Transmission line subs

[guy48065]

Over the years I've seen speakers & subs described as transmission line enclosures, but were not ported. The only examples I can find online are ported boxes.
Can an enclosure built with baffles that create a long path away from the woofer end in a passive radiator or another active driver and still be considered a T-line?

 

[ryanosaur]

Over the years I've seen speakers & subs described as transmission line enclosures, but were not ported. The only examples I can find online are ported boxes.
Can an enclosure built with baffles that create a long path away from the woofer end in a passive radiator or another active driver and still be considered a T-line?

To the best of my knowledge, like a Pipe Organ, there must be an opening in the line.
Best examples I know are Tapered Quarter Wave Tubes where the Driver is 1/3 distance from the closed end of the line, which is also the largest cross-sectional area, and tapers to the terminus which is the smallest cross-sectional area.

Perhaps, as you show above in the linked image, this may do something to control back-wave, but will do nothing to load the drivers any differently than a common sealed enclosure.

 

[DVDdoug]

Closed pipes & open pipes can both resonate. It's been awhile since I studied it but with a closed pipe, at a half-wavelength the wave should bounce-back in-phase to reinforce speaker movement.

There are also "transmission line" speakers that are supposed to absorb the sound. I believe they are usually closed and tapered.

I'm not exactly sure what a passive radiator does but I assume it lowers resonance or allows for a shorter "tube".

 

[ryanosaur]

I can't and won't dispute...
All I can really consider is my exposure to both King's and Augsperger's work which both lent toward a line open at one end, and usually tapered.

I have seen discussion of straight and flared lines, but always open at one end.

*shrugs

 

[Flaesh]

Where is this picture from? And did anyone call this a transmission line?

a passive radiator or another active driver

The picture definitely shows two active drivers at the ends of the tube.

 

[SIY]

TLs and acoustic labyrinths are often confused.

 

[DVDdoug]

P.S.
I thought about building a transmission line (or labyrinth) once (probably tuned at 20Hz). But it was going to be impractically large. 20Hz has a 56-foot long wavelength so even a half-wavelength is long.

I don't usually say "impractical". The "speaker stack" in my living room is almost 8-feet tall! Just an inch or two below the ceiling!

I'm not sure what a true acoustic transmission line would look like. In the RF world, a transmission line is impedance matched to transmit a signal efficiently with no (or minimal) standing waves or return loss.

 

[ryanosaur]

P.S.
I thought about building a transmission line (or labyrinth) once (probably tuned at 20Hz). But it was going to be impractically large. 20Hz has a 56-foot long wavelength so even a half-wavelength is long.

I don't usually say "impractical". The "speaker stack" in my living room is almost 8-feet tall! Just an inch or two below the ceiling!

I'm not sure what a true acoustic transmission line would look like. In the RF world, a transmission line is impedance matched to transmit a signal efficiently with no (or minimal) standing waves or return loss.

I don’t know if an optimized TQWT has ever been Klippelized, but measurements I’ve seen of some tubes based on King’s work are pretty legit.
Most measurements of the BMR Tower from Philharmonic are really good, and that is a MLTL that is tuned to low 20s… optimized using King’s work.

 

[guy48065]

Where is this picture from? And did anyone call this a transmission line?

The picture definitely shows two active drivers at the ends of the tube.

I drew it to represent my subs--but I don't know how many baffles or if the line tapers.

Hear me out on this...
I have a satellite-subwoofer set custom built by Unity Audio for me ~1983 and described as T-line subs. I'm beginning to think that might not be literally true.
Before these I had Shahinian Obelisk speakers which featured "hybrid transmission line enclosures"...whatever that means. They had a woofer in the front and a passive radiator in the rear.
I'm thinking it's possible my system, made to replace my Obelisks, might have been designed somewhat similarly but terminating in an active woofer.

If the backwave emerges from a 1/2-wavelength labyrinth in-phase would it not augment the low-frequency outputs of both woofers?

 

[Flaesh]

I don't know how many baffles or if the line tapers.

For example may be

(or not)

 

[SIY]

Before these I had Shahinian Obelisk speakers

Those were VERY interesting speakers, which would likely do badly on current measurement suites but were terrific for symphonic music. What happened to them?

 

[sergeauckland]

P.S.
I thought about building a transmission line (or labyrinth) once (probably tuned at 20Hz). But it was going to be impractically large. 20Hz has a 56-foot long wavelength so even a half-wavelength is long.

I don't usually say "impractical". The "speaker stack" in my living room is almost 8-feet tall! Just an inch or two below the ceiling!

I'm not sure what a true acoustic transmission line would look like. In the RF world, a transmission line is impedance matched to transmit a signal efficiently with no (or minimal) standing waves or return loss.

The main point of a Transmission Line loudspeaker is that it's NOT tuned. ALL the rear radiation is absorbed in the line, so theoretically it doesn't matter if the line is open or closed. If closed, then any energy still remaining has to travel back through the absorption such that there's really nothing left to reach the back of the cone and colour the reproduction. That's the theory. In practice, few so-called TL loudspeakers are true TLs, and the clue there is the open vent at the front, as there's still LF energy left that by the time it gets out, and with correct dimensioning, the energy left has gone through a phase change (due to the wave velocity being slowed down by the line stuffing) that now reinforces the energy from the front of the cone, helping maintain bass output. B&W's Nautilus is one of the few true TL loudspeakers, and they're pretty big. The larger PMCs also are true TLs (or very close to it).

With a subwoofer, as you say, the wavelength at 20Hz is 56ft or around 17m and even folding that still makes for a very large loudspeaker. That's why I don't know of any commercial domestic subwoofer that uses TL loading. Any that claim to do so, won't be true TLs.

S.

 

[SIY]

The main point of a Transmission Line loudspeaker is that it's NOT tuned. ALL the rear radiation is absorbed in the line

You have described a labyrinth, not a TL.

 

[sergeauckland]

You have described a labyrinth, not a TL.

In my view, a TL is so-called, because just like an RF transmission line, there is no reflection from the end when the impedance of the line is matched. In general terms, a TL loudspeaker has the TL tapering from wide to narrow, as the energy from the back is of the driver is progressively absorbed. There should be no energy left at the end of the line, so making it immaterial whether the vent is open or closed.

An acoustic labyrinth is indeed similar, but the vent deliberately allows the lowest frequencies through. In practice, I accept that most so-called TL loudspeakers are actually Acoustic Labyrinth.

S.

 

[ryanosaur]

The main point of a Transmission Line loudspeaker is that it's NOT tuned. ALL the rear radiation is absorbed in the line, so theoretically it doesn't matter if the line is open or closed. If closed, then any energy still remaining has to travel back through the absorption such that there's really nothing left to reach the back of the cone and colour the reproduction. That's the theory. In practice, few so-called TL loudspeakers are true TLs, and the clue there is the open vent at the front, as there's still LF energy left that by the time it gets out, and with correct dimensioning, the energy left has gone through a phase change (due to the wave velocity being slowed down by the line stuffing) that now reinforces the energy from the front of the cone, helping maintain bass output. B&W's Nautilus is one of the few true TL loudspeakers, and they're pretty big. The larger PMCs also are true TLs (or very close to it).

With a subwoofer, as you say, the wavelength at 20Hz is 56ft or around 17m and even folding that still makes for a very large loudspeaker. That's why I don't know of any commercial domestic subwoofer that uses TL loading. Any that claim to do so, won't be true TLs.

S.

You contradict yourself in your own words. I agree with the statement that the open end of the line will support the driver with audible output which is in phase. So how could a line absorb all rear radiation AND have such output?

Regardless, TLs are tuned to the Driver Fs, for optimal performance and both King and Augspurger show the optimal tune to always be above Fs. Unless you are going to claim TQWT designs don’t count?
According to King, the placement of the driver at 1/3 the length of the line aids in damping that 2nd harmonic (an octave and a fifth above fundamental). The proper stuffing of the line helps damp the rest of unwanted distortions and brings the phase back in time 1cycle off. It also helps smooth out the impedance.

 

[sergeauckland]

You contradict yourself in your own words. I agree with the statement that the open end of the line will support the driver with audible output which is in phase. So how could a line absorb all rear radiation AND have such output?

Regardless, TLs are tuned to the Driver Fs, for optimal performance and both King and Augspurger show the optimal tune to always be above Fs. Unless you are going to claim TQWT designs don’t count?
According to King, the placement of the driver at 1/3 the length of the line aids in damping that 2nd harmonic (an octave and a fifth above fundamental). The proper stuffing of the line helps damp the rest of unwanted distortions and brings the phase back in time 1cycle off. It also helps smooth out the impedance.

It can't. If a line absorbs the whole output, which it should in a real TL, then there's nothing to come out, which is why it doesn't matter if the line is open or closed.

Secondly, TLs are NOT tuned. If they are, then they're not TLs. Practical, real world so-called TLs are, but then they're not real TLs. At this point, TL is just a marketing name, not a technical name.

A TQWT isn't a Transmission Line, whatever the initials stand for. Any loudspeaker that relies on tuning isn't a Transmission Line, which by definition is untuned.

S.

 

[ryanosaur]

Well, clearly, everything I've read, and the designers I've discussed this topic with are all wrong. *shrugs

 

[SIY]

Well, clearly, everything I've read, and the designers I've discussed this topic with are all wrong. *shrugs

Bud Fried and Martin King nod.

Speaking of which, King has removed access to his design spreadsheets. Does anyone know a good alternative?

 

[ryanosaur]

Speaking of which, King has removed access to his design spreadsheets.

It's been quite a while now since he took that down. Bummer, too!

George Augspurger's program is available to download online. One designer I know who uses King's MathCAD program agrees that Augspurger's is a very good alternative. They effectively each confirmed the other's approach to solving the design issues. (Strangely, the guy I know that is into Augspurger's work really dislikes King's solution. *shrugs Again, most seem to agree they are pretty comparable.)
Interestingly, the King program seems to be able to do a little more than Augspurger's, and some lines I've seen modeled by this gentleman come out much more compact than I would otherwise expect.
One example he showed using the ScanSpeak 13" Subwoofer Driver was not really any larger than a big Tower Speaker with a single fold in the line (modeled and laid out in a CAD program only). Comparatively, a 12" Dayton HF Sub built using the Augspurger program by a different designer resulted in a much larger enclosure, still with a single fold in the line.

A google search should take you to the site where you can download Augspurger's program. PC only...

 

[SIY]

A google search should take you to the site where you can download Augspurger's program. PC only...

Fortunately, I hate Apples with a deep, white-hot, burning fury.

Thanks for the tip!