Transmission-line speakers

[Flak]

Eh... is not it called that in English? Or did it go wrong, ... ignore the ditto.

An example of a good TL speaker that goes down to 40 Hz. I'm curious to see some example.

The Sanders Model 10 are declared to go as low as 20 Hz... they use DSP for EQing and a lot of power though

 

[Everett T]

The new Philharmonic BMR Towers, as well as the predecessor Phil3 are MLTL designs. Both are rated to mid 20s at the bottom end. Along with several of the Salk builds, all the TL designs are optimized by Paul Kittinger.

The plethora of TLs Paul has designed for Dennis and Jim is pretty impressive IMO.

 

[witwald]

Your comments have raised a few questions in my mind, which I hope can be answered.

A TL is not a simple resonator, and its response can be shaped quite freely to adapt to a chosen roll-off,

Can you please describe how this roll-off response shaping is freely achieved in a TL loudspeaker system? If possible, some simulations would be great, where one or more parameters are varied to demonstrate the ability to response-shape the roll-off.

But it is not the resonance at roll-off that is problematic for practical speaker designs, it is the dynamic non-linear behavior and room interaction that determines transient performance.

If there is "dynamic non-linear behavior" present in the system's response, isn't that distortion? Distorted transient performance would seem to be unwanted. And what sort of room interaction is being referred to here? Don't all loudspeakers interact with the room, albeit in different ways depending on their electroacoustic design?

A TL can be designed with a very low roll-off, and thus any ringing at resonance will never be a problem, because it is never excited.

How low is that "very low roll-off"? It's important to have some quantitative details on that score. Are we talking 3 Hz or 30 Hz here? Some music has demonstrable significant signal content down to 30 Hz, and there are examples with plenty of energy at 20 Hz or so. So, a 30 Hz transient pulse will definitely excite the ringing of a loudspeaker system with a 30-Hz low-frequency cut-off.

Acoustic loading can reduce non-linear effects and dynamic compression, when done properly, which means better transient response.

Which "acoustic loading" is being referred to here? That of the loudspeaker enclosure or that of the room, or both? For example, are you referring to a vented enclosure being able to reduce driver excursion at vent resonance, hence reducing non-linear driver displacement effects, thus producing less dynamic compression at higher sound pressure levels?

 

[Kvalsvoll]

Which "acoustic loading" is being referred to here? That of the loudspeaker enclosure or that of the room, or both? For example, are you referring to a vented enclosure being able to reduce driver excursion at vent resonance, hence reducing non-linear driver displacement effects, thus producing less dynamic compression at higher sound pressure levels?

Acoustic loading means to present the driver with acoustic impedance more equal to the acoustical-mechanical impedance of the relatively small and heavy cone and powerful motor system. Air does not couple well to small surfaces at low frequencies.

A long channel with small cross section at the driver end and very large at the output, will transform small and powerful movement out to large movement that couples better to the air outside the exit. This is, very simplified, a horn.

Acoustic loading can be done using horns, channels with smaller cross section, ports like in a typical ported enclosure.

Smaller cross-section of the channel reduces efficiency, so there is a trade-off between efficiency and size. A true full-size horn is not practical for very low frequencies.

A longer channel more like a horn can work across a wider frequency range compared to a ported box.

Performance improves due to 2 things - the cone does not need to move, so less distortion and increased maximum output capacity, and then this allows for choosing driver parameters that gives fewer compromises, as there is no need for long excursion and a heavy moving mass to push resonance frequency down.

 

[Kvalsvoll]

How low is that "very low roll-off"? It's important to have some quantitative details on that score. Are we talking 3 Hz or 30 Hz here? Some music has demonstrable significant signal content down to 30 Hz, and there are examples with plenty of energy at 20 Hz or so. So, a 30 Hz transient pulse will definitely excite the ringing of a loudspeaker system with a 30-Hz low-frequency cut-off.

By moving the cut-of down to well below 20Hz, a system that is free of any ringing within the audible range can be achieved. For smaller systems and reduced (normal) listening levels, even 30Hz will be enough. Also, most rooms will have decay longer than the ringing of the speaker system itself, at low frequencies, especially true for larger spaces.

 

[Kvalsvoll]

If there is "dynamic non-linear behavior" present in the system's response, isn't that distortion? Distorted transient performance would seem to be unwanted. And what sort of room interaction is being referred to here? Don't all loudspeakers interact with the room, albeit in different ways depending on their electroacoustic design?

All non-linearities including dynamic compression can be measured. No mystery.

The room will have a huge influence of the behavior of all speaker systems in the time domain at low frequencies. Even below the lowest mode, there will be a decay that is significant.

This was the 3. question, counting from bottom-to-top. The last one must wait until tomorrow.

 

[DanielT]

Are these TL, or is it "just" a long BR slot port?

By the way, for those who have access to a 3d printer, maybe it can be a fun DIY project?

LCM Waveguide Speaker - 3D Printed Near-field Studio Monitor With DSP and Bluetooth

LCM Waveguide Speaker - 3D Printed Near-field Studio Monitor With DSP and Bluetooth: This is a near field mini-monitor speaker, designed for excellent bass response, and intended for use at a desk or mixing console. This is not the cheapest project to build, but it is a scientifically designed...

www.instructables.com

 

[DanielT]

The issue with pre-made transmission line speakers is everybody wants a very high price for them. There are many kit transmission line speakers but they can get expensive too. I've owned 2 different models of transmission line speakers and they where warm sounding deep bass thumpers. One pair I used in a tri-amp'd system and the other was a set of Mirage that I can't remember the model number it was so long ago.

Plenty have been made, it depends on the design which you're looking for. One of the reasons that they have been used is to extend the frequency response, which you see in single driver systems. You have speaker like the Salk Song Towers that take a 5 1/4 driver and get into the low 40s F3. Many bass reflex units end up being transmission line speaker by accident, Jeff Bagaby had two that he mentioned.

It's not a simple subject, but most agree that no matter the version of a TL, the bass always sounds good.

The Sanders Model 10 are declared to go as low as 20 Hz... they use DSP for EQing and a lot of power though

Thanks for the tips and advice! I am open to TL, but I am or have been a little skeptical about whether you get enough performance from a couple of such, in relation to price and size compared to ported or sealed speaker design.But I have an open mind, no preconceived notions.

My friend Björn, if you are reading this do not take it badly. I liked your big ass TL subwoofer ... It had good sound, but the actual bass result could be obtained via a ported, or sealed subwoofer. Which would have been much smaller. But I know you will use it as a table, so it also serves another purpose.

 

[Everett T]

Eh... is not it called that in English? Or did it go wrong, ... ignore the ditto.

An example of a good TL speaker that goes down to 40 Hz. I'm curious to see some example.

Example

Salk Sound

www.salksound.com

 

[Everett T]

Eh... is not it called that in English? Or did it go wrong, ... ignore the ditto.

An example of a good TL speaker that goes down to 40 Hz. I'm curious to see some example.

Not sure if the link was already posted. This is Martin's website on TL speakers and his math work sheets are used by many speaker designers, including the speaker I previously linked

Quarter Wavelength Loudspeaker Design

Derivation and Correlation of a General Acoustic Model for a Fiber Filled Transmission Line Loudspeaker.

www.quarter-wave.com

 

[ryanosaur]

Are these TL, or is it "just" a long BR slot port?

By the way, for those who have access to a 3d printer, maybe it can be a fun DIY project?

LCM Waveguide Speaker - 3D Printed Near-field Studio Monitor With DSP and Bluetooth

LCM Waveguide Speaker - 3D Printed Near-field Studio Monitor With DSP and Bluetooth: This is a near field mini-monitor speaker, designed for excellent bass response, and intended for use at a desk or mixing console. This is not the cheapest project to build, but it is a scientifically designed...

www.instructables.com

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No. I would not call that a Transmission Line design.

 

[ryanosaur]

I’ve seen many definitions of a Transmission Line Speaker.
Some show the Driver at one end of the pipe, however this could more accurately be classed a rear-horn loaded design, especially if it flares to a larger Cross Sectional Area at the mouth.
Placing the Driver along the Length of the pipe is more akin to what a TL design is (as I’ve learned it). This is much more common to a Pipe Organ style Gedakt Pipe. Where the Driver is placed affects the loading of it and the resulting acoustic performance.
With the work of King and Augspurger, it seems to be more largely agreed upon that the Driver be placed at a point about 1/3 the length of the 1/4 wave tuning for the driver being used. This placement dampens the most destructive odd order harmonic (3rd). (I have seen some references that a second matching driver should be placed at 1/5 the length of the line to dampen the next most destructive harmonic inside the line, but this seems somewhat at odds with some experts experience in designing multi-Driver lines.)
Such Transmission Line “pipes” can be straight (with a constant CSA for the full length), flared (closed with a smaller CSA at the closed end and larger CSA at the Terminus), or tapered (larger CSA at the closed end and narrowing to a smaller CSA at the Terminus).
It also seems to be largely agreed that a Tapered line performs better, acoustically, that the straight or flared lines. By "better," we are talking about smoother FR and dampened harmonics in the line itself. (There have also been some resources touting such benefits as lower tuning frequency or higher output for certain "shapes" of the line, but this too cannot be backed up with any actual evidence I've found.)
One other version of the TL is Mass Loaded which relies on a restricted terminus at the end of the line. One person I have discussed this with claims he gets better performance from MLTLs than he does TQWTs. (I will consider this person an expert in that he does design fully operational Speakers currently sold on the market by a couple "boutique " ID companies.)
Other aspects affecting performance of a TL Speaker are the ratio of the Flare or Taper, the size of the CSA of the line, the volume of the line, and the "effective length" vs the "actual length" of the line. Another characteristic of a properly executed TL Speaker is that the first two thirds of the length will be Damped at a moderately heavy level. Folding the line also helps control some of the unwanted internal resonances.
Proper TL loading will also have a unique Impedance characteristic. For example, here is a MLTL measured impedance graph alongside the impedance model:

And this is the Impedance plot of the THOR TL Speaker:

One famous design with a permutation is the Voigt Pipe and the Weems design. The Voigt Pipe is a flared TL with a closed CSA of 0 and a driver placement at 1/2 the length of the line. It is commonly seen not folded. Voigt himself deemed this a poor design and never intended for it to be used.

The Weems version of the Voigt Pipe introduced a closed end CSA >0, folded the line and also changed the Driver placement to less than 1/2 the line length.

Sadly, proper TL design has been confused a lot. The two best sources are the Augspurger program and the King MathCad worksheets. Kings work is no longer available, while the Augspurger program can be downloaded. Moreover, there is a lot of misinformation out there, too, which makes understanding much more difficult. Not being a mathmetician or physicist seems to complicate the "understanding" much more. Many designs that may include a 1/4 wavelength tuning also get lumped into the category. Rear Horns and Lowther Horns, for example, as well as Speakers that sometimes get called Labyrinths.

Regardless, I've spent too much time trying to learn about TL designs. My non-math/non-physicist self struggles with the complexities. As an owner of a MLTL, I can state unequivocally that there is indeed something special about a well designed TL Speaker. That I want to build my own, better suited to my desires, is my own personal madness.

 

[Kvalsvoll]

Are these TL, or is it "just" a long BR slot port?

By the way, for those who have access to a 3d printer, maybe it can be a fun DIY project?

LCM Waveguide Speaker - 3D Printed Near-field Studio Monitor With DSP and Bluetooth

LCM Waveguide Speaker - 3D Printed Near-field Studio Monitor With DSP and Bluetooth: This is a near field mini-monitor speaker, designed for excellent bass response, and intended for use at a desk or mixing console. This is not the cheapest project to build, but it is a scientifically designed...

www.instructables.com

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What you call it is not so important, the graphs show behavior of a high-q ported design where the port resonance in the low mid is a classic example of faults that can and should be fixed.

 

[ryanosaur]

What you call it is not so important,

I would kindly argue that how you call a thing IS important. That example does not behave like any properly executed TL I have seen, and calling it such pollutes the definition.

 

[Kvalsvoll]

Can you please describe how this roll-off response shaping is freely achieved in a TL loudspeaker system? If possible, some simulations would be great, where one or more parameters are varied to demonstrate the ability to response-shape the roll-off.

Not exactly freely, because everything is linked, so that changing one thing requires adjustment to something else. But compared to a simple closed or ported box, it is possible to choose the response, within limits of what is physically possible.

So, as an example, it is possible to achieve output down to say 20hz using a small 6" driver, but the cabinet will be large, and maximum output will typically not be greater than what could have been achieved in a similar sized cabinet with normal ports and a larger driver.

Shape and cross-section and length of the channel, shape and size of the back chamber, damping material, driver parameters, all affect the response. Changing the response requires adjustment of several parameters, so that the new end result still has a reasonably smooth response in the pass-band, while changing the low-end roll-off.

 

[Kvalsvoll]

I would kindly argue that how you call a thing IS important. That example does not behave like any properly executed TL I have seen, and calling it such pollutes the definition.

Yes, calling this a TL or "line" or horn of any kind, really is wrong, because the length of the port is so short compared to wavelength that it is acoustically functioning as a lumped-element acoustic mass.

 

[Everett T]

Here's another thread I've referred to a few times in the discussion. Hearing designers, even with differing opinions, provokes plenty of thought and discussion.

Transmission line bass roll-off - Techtalk Speaker Building, Audio, Video Discussion Forum

Want a second or third opinion about your speaker cabinet design or other audio related problem? Post your question or comment on the Technical Discussion Board. Hundreds of technicians, engineers, and hobbyists, nationwide read and discuss electronics related questions each week. We welcome...

techtalk.parts-express.com

 

[witwald]

It also seems to be largely agreed that a Tapered line performs better, acoustically, that the straight or flared lines. By "better," we are talking about smoother FR and dampened harmonics in the line itself.

Below is an example of a commercial loudspeaker making use of the tapered-line folded-line approach for a transmission line loudsapeaker, the IMF TLS 80. A slightly smaller design is the IMF TLS 50, and there were also larger examples in the IMF and TDL product ranges.

The Distortion and Frequency Response performance of the IMF TLS 80 are detailed in the measurements shown below:

The following plot shows the Impedance Response of the IMF TLS 80:

(There have also been some resources touting such benefits as lower tuning frequency or higher output for certain "shapes" of the line, but this too cannot be backed up with any actual evidence I've found.)

That's not surprising. When IMF became TDL, their measurements were no longer published. From the impedance curves published for IMF transmission line loudspeakers, it would seem that the "transmission line" loudspeaker could just as easily be a vented loudspeaker, with a very large port that is heavily damped to reduce port resonances as much as possible.

Compare the above results with those of the B&W DM6 loudspeaker from the same era, which are shown below. Here we have a 10-inch woofer in a closed-box enclosure. This enclosure is much more compact than those found in transmission line loudspeakers, while still producing similar bass-response extension with relatively low distortion. The closed-box enclosure design seems to be closer to the ideal, with fewer response aberrations in the lower passband, than what can typically be achieved by a transmission line loudspeaker system.

 

[Kvalsvoll]

From the impedance curves published for IMF transmission line loudspeakers, it would seem that the "transmission line" loudspeaker could just as easily be a vented loudspeaker, with a very large port that is heavily damped to reduce port resonances as much as possible.

The channel is shorter than 1/4 wavelength at tuning, we can see that from the picture. The driver sort of sees a longer channel due to the acoustic properties of the channel that starts very wide with something similar to a larger volume, and then narrows down towards the exit.

The problem here is just what post #1 describes - the channel must be damped to give a smooth response and to attenuate output at higher frequencies where the port output cause destructive interference with the output from the front of the driver. The damping reduces efficiency. Still, it sort of works, and those speakers were highly appreciated.

 

[IPunchCholla]

Interesting thread! I am slowly working towards rehousing my drivers, so after reading the thread, I decided to model a 1/1 transmission line (very roughly, as I am still waiting on a couple resistors to measure my current drivers). I wanted to keep it as simple as possible so the speaker is a 17.4 meter tapered rectangular tube sliced into equal length sections with the paired ends (from inverting the even sections) capped at heights equal to the width of the section. Woofer is 12”.

It comes in at 2.777 meters tall and 1.068 meters deep. If it was made from 1.5mm thick aluminum, the housing alone weighs 59 kilos. Need a room with tall ceilings!

Cost and complexity wise, and think I’ll stick with a sealed or ported box. But it was fun to do.