Jordans in GM's MLTL boxes sounding great

[KCHANG]

Two weeks ago I built two boxes for my Jordan JX-92S drivers based on GM"s 40" MLTL design. The boxes have a triangular cross section. One additional twist I put in was to use a relatively wide and tall front baffle, about 17.5" wide and 60” tall.

The result was surprisingly good. The bass, although a bit shy and not too deep, is really not bad considering the size of the drivers and that the speakers are far away from the walls. The imaging is very good, and the soundstage is wide and deep. The tonal balance is quite pleasing. I did not use any baffle-step correction.

Before I tried GM's MLTL design, I was not too happy with what I got from the Jordans on open baffles or sealed boxes. I was about to give up on them. I decided to try the MLTL design mainly because it was such a simple project, and I did not expect much from the design. Somehow the MLTL design works, and I need to figure out why, but I am surely glad that I did not give the Jordans away.

As testament to the good sound of the Jordans in the MLTL boxes, my wife was listening to two Pink Martini CD's she just bought through the Jordans, and she was very impressed by what she heard. She commented on how clear the sound was, and how she could sense that the music was performed in a hall. She is not an audiophile and is normally very picky and was rarely impressed by the many speaker projects I tried, and I was surprised that she talked about the “hall” sound like an audiophile would do. She thought that the Pink Martini CD’s sounded better through the Jordans than through my big system using B&G RD75 drivers and eight 18” woofers for dipolar bass, although (apparently to avoid hurting my feelings) she emphasized that her preference was limited to the two particular CD’s. I have to say that listening to the Jordans in the MLTL gave me the urge to go back to tweaking the B&G system, and perhaps with some digital EQ the B&G system will leap-frog over the Jordans to be again the clear winner. We shall see.

A big thank-you to GM.

Kurt

[GM]

Greets!

You're welcome! I assume you mean the 48" version. WRT tweaking your reference system, I imagine it's much more 'accurate' down in the midbass/bass than the pipe, which is somewhat 'colored' (resonant), giving you that 'hall' effect. For a somewhat more 'accurate' reproducer, consider my original 30+" pipe design, which AFAIK is now posted on the Jordan site.

FWIW, I've never built/auditioned any of the various versions, but if I did, it would have at least a 30" wide baffle if not up against a wall or near/at a corner.

GM

[Cortez]

> Somehow the MLTL design works, and I need to figure out why
Mee too...
Is there any base article abount MLTL ? What is it exactly, how it works acoustically, etc ?
(Till now i didnt found any usefull stuff on the net about this type of enclosure...)

[MJK]

I guess I'll take credit for the ML TQWT and ML TL labels. Basically I got the idea when I was designing my first full range driver speaker a few years ago. I was working with a pair of Fostex FE-164 drivers and trying to design a TQWT enclosure for them. I found that restricting the open end using a port had some real advantages for controlling the TQWT's open end output. The port mass loaded the standing wave that existed in the TQWT hence the ML TQWT abbreviation

A few years later I used the ML TL label to describe a tall "thin" tower I was designing for the Fostex FE-208E Sigma driver. I studied some finite element models of the air motion in the enclosure to see how it was behaving at resonance and found that a standing wave existed in the tall direction as apposed to the uniform compression found in a classic bass reflex enclosure. This motivated me to call the design a mass loaded transmission line or ML TL. These types of designs seem to work very well and have bcome quite popular with full range driver DIYers.

If you read the description of the enclosure design used for my Project 4, you will find the following summary of how a ML TL works.

"I have been asked many times about the difference between a ML TL design and a simple bass reflex enclosure. From the outside the two look very similar and performance wise there is not a large difference. I think that the principle difference is the way the air volume in the cabinet is used to provide the spring that interacts with the mass of air in the port to form a resonant system.

In a bass reflex cabinet, the air in the box is compressed to a uniform pressure to form an air spring. Typically no damping material is added to the inside of the box so that the Q of the box remains high and the effective volume of air is predictable from the internal dimensions of the box. The shape of the bass reflex box is not that critical, only the internal volume matters. A bass reflex enclosure can be represented as a lumped mass hanging on a spring. If you displace the mass the entire spring stretches. When you let go, the mass oscillates at a predictable frequency that is a function of the springrate and the mass of air in the port. The key point is that the entire spring stretches linearly. This is a simple one degree of freedom mechanical system.

In my opinion, one of the negative attributes of a bass reflex enclosure is that any strong standing wave resonances in the enclosure will not be sufficiently damped. The lack of fiber in the center of the air volume allows energy from the back of the driver to potentially excite resonances and produce unwanted acoustic output that escapes through the port opening. Some people try and mitigate this problem by placing the port on the back of the enclosure. Placing the port on the back of the bass reflex enclosure may require more standoff from the rear wall and lead to room placement problems. The ML TL enclosure design requires stuffing in the internal volume of the enclosure. The presence of this stuffing is part of the design cycle and the amount and location is accounted for in the design process.

The ML TL enclosure can be thought of as a form of transmission line where quarter wavelength standing waves are used to provide the spring for the mass of air in the port. To physically model a straight uniform TL, clamp a yardstick to the edge of a counter or desk and pluck the free end so that it starts to vibrate. The vibration pattern is analogous to the air velocity in a TL. The TL's air velocity is zero at the closed end as is the yardstick's motion at the clamped end. The TL's air velocity is a maximum at the open end as is the yardstick's velocity at the free end.

There are two ways of changing the frequency of vibration for the yardstick. If you shorten the length cantilevered off the counter, the frequency of vibration will increase. Make the length longer and the frequency decreases. This is how straight TL's have traditionally been tuned by adjusting the length. The second way of tuning the frequency of the yardstick is to add a lump of mass to the free end. Put a piece of modeling clay on the free end and watch the frequency decrease. What I have done to the classic TL is put a lump of mass at the terminus end using a restrictive port. For a given frequency, I can shorten the TL (make it stiffer) increasing the tuning frequency and then add mass (air in a port) to pull the frequency back down and get a similar tuned result. One other benefit of having a lump of mass at the terminus is a rolled off port output above the first quarter wavelength resonance. This result is similar to a bass reflex port's response. I did this first with the ML TQWT and then with a straight TL. If you try the yardstick analogy, I think by changing the length and adding mass to the end you can demonstrate to yourself exactly what I am doing in my MathCad computer models."

Hope that helps,

[Cortez]

Hi Martin !

> I studied some finite element models of the air motion in
> the enclosure to see how it was behaving at resonance
Oh, it would be cool, to learn more about acoustic with this method...
In your opinion is this method enough precise to modell the sound ?

> Hope that helps
Of course, and thanks for the explanation !
I would like to ask a few thing more:

If understand correctly, a MLTL is a BR + a TL, with a common tuned resonant freq.
Maybe the tapered type TL-s (eg: Sd -> Sd/2) are similar to an MLTL ?

What are the MLTL's drawbacks related to a classic BR, or a classic TL ?
And whats the difference in efficiency between them, whats the order ?
If i'am right The benefits are: shorter line (smaller box), and a stronger
cutting of higher frequencies (above the tuned freq).

There is a site about resonances in air columns:
http://hyperphysics.phy-astr.gsu.edu...es/opecol.html
But i cant decide, that a TL is wich type of these...
The side with the speaker is an opened or a closed end ?
At the closed end the air doesnt move, but at the speaker there is > 0 speed, isnt ?

A closed pipe resonates deeper related to an opened type. When we put the port to the end,
we are getting closer to the closed state and therefore our resonance getting lower too ?
Or maybe the port is halfway a closed end, and halfway an open end, so the half of the energy
radiated by the cone's rear surface is going out from the box through the port, and the other
half part is reflected due to the acoustical impedance of the port and therefore halfway it behaves
like a closed air column resonator, and halfway like a BR that radiate the sound from the port in
the correct phase. Am i right ?
I guess, i understand a pure BR, and a pure TL alone, but i'am unable to imagine they together.

Otherwise when there is a standing-wave in a box, like in a MLTL, from where we hear it,
through the cone, or just from the vent, when the phase is correct to the front radiation ?

I have a pair of Fostex FE-166, and now they are in a small, self-designed horn, but i'am
not completely satisfied. In your opinion, an MLTL could give a better bass performance ?

[MJK]

Oh, it would be cool, to learn more about acoustic with this method...
In your opinion is this method enough precise to modell the sound ?

I was using the FEM model to calculate and correlate the frequencies and mode shapes of the standing waves in different enclosures. To extend this to predict SPL at a specific position could probably be done but I have never tried.

If understand correctly, a MLTL is a BR + a TL, with a common tuned resonant freq.
Maybe the tapered type TL-s (eg: Sd -> Sd/2) are similar to an MLTL ?

Yes, I guess a good analogy would be to combine the BR and TL behavior to explain the ML TL performance. A classic tapered TL is not a ML TL since it will produce response at the open end from the higher order standing waves while the ML TL will suppress this output. A ML TL can have a tapered internal geometry with a prot at the open end. The best way to try and understand the differences is to model some of these geometries in the MathCad worksheets and review the results. Keep in mind that there is not clear line that separates the different designs but more of a blurry region as one transitions froms a TL to a ML TL to a BR.

What are the MLTL's drawbacks related to a classic BR, or a classic TL ?
And whats the difference in efficiency between them, whats the order ?
If i'am right The benefits are: shorter line (smaller box), and a stronger
cutting of higher frequencies (above the tuned freq).

I think you have the basic advantages stated above. Everything is a compromise and I do not believe any one is beat in all situations. All three will have the same efficiency and all roll of at 24 dB/octave below the tuning frequency.

The side with the speaker is an opened or a closed end ?
At the closed end the air doesnt move, but at the speaker there is > 0 speed, isnt ?

Typically the speaker is placed at or near the closed end. The air at the closed end does not move and is the location of maximum pressure. At the quarter wavelength frequencies, the pressure at the closed end becomes large enough that it will significantly attenuate, and almost stop, the driver motion just like in a BR enclosure.

I guess, i understand a pure BR, and a pure TL alone, but i'am unable to imagine they together.

The best explanation/expereiment is the ruler clamped to the edge of a table. The ruler will vibrate with quarter wavelength displacement shapes. Adding mass (the air in the port for a TL) with a lump of clay will lower the resonant frequency.

Otherwise when there is a standing-wave in a box, like in a MLTL, from where we hear it,
through the cone, or just from the vent, when the phase is correct to the front radiation ?

When there is a standing wave in a TL, the driver motion is attenuated and most of the sound is produced at the open end or at the port for a ML TL. The driver's contribution to the SPL is minimized. Again, if you run the MathCad worksheets for a classic TL with no driver offset you can see this in the plotted SPL responses.

I have a pair of Fostex FE-166, and now they are in a small, self-designed horn, but i'am
not completely satisfied. In your opinion, an MLTL could give a better bass performance ?

The FE-166E driver has a very low Qts so if you put it in a ML TL you will definitely need a correction circuit to balance the SPL response. I really don't know anything about the FE-166E or the horn you are using so I really cannot offer any opinion about your results. If the bass output from the horn is weak, you might try a correction circuit to see if just rebalancing the SPL helps the situation.

Hope that helps,

[Cortez]

Typically the speaker is placed at or near the closed end. The air at the closed end does not move and is the location of maximum pressure. At the quarter wavelength frequencies, the pressure at the closed end becomes large enough that it will significantly attenuate, and almost stop, the driver motion just like in a BR enclosure.
...
When there is a standing wave in a TL, the driver motion is attenuated and most
of the sound is produced at the open end or at the port for a ML TL.

Aham, i guess, i starting to understand it... Its just like the mass on the spring.
On the resonant frequency its hard to move this system, even so the mass moves with a huge amplitude.
But when the front radiation doesnt matter, why is the TL better, related to a simple closed box,
where only the speaker radiates ? Or due to the resonance, it means double energy ?

Still a question for me: Ok, the speaker is a closed end, but the open end is opened, isnt it ? :-)
If yes, this is just a halfway opened air column, which ideally resonates only on L/2 and not on L/4.
I would understand if we used an L/4 TL + a port (mass loading) cause these together would form a L/2 TL.

A MLTL also resonates on upper harmonics, cause the port
doesnt pass these freqeuncies, but reflect back, doesnt ?

[Colin]

Rewinding the thread a little ...

>consider my original 30+" pipe design, which AFAIK is now posted on the Jordan site

It's at

www.ejjordan.co.uk/diy

There's also a link to a page with GM's MLTL designs for the JX125 and JX150 bass drivers

[MJK]

If yes, this is just a halfway opened air column, which ideally resonates only on L/2 and not on L/4.
I would understand if we used an L/4 TL + a port (mass loading) cause these together would form a L/2 TL.

I am sorry but I have no clue what you are saying above. I think that the best way for you to gain an understanding of TL's is to do the math and determine what geometries and boundary conditions generate half wavelength and quarter wavelength resonances.

[Cortez]

I am sorry but I have no clue what you are saying above.

I cant understand, that a TL with a vent at the end, how can resonate at the same
frequency, like a closed air-column. Or a simple TL is not an opened air column ?!

Otherwise, when the front radiation doesnt matter, why is a TL better, related to
a simple closed box ? Maybe due to the resonance, it means double sound-energy ?

A MLTL also resonates on upper harmonics, cause the port
doesnt pass these freqeuncies, but reflect back, doesnt ?

Thanks !