Over the past few months, I've been approached -actually, harassed would be a more apt description, by some folks claiming to be Transmission Line "experts" after I offered some observations regarding apparent errors in published data by a well regarded industry expert - George Augspurger. So called TL "experts" came out of the woodwork, demanding that I - and everyone else delete from our hard drives, a 1997 paper published by a university professor which can be found here:
http://www.kt.agh.edu.pl/~natkanie/papers/TLS.pdf
A rather heated discussion followed between myself (defending the above paper) and the "experts" on another DIY blog site. The crux of the disagreement centered on the "expert's" oversight of a critical element to Transmission Line design - that a spectrum of frequencies about the driver's resonance point are sought to be dealt with - not just the peak resonance. This lead to the conclusion that an "ideal" transmission line length corresponds to 1.5 times the driver's peak resonant frequency. The disagreement raised further questions about "new" transmission line theories - contradicting current thinking that the transmission line length alone determines the resonant behavior of the system instead of the driver's resonant behavior. The new TL experts could not explain with their equations published measured data that proved sound emitted from the port of a TL possessed resonance multiples tied strictly to the driver's resonance not the transmission line pipe's resonance. Anyone whose experimented with and measured TL's for a length of time knows this - but not the "new experts". To add further icing to the disagreement cake - the new TL "experts" appear to be suggesting that it's ok to morph a TL into a bass reflex loading - and still call it a transmission line - that the term "transmission line" is just audio slang anyway.
Frankly, I'm fed up with computer geeks and those who suddenly become acoustics experts after downloading a cheesy software simulation program to their laptops. It wouldn't be so bad if they kept their technical opinions to themselves - but they insist that dozens of engineers and scientists who came before them are wrong and all their work should be deleted.
A real transmission line loudspeaker loading configuration is different from a bass reflex in a number of important ways. Primarily, a TL is designed to suppress to the greatest possible extent the driver's total resonance (not just a narrow band as with the bass reflex) and in doing so, to achieve as close to critical damping loading as is theoretically possible.
If you want a real understanding of Transmission Lines - one that is not only "modern" but takes into account everything that has been learned over the past half century - I suggest you look at the paper referenced by the link above. On the other hand, if you want to build bass reflex speakers and call them transmission lines - feel free to ignore this thread. 🙂
http://www.kt.agh.edu.pl/~natkanie/papers/TLS.pdf
A rather heated discussion followed between myself (defending the above paper) and the "experts" on another DIY blog site. The crux of the disagreement centered on the "expert's" oversight of a critical element to Transmission Line design - that a spectrum of frequencies about the driver's resonance point are sought to be dealt with - not just the peak resonance. This lead to the conclusion that an "ideal" transmission line length corresponds to 1.5 times the driver's peak resonant frequency. The disagreement raised further questions about "new" transmission line theories - contradicting current thinking that the transmission line length alone determines the resonant behavior of the system instead of the driver's resonant behavior. The new TL experts could not explain with their equations published measured data that proved sound emitted from the port of a TL possessed resonance multiples tied strictly to the driver's resonance not the transmission line pipe's resonance. Anyone whose experimented with and measured TL's for a length of time knows this - but not the "new experts". To add further icing to the disagreement cake - the new TL "experts" appear to be suggesting that it's ok to morph a TL into a bass reflex loading - and still call it a transmission line - that the term "transmission line" is just audio slang anyway.
Frankly, I'm fed up with computer geeks and those who suddenly become acoustics experts after downloading a cheesy software simulation program to their laptops. It wouldn't be so bad if they kept their technical opinions to themselves - but they insist that dozens of engineers and scientists who came before them are wrong and all their work should be deleted.
A real transmission line loudspeaker loading configuration is different from a bass reflex in a number of important ways. Primarily, a TL is designed to suppress to the greatest possible extent the driver's total resonance (not just a narrow band as with the bass reflex) and in doing so, to achieve as close to critical damping loading as is theoretically possible.
If you want a real understanding of Transmission Lines - one that is not only "modern" but takes into account everything that has been learned over the past half century - I suggest you look at the paper referenced by the link above. On the other hand, if you want to build bass reflex speakers and call them transmission lines - feel free to ignore this thread. 🙂
OK, Let's wade into this. I built my 1st TL in 1973. And have done a lot more since. I have studied everything i can find (i haven't seen the paper you refer to).
I was in New York when Augspurger presented his seminal paper on TL Modeling. At the same time i was putting the finishing touches on the web publication of Martin King's parallel modeling software. That was it for Augspurger, King has been continuosly refining his model. 10 years of model, build, test show that King's software is very accurate, but that the tool is best used in the hands of someone who can interpret the gotchas. Actual TLs built using the models have exploded some historical myths wrt TLs.
Let's examine the way the term transmission line is used. In the strictest sense -- ie the way i interpret your meaning, a transmission line is one that, to completely kill the fundemental resonance, has stuffing appropriate to the line geometry such that the line is aperiodic. Such a line has little or no output from the terminus. By this strict definition Bailey's seminal line and all its descendents (Radford, IMF, TDL, and most of the Wireless World and HiFi news diy projects) do not qualify as transmission lines, as they aim to use the fundemental line resonance to augment the bass and to attempt to suppress all the upper harmonics (the mentioned designs define what people generally have refered to as TLs).
In its broadest use, transmission line is used to categorize the entire family of quarter-wave resonators. This would include designs traditionally called voigt pipes and even horns, and brings an understanding of how a bass-reflex can morph into an ML-TL by increasing the aspect ratio, careful placement of a restricted terminus. and volume filling the enclosure volume.
I usually use the term TL in almost the broadest sense, using the more definitive aperiodic TL for the tightest definition. I'll usually use the term horn for rear-loaded systems that are horn-like even thou most of them achieve their lowest bass using TL-action.
That to get the most bass out of a line it needs to be shorter than the driver's resonance divided by 4 has also been illustrated with the software (straight line). Software has also led us to the understanding that a line that tapers from big to small needs to be shorter to acheive the same fundemental resonance and the steeper the taper the shorter it gets (something in the past erroneously attributed to stuffing slowing down the speed of sound). The reverse is true of a line that tapers from small to large.
Further there is an entire train of harmonic resonances that need to be dealt with -- usually suppressed using damping and line geometry, but sometimes using other tricks.
Differences between early modeled performance & measured performance also led to understanding how the offset of the driver from the closed end of the line affected the resonance structure of the line. Careful placement allowed for the almost complete suppression of the 1st unwanted 1/4 wave harmonic (at a bit of a sacrifice of the strength of the fundemental), allowing for less damping material required to damp the unwanted harmonics leading to less ripple or more bass or somewhere in-between.
King's modeler has opened up a much larger world of transmission line systems (used in the broadest sense), allows virtual prototyping using recyclable electrons & pixels, instead of making a pile of scrap wood, and has allowed for a much richer choice of speaker system designs available to the diyer. Augspurgers software is, unfortunately, way out of date and crude in comparison. Both need a skilled designer to get the most out of them.
The model takes into consideration the driver's characteristics and the lines, giving an end view of the system's performance.
dave
I was in New York when Augspurger presented his seminal paper on TL Modeling. At the same time i was putting the finishing touches on the web publication of Martin King's parallel modeling software. That was it for Augspurger, King has been continuosly refining his model. 10 years of model, build, test show that King's software is very accurate, but that the tool is best used in the hands of someone who can interpret the gotchas. Actual TLs built using the models have exploded some historical myths wrt TLs.
Let's examine the way the term transmission line is used. In the strictest sense -- ie the way i interpret your meaning, a transmission line is one that, to completely kill the fundemental resonance, has stuffing appropriate to the line geometry such that the line is aperiodic. Such a line has little or no output from the terminus. By this strict definition Bailey's seminal line and all its descendents (Radford, IMF, TDL, and most of the Wireless World and HiFi news diy projects) do not qualify as transmission lines, as they aim to use the fundemental line resonance to augment the bass and to attempt to suppress all the upper harmonics (the mentioned designs define what people generally have refered to as TLs).
In its broadest use, transmission line is used to categorize the entire family of quarter-wave resonators. This would include designs traditionally called voigt pipes and even horns, and brings an understanding of how a bass-reflex can morph into an ML-TL by increasing the aspect ratio, careful placement of a restricted terminus. and volume filling the enclosure volume.
I usually use the term TL in almost the broadest sense, using the more definitive aperiodic TL for the tightest definition. I'll usually use the term horn for rear-loaded systems that are horn-like even thou most of them achieve their lowest bass using TL-action.
That to get the most bass out of a line it needs to be shorter than the driver's resonance divided by 4 has also been illustrated with the software (straight line). Software has also led us to the understanding that a line that tapers from big to small needs to be shorter to acheive the same fundemental resonance and the steeper the taper the shorter it gets (something in the past erroneously attributed to stuffing slowing down the speed of sound). The reverse is true of a line that tapers from small to large.
Further there is an entire train of harmonic resonances that need to be dealt with -- usually suppressed using damping and line geometry, but sometimes using other tricks.
Differences between early modeled performance & measured performance also led to understanding how the offset of the driver from the closed end of the line affected the resonance structure of the line. Careful placement allowed for the almost complete suppression of the 1st unwanted 1/4 wave harmonic (at a bit of a sacrifice of the strength of the fundemental), allowing for less damping material required to damp the unwanted harmonics leading to less ripple or more bass or somewhere in-between.
King's modeler has opened up a much larger world of transmission line systems (used in the broadest sense), allows virtual prototyping using recyclable electrons & pixels, instead of making a pile of scrap wood, and has allowed for a much richer choice of speaker system designs available to the diyer. Augspurgers software is, unfortunately, way out of date and crude in comparison. Both need a skilled designer to get the most out of them.
The model takes into consideration the driver's characteristics and the lines, giving an end view of the system's performance.
dave
There are differing definitions of 'Transmission Line', and each has different objectives - this has been covered here before. MJK's models cover both the traditional and newer MLTL versions.
The link does not seem to work for me.
The statement above is not correct so thats a bad start for your position.
Dave,
I'm confused.
If an aperiodic transmission line is designed to completely kill the primary resonance (of the line or the driver?), how does it augment the bass output?
Looking at it another way, it appears to me that a true aperiodic line with zero terminus output is the equivalent of an infinite baffle. All of the rear output of the driver is absorbed / dissipated / canceled somehow. If so, there can be no bass augmentation. The bass response curve depends purely on the Qts of the driver.
If the aim is to suppress the output at the terminus, it appears to me that the only way to augment the bass output is to increase the excursion of the driver. For this to occur, there has to be some sort of resonance in the line in phase with the primary force applied by the voice coil. And if there is such a resonance, there must be output at the terminus.
So would it be true to say that:
- A true aperiodic transmission line has no output at the terminus, no internal resonances and its response is that of the driver mounted in an infinite baffle.
- A "normal" or "classic" transmission line uses resonances to augment the bass response of the system. Output from the terminus is suppressed as far as possible, leading to the assumption that most of the augmentation occurs by the resonance adding in phase at the driver with the force applied by the voice coil. (Thereby causing the driver excursion to continue to increase for a while below its resonant frequency, instead of leveling off.)
- A "bass reflex" system also uses resonance, but applied to the back of the driver driver out of phase and instead augmenting the bass by exiting via the terminus (port).
I haven't seen any substantial evidence of "exploded myths" you talk about - TL theory has evolved over many decades. King hasn't produced any new, miraculous, or even substantial understanding of TL theory. His contribution has been limited to modeling of observed behavior. If you asked the average Martin King "disciple" to explain why the offset TL requires the driver to be placed at 1/5 the distance from the closed end - most couldn't tell you. It's simple physics and math, but the fact is most people who use his spread sheet couldn't cite the science behind such fundamental principles - principles that have existed long before Martin King hit the scene. If you listen to the typical King disciple, Martin King invented offset TL's. This is absurd.
"Differences between modeled performance & measured performance also led to understanding how the offset of the driver from the closed end of the line affected the resonance structure of the line."
Would it surprise you that heating ducts have used this "offset" principle to reduce noise in HVAC systems for decades? From your quoted statement above, you seem to suggest that before "computer modeling" and "software", this concept was neither documented nor fully understood - ridiculous.
"That to get the most bass out of a line it needs to be shorter than the driver's resonance divided by 4 has also been illustrated with the software (straight line). Software has also led us to the understanding that a line that tapers from big to small needs to be shorter to acheive the same fundemental resonance and the steeper the taper the shorter it gets (something in the past erroneously attributed to stuffing slowing down the speed of sound). "
This quote is equally simplistic and as inaccurate as the one before it. The paper from Natkaniec at the beginning of this thread very clearly outlines where maximum backwave energy is centered - at 60 degrees phase distance along the pipe from the driver. As a result, the peak resonance of the driver/pipe system occurs when the pipe length corresponds to this location - or a "pipe" frequency of 1.5 times the driver's peak resonant frequency. Natkaniec's paper also tells you specifically why a driver in an offset design is located at 1/5 the length of the line.
What both you and King fail to understand is that transmission lines are acoustic impedance matching structures. The name "transmission line" owes its existence to high power transmission lines that use impedance matching to transport electricity at peak efficiency and minimal loss by matching the impedance of the load with the impedance of the electrical source. Transmission lines - unlike any other loading design, were intended to approximate as closely as possible the theoretical limit of infinite baffle loading by minimizing the effect of reverberant backwave energy on the transducer element. It carries this backwave most effectively (approximating an infinite baffle) by tuning the resonance of a pipe/driver coupling to the resonance of the driver - which in the real world is not a single frequency but a distribution - the center of which is at 1.5 times the driver's resonant peak frequency. Setting pipe frequency lower or higher reduces the maximum effectiveness of the backwave's "transmission".
These are fundamental acoustical concepts which seem to have been lost in today's "software" focused designers. The laws of physics and acoustics have not changed since Martin King hit the scene - despite all the hype and exclusive claims to knowledge.
"Differences between modeled performance & measured performance also led to understanding how the offset of the driver from the closed end of the line affected the resonance structure of the line."
Would it surprise you that heating ducts have used this "offset" principle to reduce noise in HVAC systems for decades? From your quoted statement above, you seem to suggest that before "computer modeling" and "software", this concept was neither documented nor fully understood - ridiculous.
"That to get the most bass out of a line it needs to be shorter than the driver's resonance divided by 4 has also been illustrated with the software (straight line). Software has also led us to the understanding that a line that tapers from big to small needs to be shorter to acheive the same fundemental resonance and the steeper the taper the shorter it gets (something in the past erroneously attributed to stuffing slowing down the speed of sound). "
This quote is equally simplistic and as inaccurate as the one before it. The paper from Natkaniec at the beginning of this thread very clearly outlines where maximum backwave energy is centered - at 60 degrees phase distance along the pipe from the driver. As a result, the peak resonance of the driver/pipe system occurs when the pipe length corresponds to this location - or a "pipe" frequency of 1.5 times the driver's peak resonant frequency. Natkaniec's paper also tells you specifically why a driver in an offset design is located at 1/5 the length of the line.
What both you and King fail to understand is that transmission lines are acoustic impedance matching structures. The name "transmission line" owes its existence to high power transmission lines that use impedance matching to transport electricity at peak efficiency and minimal loss by matching the impedance of the load with the impedance of the electrical source. Transmission lines - unlike any other loading design, were intended to approximate as closely as possible the theoretical limit of infinite baffle loading by minimizing the effect of reverberant backwave energy on the transducer element. It carries this backwave most effectively (approximating an infinite baffle) by tuning the resonance of a pipe/driver coupling to the resonance of the driver - which in the real world is not a single frequency but a distribution - the center of which is at 1.5 times the driver's resonant peak frequency. Setting pipe frequency lower or higher reduces the maximum effectiveness of the backwave's "transmission".
These are fundamental acoustical concepts which seem to have been lost in today's "software" focused designers. The laws of physics and acoustics have not changed since Martin King hit the scene - despite all the hype and exclusive claims to knowledge.
From what i make of the OP's definition, to use the title of Bailey's article "A Non-Resonant Loudspeaker Enclosure Design". In the article Bailey then goes on and illustrates a 1/4 wave resonant design starting a long standing misunderstanding of what the term TL means.
dave
the Fulmer or Karlsonized partition in Fig.9 is interesting - wonder if it has much effect upon the response?
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?
I managed to download the paper before it went offline - a quick squiz shows it's a traditional 'rule-of-thumb' approach tarted up with a few equations. Martin's work is a considerable advance on this approach.
It doesn't augment the bass. Unlike an infinite baffle some of the 1/4 wave energy is used to suppress the drivers resonant impedance peak.
That is not the goal, it is a side effect. The goal is to flatten the impedance peak.
That is more usually what people think of when they think TL -- ie what Bailey illustrated and not what the title of the article said.
I would clarify a bit more, the idea being to preserve the fundemental resonace of the line as much as possible to augment the bass, and to suppress all the higher harmonics to minimize the ripple.
dave
As i said to get the most out of the software you have to understand the principals. King may not have invented the offset TL, but he certainly was the significant contributor to quantifying it so that it could be very effectively used.
Historically i can't think of many TLs that weren't end-loaded (ie Zd = 0), can you provide some classic examples that are offset for any other reason than convienient placement of the driver?
Anyone who understands how offset works would know that Zd at 20% is an approximation for straight lines to suppress the 2nd harmonic (not the harder to control & preferred 1st harmonic), that Zd is dependent on line taper, and that differences of inches can change things.
I'll comment on the paper after i read it.
It really matters not what came before, Martin certainly spent a lot of time absorbing as much historical information as possible including duct theory, what matters is that is reliably models a quarter-wave (aka TL) loudspeaker system. It is a specific modeler of a loudspeaker and a resonant pipe, with output useful to someone wishing to design a speaker.
dave
Yea - a lot of hand waving and not very accurate.
The idea behind a TL is not much different than most other ways of dealing with the back radiation. The rearward radiation is out of phase from the front so if we can somehow change that phase then we can theoretically double the systems output. A ported box does this through a resonance, but that only occurs at a single frequency, however judiciuosly choosen it can help to augment the LF. A transmission line attempts to change the phase by a delay, but this too has some limitations because a constant delay continuously changes the phase so once again there are only limited frequency ranges where the output from the front and back are in phase - at other frequencies they are out of phase again
Ideally we would want the delay to be frequency dependent and thats where the concept of stuffing the line comes in. Stuffing also attenuates the wave so that the periodic add/cancel cycle gets broken. Otherwise there are large peaks and dips in the response.
Any idea of "impedance matching" is simply not the case.
Anything beyond this basic idea is simply someones "hope" as to how things will work.
TL have not been very sucessful because they simply don't offer a significant improvment.
And at LFs, who cares anyways. Just use three or four subs - of any kind - spaced arround the room and be done with it.
"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. Every properly designed transmission line that's ever been measured disproves that statement beyond a reasonable doubt. The most effective method for suppressing the energy storage characteristic of low frequency drivers has been proven time and again to be the transmission line. A cursory look at any TL's impedance curve would reveal that it is more effective at eliminating/reducing impedance associated with resonance than either the bass reflex (usually results in split peaks) or acoustic suspension (impedance curve largely unchanged). Truly, I cannot believe you just said that!
😱
The above quote is so utterly incorrect, I'm having a hard time believing you just said that. Every properly designed transmission line that's ever been measured disproves that statement beyond a reasonable doubt. The most effective method for suppressing the energy storage characteristic of low frequency drivers has been proven time and again to be the transmission line. A cursory look at any TL's impedance curve would reveal that it is more effective at eliminating/reducing impedance associated with resonance than either the bass reflex (usually results in split peaks) or acoustic suspension (impedance curve largely unchanged). Truly, I cannot believe you just said that!
😱
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. 😕
Thanks - Godfrey
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.
Pretty much my experience too. I'd think that if it was doing impedance matching that the efficiency would be better than reflex.
BTW, electrical transmission lines can be used as transformers (quarter wave lengths) and those can do impedance matching from say 50 ohm sources to 100 ohm loads. But not if you put huge losses inline (like stuffing gives you). About all you'd get then would be matching of the impedance to the internal losses, and there are way cheaper ways do that.
Curious question -- just what is the source impedance of that power company generator?
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