Mass Loaded Transmission Line Speakers

[raycarr00]

if the sound works for you...like using 4 different preamps n getting 4 different sounds. just googled MLTL-10,and at pinterest, they show some really wild designs with some showing internal damping materials and ports....man! what recess of a person's mind comes up with this stuff.... amazing.

 

[JimPA]

I have built two pairs of mass loaded transmission line speakers; both using 8" woofers. The bass and midrange clarity, and the bass extension are simply the best I have ever heard. I have been listening to hifi audio for over 32 years, and these speakers are as good as I could ever imagine.

I used an acoustic modeling program called Hornresp ( http://www.hornresp.net/ ) along with DataCAD, which I use in my day job, to settle on the designs, for the two woofers. The first used the Dayton Audio RS225P-4A, and the second has the SB Acoustics SB23RNXS45-4 - both are 92dB sensitivity, 4 ohm nominal, paper coned 8" woofers.

I have been using a miniDSP 2x4 HD active crossover initially. This is easy enough for me to refine the crossover to a fine level, and on the first speakers, I used the PEQ with a UMIK-1 calibrated mic and REW, to make custom filters to get very flat response.

On the second design, which I am calling MLTL-10 (after the Linaeum Model 10) I was able to use just the crossover and adjust the volume on the tweeters; as a model for an analog crossover. I have now done this, as well, and the parts are ordered. I used XSim to match the measured response of the speaker, using the individual driver response measurements. The crossover is 1st order on the tweeter, plus an L-pad to lower the level and balance the impedance - and 2nd order on the woofer.

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Does the photo show all of the damping material used in the enclosure?

 

[onwardjames]

Sure look like a nice set of speakers, and I'd love to hear them. Congrats on some fine work.

 

[NeilBEN]

Does the photo show all of the damping material used in the enclosure?

No - this is what I had to put in before gluing on the side. Hornresp gives a total weight of poly-fil used in the acoustic model, which is 0.295kg. Of that 9.1% goes into the closed end below the woofer, and the rest into the top of the cabinet, right behind the woofer. This translates to ~0.95oz in the first segment, and ~9.5oz in the second.

I used loose poly-fil, but this is hard to distribute in the first segment - so if anyone knows a good source for poly-fil batts (maybe 1" thick?) please let me know?

 

[NeilBEN]

Sure look like a nice set of speakers, and I'd love to hear them. Congrats on some fine work.

Thanks very much!

The rest of the crossover parts should arrive Tuesday - ironically, the additional resistors I ordered the next day, after realizing I should use an L-pad, rather than just a series resistor on the tweeter - arrived today, by USPS.

 

[NeilBEN]

Did you use Martin's stuff, too? I've been out of touch with any of that really since he moved to yahoo let alone after that all folded (and am not a FB person). Fine work, at any rate, and thank you for posting.

And yeah as Neil stated, it's a continuum and there's a lot of gray. I just call everything "pipes" and it keeps life simple. There are closed-ended pipes and open pipes, each with different harmonics, but everything resonates. Increasing pipes trade bandwidth for gain, decreasing pipes trade gain for bandwidth. Unradiused corners lowpass because they're expansions. Mass-loading is another damping tool in a designer's box. It becomes about best-fitting a pipe for an era of driver manufacturing (eg very old big Vas stuff vs. more contemporary stuff) and cab goals. People are trying to get a lot of different things done in their differing designs and the semantics get in the way sometimes.

Another point I meant to make, is that one of the things that has been figured out about folding transmission lines - is that you do NOT need to put an angle in the corners of turns. The air forms a still pocket in the corner, and the "radius" is formed naturally by the air. The behavior / response of a folded TL is unaffected by the presence or absence of "smoother" corners. This is not all that different from the way that air flows in a duct or chimney - a square section acts almost like a circle of the same dimension; and the air forms still pockets in the corners. Rectangles act like an ellipse of the same vertical and horizontal dimensions - so the narrower in one direction a rectangle gets, the less air flows through it; even with the same area. So a 12" diameter round duct (113 sq in) passes the same amount of air as a 12"x12" square (144sq in). And a 6" high rectangle would need to almost 19" in the other direction to have the same air flow as the 12" round duct.

In my speakers, I did radius the *inside* of the turns, so the air will have less turbulence. The thing to remember (like the AC music signal signal) is the air flows in *both* directions in the portion of the TL between the woofer and the terminus opening. The closed end has slightly higher pressures, and it has compression resonances, like most other speaker cabinets. I think that TL cabinets are not going to resonate as much, due to the lower internal air pressure.

 

[Pete B]

Nice work!

What is the total volume of the enclosure for the system that you show the simulation?

28Hz is a good choice for the l/4 tuning frequency.

 

[Pete B]

You may have great bass, but it won't be tight and accurate. High distortion levels and doppler distortion, too. Any time you port a speaker you will be having time arrival and increased distortion issues just so you can extend your bass a bit.

If you look at the simulation of this system, you'll see that the output from the woofer,
in yellow, dips significantly at the l/4 tuning frequency (28 Hz). This reduction significantly
lowers all the distortion mechanisms due to the displacement of the cone. You're way off
on all of your statements. Read the classical literature, even a simple bass reflex (ported)
system has this property.
Properly designed ported boxes and TLs have significantly lower bass distortion around
and above the tuning frequency.

 

[JimPA]

Another point I meant to make, is that one of the things that has been figured out about folding transmission lines - is that you do NOT need to put an angle in the corners of turns. The air forms a still pocket in the corner, and the "radius" is formed naturally by the air. The behavior / response of a folded TL is unaffected by the presence or absence of "smoother" corners. This is not all that different from the way that air flows in a duct or chimney - a square section acts almost like a circle of the same dimension; and the air forms still pockets in the corners. Rectangles act like an ellipse of the same vertical and horizontal dimensions - so the narrower in one direction a rectangle gets, the less air flows through it; even with the same area. So a 12" diameter round duct (113 sq in) passes the same amount of air as a 12"x12" square (144sq in). And a 6" high rectangle would need to almost 19" in the other direction to have the same air flow as the 12" round duct.

In my speakers, I did radius the *inside* of the turns, so the air will have less turbulence. The thing to remember (like the AC music signal signal) is the air flows in *both* directions in the portion of the TL between the woofer and the terminus opening. The closed end has slightly higher pressures, and it has compression resonances, like most other speaker cabinets. I think that TL cabinets are not going to resonate as much, due to the lower internal air pressure.

Having angles instead of straight lines diffuse standing waves.
You don't want any eigenmodes reflecting back hitting the rear of the cone.
This is why so many transmission line designs have the lines angled.

In one of your earlier posts you mentioned determining the vent area.
Making the vent smaller does lower the cut off frequency
It still comes down to the taper ratio of the line relative to driver Sd

 

[Pete B]

I call a TL with the woofer moved down the line away from the closed end an
offset TL. You all probably won't believe which company popularized this.
There's probably more misinformation about TLs out there than accurate but
this thread discusses offset TLs with pictures YMMV regarding accuracy:
https://www.diyaudio.com/forums/full-range/243483-improved-transmission-line-alignment.html

If anyone knows of a company that did offset TLs before Bose did please let
us know about the company and system.

Someone in that thread claimed that Olney (Stromberg Carlson) did offset TLs
which I believe is wrong but it's been a very long time since I read the papers.
The Olney paper is one of the oldest that I know of from the 1930s. My project
advisor pointed it out to me and I informed people on the net in the 1990s.

 

[JimPA]

Instead of using poly fill which is difficult to keep in place have you thought of using low density polyurethane foam?
I am not stating to line the walls with foam but place the foam in the lines.

Many commercial designs like IMF and Fried used foam.
Richard Wright and Irving Fried preferred the acoustical properties of foam over fiberglass and poly fill.

 

[NeilBEN]

Nice work!

What is the total volume of the enclosure for the system that you show the simulation?

28Hz is a good choice for the l/4 tuning frequency.

The system volume that Hornresp is reporting is 77.85 liters. It uses a round shape, as I understand it. It reports the length as 213.38cm, and this (not coincidentally!) is the exact same length of the centerline I have drawn in my CAD drawing.

The Fs of this driver is 27Hz, and I was very happy to get the fundamental frequency down to 28Hz. The in-room response is quite usable to essentially 20Hz. The terminus opening is right at the floor for a reason.

From the Madisound spec page on this SB Acoustics woofer:

Suggested Box Alignments

• 0.4 to 0.7 cubic foot sealed box for 3dB down of 70Hz, F6 of 51Hz
• 1.0 cubic foot vented box with 2" diameter vent by 4" long for F3 of 40Hz
  
• 1.25 cubic foot vented box with 2" diameter vent by 3.25" long (or 3" x 8.5") for F3 of 38Hz
  
• 1.5 cubic foot vented box with 2" diameter vent by 3" long (or 3" x 8") for F3 of 35Hz

F3 of 28Hz in a 2.75 cu ft MLTL cabinet is very nice, indeed.

 

[JimPA]

I call a TL with the woofer moved down the line away from the closed end an
offset TL. You all probably won't believe which company popularized this.
There's probably more misinformation about TLs out there than accurate but
this thread discusses offset TLs with pictures YMMV regarding accuracy:
https://www.diyaudio.com/forums/full-range/243483-improved-transmission-line-alignment.html

If anyone knows of a company that did offset TLs before Bose did please let
us know about the company and system.

Someone in that thread claimed that Olney (Stromberg Carlson) did offset TLs
which I believe is wrong but it's been a very long time since I read the papers.
The Olney paper is one of the oldest that I know of from the 1930s. My project
advisor pointed it out to me and I informed people on the net in the 1990s.

Fried claimed he did give advice to Amar Bose when it came to the Wave Radio design.
IIRC there is a circuit to boost the bass in the wave radio design.

The Fried O1 and SM1 subwoofer designs were offset a little.
They were the wide less deep enclosure designs.
I don't have the plans on my computer.

 

[NeilBEN]

Having angles instead of straight lines diffuse standing waves.
You don't want any eigenmodes reflecting back hitting the rear of the cone.
This is why so many transmission line designs have the lines angled.

In one of your earlier posts you mentioned determining the vent area.
Making the vent smaller does lower the cut off frequency
It still comes down to the taper ratio of the line relative to driver Sd

Right, and that is why I have the top rear corner of the cabinet angled; to reduce the higher frequencies from being reflected back out through the woofer cone. In some iterations of this design, I have the bottom of the closed end angled at 45 degrees - I took the area of the beginning of the the TL, as if it were a rectangle perpendicular to the center of the TL. And the length was to that center point of the angled end - this would be the average length and the functional area.

But in this "final" version, I needed the closed end to be shorter (about 23% in this design - typical is about 1/3rd of the overall) and having the open end longer resulted in the response going deeper. So, the void that was needed to do that angled end was not possible to fit inside the box.

This second design is only 1/2" wider and 2" deeper than the first one (and 3 3/8" shorter in the TL portion of the cabinet) and it seems like it is much bigger than that. I had to still be able to layout the panels for each speaker on a 5'x5' Baltic birch sheet.

As I understand it, there are several things going on in the open end - the sides don't need to literally be tapered in order to function like they are - the segments are progressively smaller in area, and the air functions the same as if the sides were tapered. There can be steps in the design - there is in this one - but the function of the system is equivalent to the taper. The lengths and segment areas describe (in the Hornresp model) a way that has been shown to be very accurate. The job comes down to how it gets folded in reality.

There are certainly standing waves in the closed end but they diminish over time, as some of the reflected pressure moves into the open end of the TL. And I think there are far less significant standing waves in the open end?

On making the terminus open (much) smaller - it definitely lowers the fundamental frequency - but Hornresp calculates the mouth velocity, and the driver displacement. You can change the input power to test the limits of these. At 14V, the predicted output is over 105dB for this speaker, and the mouth velocity is a tame 8 meters/second. They recommend staying below 12m/s and an absolute limit is about 17m/s. And the displacement is less than 10mm at the same (very high) power level all the way down to about 25Hz - so if it is played at 105dB and you get a note below 25Hz, the driver would be pushed past its excursion P-P limit of 13mm.

 

[NeilBEN]

Instead of using poly fill which is difficult to keep in place have you thought of using low density polyurethane foam?
I am not stating to line the walls with foam but place the foam in the lines.

Many commercial designs like IMF and Fried used foam.
Richard Wright and Irving Fried preferred the acoustical properties of foam over fiberglass and poly fill.

I had not considered it, because I have not heard of this. I would like to try better material in the closed end. The poly-fil at the top is held in place by the wires and the fact that it is dense enough to be self supporting.
The other thing is poly-fil batt which seems harder to come by.

 

[JimPA]

I had not considered it, because I have not heard of this. I would like to try better material in the closed end. The poly-fil at the top is held in place by the wires and the fact that it is dense enough to be self supporting.
The other thing is poly-fil batt which seems harder to come by.

Walmart usually has the polyester fiber fill in stock.
20713592

When it comes to low density polyurethane foam it has a much higher bulk density than poly fil.
The last two folds of the line would require one piece in the middle placed vertically in the line.

I have experimented with poly fill and fiberglass and found foam gives the best measured response.
I tried this with one of the many pairs of Fried C3L enclosures I have built over the years.
The foam allows a free flow of air in the lines.
You can see the photos of the enclosures and stuffing placement in lines using the foam in the link below.
index.php

BTW you did an excellent job with the speakers.

 

[NeilBEN]

Thanks, Jim!

I am hoping to find a poly-fil batt - I have pulled this from various OEM speakers over time, and I like how it stays in place better than the loose poly-fil. Hornresp is able to model poly-fil, but not foam. I am adding the eggcrate foam by the seat of my pants. Is that the type of foam you are calling low density polyurethane?

My first pair of speakers, I had gone a bit too large again on the terminus opening, and I measured a fair bit of higher frequencies (700-900Hz or so) coing out of the opening, so I adjusted it both for a bit better bass extension, and to reduce the leakage of the higher frequencies. On this second pair, I am filling the part of the TL that is getting reduced anyway, as well as those higher frequencies. Though the last vertical baffle on the second design is doing that latter part, as well. My best guess is that foam along the wall (in the direction of the TL, so to speak) only damps the flow, and at the bottom, it damps the reflection, without changing the effective length of the segment?

 

[kfa888]

I've always heard the issue with these designs is the delay at the port making the group delay bad and imparting a looseness to the sound as the timing is off. Do you experience any of that and what are your feelings about it? There are trade offs to everything in life.

 

[twiiii]

Yes the cone movement is reduced at the tuning frequency but what about off the tuning frequency. Just look at the impedance curve. You still have timing distortion and you still have much higher distortion that todays well designed critically damped woofer enclosure systems don't have. You also have higher frequencies leaking back from the woofer to cancel the front wave causing further issues. And AR 1W had about 6 % distortion back in the 50's, Mcintosh ML series woofers had less tha 3% at 30 Hz in the early 70's. B&O had some speakers in the 70's that had less than 2 % distortion when every one else making point source speakers were more than 3 times that much. Line arrays really reduced distortion below 1% which a lot of speakers today can't match. Even JBL's DD 67000 has distortion and using the magnetic shorting rings that Mcintosh developed in the 80's. Magico uses the technology too to get less than 2% from their sealed woofers systems. Yes porting reduces distortions in some areas but there are other issues created that deter from proper reproduction. All speakers are compromises, you just have to choose which you can tolerate and which ones you reject.

 

[Pete B]

You cannot quote distortion numbers for a system where they are highly frequency
and level dependent without stating the level and frequency. AR 6%? What level?
I can show you the AES paper that has the curve for this one by the way.
B&O? Have you ever listened to them, they have no deep bass to speak of, 2% at
what level and frequency. You are just throwing numbers and jargon around.