you stuff it sufficiently and taper it to ridiculous levels (approaching 20:1) you wind up with a bass reflex that is so overdamped - it's almost the equivalent of an acoustic suspension design.

[smokinghot]

If anyone cares to get any background on what to expect...

HTGuide Forum - Issue Versions of Augspurger Tables

[planet10]

Originally Posted by Loren42

I am limited to 50 to 60 WPC, so having efficient speakers goes a long way.

Limited to?

Here the big tube amp is 20 or 40 w (triode/pentode). The others are closer to 5W.

dave

[scott wurcer]

I think the fundamental misunderstanding here is that the acoustical impedance simply coupling to the speaker and affecting the electrical impedance is being mistaken for some kind of "matching". As was pointed out a short acoustic TL is usually terminated with an open or short as far from a match as possible.

[godfrey]

Originally Posted by bwaslo

Curious question -- just what is the source impedance of that power company generator?

IIRC it works out to about 1,274 erg pounds per square furlong (in imperial units, that is - I prefer metric myself)

Popcorn, anyone?