Transmission line speaker design – Acoustical labyrinth
What is a transmission line speaker ?
Also called acoustic labyrinth or maze, the transmission line speaker design is a type of enclosure that follows a simple concept, yet hard to achieve. The design is pretty straight forward, but the hard part is that there is no reliable software to model the enclosure results in an accurate fashion. Of course, you can model the dimensions and internals of the box, but the problem is the damping material. The transmission line speaker design relies on heavy use of dampening material. The different types of material used, the amount, the thickness, the location where it is placed, all contribute to a different end result.
That’s why it is very hard to predict the result with a transmission line. The ways you can place damping material inside the enclosure are practically endless. How you do so, will dictate whether the enclosure will sound good or bad. Trial and error is key to making a good transmission line. This makes it difficult for a mass producer (which is the reason why you rarely see transmission lines in your local audio store), but some DIY-ers love to tinker, and the fact that the transmission line speaker design is difficult and time consuming, will only attract them even more.
How does a transmission line speaker design work ?
Transmission lines (TL for short) work a bit differently, compared to sealed and ported boxes. When you make a sealed or bass reflex enclosure (passive radiator slips into this category as well), you alter the resonant peak of the speaker. Transmission line just takes the back waves generated from the speaker, inverts their phase, and throws them back in front of the speaker, to combine with the front waves. This is the main principle around how the transmission line works. You have to understand that this is just the concept behind how the transmission line is designed. You will soon discover, that TL has some serious problems with resonant modes, which need adequate damping.
Here is how to design the transmission line :
- Take your speaker and find out what is the resonant frequency in free air (Fs).
- Find the corresponding wavelength for that Fs.
- Find the length of the transmission line by dividing that wavelength by 4 (the quarter wavelength).
- Make a path from the back of the speaker to the front of the speaker, which is exactly the length you just calculated.
- The path can take different shapes. A labyrinth is popular because it saves spaces (so enclosure doesn’t get enormous).
- Damp the path with different materials of various thicknesses.
- The damping material will absorb the upper frequencies, which introduce some resonance problems.
- If the upper frequencies are successfully absorbed, all that is left is the low frequencies from the back of the speaker, which will combine with those in the front.
These are the general milestones you need to go through, when you are designing and building your transmission line enclosure. Of course, there are other things to worry about, but let’s just keep it simple for now, so you can get the main idea on how the transmission line speaker design works.
What shape should the line be ?
Various shapes are available for you to choose. Using different techniques will yield different results, but sometimes the physical aspect is just plain ugly. Most of the time, you cannot turn a blind eye to this issue. One of the first transmission line speaker design, featured a pipe to guide the back waves. The length of the pipe was 25% of the wavelength of the resonant frequency (Fs) of the speaker. The diameter of the pipe was in correlation with the diameter of the speaker. To be more exact, the cross section of the pipe was equal to the Sd (cone area + half of surround) of the speaker.
I know this is hard to imagine, because you probably never seen such intricate design. No boxes, no baffles, just a pipe which is attached to the back of the speaker, at one end, and the other end pointing in the same direction as the front of the speaker. You could make the look more industrial, and fit them in an steampunk style room, otherwise, you will have a bad time explaining those “speakers” to your wife.
While you could be a little eccentric, and make a transmission line using just a pipe, reason will guide you to more traditional solutions. Here are the usual shapes for the line :
- Straight line.
- Tapered line. Line larger near the speaker and narrower near the exhaust (like an inverted horn).
- Flared line. Line is narrower near the speaker and larger near the exhaust (similar to horn).
- Folded versions of the previous mentioned types of transmission lines.
There are studies made on the transmission lines we just mentioned, without dampening material, and here are the conclusions :
- All types suffer from resonant response anomalies above 100 Hz.
- Tapered line lowers the f3 of woofer, which makes it an attractive line design.
- Folding the line will reduce the upper frequency response anomalies.
- Folding the line will also increase the f3, but insignifican
- Flared line raises the f3 of the woofer, which practically excludes it from the selection.
When you are folding the line, like in the above examples, you are essentially making the box more compact. Let’s say, your line needs to be 2 meters long. A straight 2 m transmission line would be ridiculous. To decrease the length you will have to fold the line. On the other hand, it will increase the height or the width (depending on how you design it), to make it a bit more proportional. Tapered line is a popular choice among those who wish to make a transmission line enclosure, because of the several benefits we mentioned earlier.
To make the enclosure an acceptable size and shape, the tapered line is folded in such a way, that is similar to folded horn, but in a reverse manner. Folded horn, shapes a path for the back waves to travel from a narrow opening to a larger one (at the exit). Folded tapered line, is the other way around, starting from a large opening (near the speaker), and leads to a progressively narrower path, till the exit.
Another advantage of folded lines is the direct effect on the overall sound quality. There needs to be some kind of phase correlation between the speaker and the exhaust. In a perfect world, they would radiate sound from the same spot. Since that is not physically possible, compromise is the solution. The two need to be firing in the same direction and need to be as close as possible to each other. Now imagine a straight line, which has the port and the speaker on opposite sides, leading to a significant non-correlation.
Damping materials and techniques
The damping of the line is an important step in achieving a successful transmission line speaker design. The purpose is to eliminate the anomalies caused by the upper frequencies. As the line gets more complex, and factoring the numerous types of damping materials available, we can conclude that the damping possibilities are endless. Regardless of that, we can make a few observations that happen consistently, when adding more damping material.
Increasing the density of the damping material has several effects :
- Smoother frequency response (as the high frequencies are absorbed).
- Exhaust output decreases.
- Impedance becomes progressively damped.
- F3 increases.
This is somewhat similar to the effects of damping a sealed box. Using damping material in a sealed box, alters the speed of sound, which is similar to making the box bigger. This decreases Qtc, which makes roll-off smoother and increases the f3. If we make a quick analogy, we can conclude that by damping the line, we are changing the speed of sound, thus we can make the line shorter for similar results. While this is true to some degree, in practice it is better to keep the line length at the original dimensions.
Types of damping materials
There are various materials which can be used to damp the line. Depending on the material, you will have to use different quantities, because they have different densities. The whole point of the damping material is to reduce the ripple as much as possible. Considering ±1 db of ripple acceptable, here are the quantities needed to reach this effect, for different materials and different path lengths. This data was confirmed to have some consistency over various trials with tapered lines.
All of these materials will work well in suppressing the upper frequencies. If you use the correct quantities they will have a similar effect. What is not so similar is the effect they have on the overall frequency response. This is the part that is hard to predict, because there are a lot in inconsistencies in overall frequency response when using damping material. Because of this, the conclusion about which material is better is purely subjective. Experienced transmission line builders will rank long-fiber wool as one of the best damping materials for TL, followed by Acousta-stuf. Polyester fill and fiberglass will yield inferior results. Again, these are all subjective conclusions.
Another variation of damping is to place different materials along the line. Starting with lower density near the speaker, and progressively increasing the density of the damping material as we reach the exit. Not uncommon is to leave the last 20% of the line without damping. Final tuning will result in just making adjustments to the last few feet of the transmission line.
The transmission line speaker design is definitely something that is more on the exotic side. Pretty difficult to design, to build and with no guarantees for the end result. I have to mention that there are more line shapes out there that have been studied (like coupled chamber and offset lines), which I didn’t cover in the article. The subject is very vast, with lots of unknown variables. This is definitely not the enclosure you should try if you are an absolute beginner. However, if you have some speaker building skills, love trial and error, and wish to work for your prize, then I encourage you to try it.
- Loudspeaker Design Cookbook 7th Edition by Vance Dickason (Audio Amateur Pubns, 2005).
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