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Please explain: Transmission line bass loading.

208.221.218.82
Posted on March 30, 1999 at 17:06:31
mikem


 
In my serach for my next speakers.. I came across a design that uses transmission line bass loading? Not quite sure if this is just a "buzz" phrase or a technique that I am not aware of.. Any explanations?
thanks
mike

 

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Re: Please explain: Transmission line bass loading., posted on March 30, 1999 at 19:59:39
Clayton Oxendine


 
T/Ls work well for midrange and bass. There are tradeoffs with all enclosure types, and the one for T/Ls is size. The benefits of the design are unparalleled neutrality, uncolored sound,moderate efficiency, and powerful extended low frequency support.
As the name implies, the enclosure is a transmission line. The line can be either 1/4 or 3/4 wavelength long at some critical frequency. The shorter lines are more popular, although, they are a bit more critical to stuff. Stuffing the line makes it possible to make the physical size of the enclosure smaller because the speed of sound through the line is reduced. The reduced speed of sound within the line makes the overall line length shorter than the equivalent undamped wavelength by a significant percentage. The overall size of the enclosure can be reduced even further by folding the line back upon itself, hence the name: "folded T/L".
The "critical frequency" is that frequency at which you want to have a 180 degree phase reversal. The phase of the rear wave is reversed, exits the terminus and provides bass augmentation for the sound emanating from the front of the driver's cone. Broadband support is provided by the frequencies close to and on either side of the 180 degree phase shift.
Transmission Lines are not resonant enclosures in the same sense that ported or reflex enclosures are. The opening at the end of the line is properly referred to as a "Terminus" rather than a "Port". Because the air pressure within the line is nearly that of ambient, box colorations are essentially non-existent. System resonances can be eliminated by tapering one wall of the line. Doing so creates an infinite number of very minute resonances that are then absorbed by the stuffing. The stuffing also contributes to the driver's exceptional damping. There is a book from Old Colony that is the only book I know of on T/L boxes. They are difficult to get right and take lots of fine tuning.

Clayton

 

Re: Please explain: Transmission line bass loading., posted on March 31, 1999 at 00:04:43
Sean


 
Transmission lines (T/L) and Labyrinths are more common in British speaker designs than anywhere else.

The "transmission line" uses a woofer that radiates out of the front like a conventional speaker design, but also radiates the sound produced from the back of the woofer into a box that is actually a tuned tunnel. In order to conserve space in many designs, the tunnel is folded inside the box like a small maze. The maze feeds a large opening, much larger than a port typically. The hole should just about equal the cone area of the woofer being used. Internal dimensions are VERY critical to tuning. This makes it hard to mass produce.

The impedance peak is very small and well damped, letting the amp control the woofer much better than in most ported or passive radiator designs. Even though it does use the back wave of the woofer like ports and passives do, T/L's typically don't suffer from the the same side effects such as ringing and "slow" or "sloppy" bass. Port noise is also minimized due to the use of such a large vent hole.

Bass response is typically VERY smooth and extended with excellent control IF properly designed. T/L's typically suffer from lower efficiency though and do take more power to play loud. They are an "easy" load to drive though, as their impedance stays pretty consistent throughout the drivers range.

As Clayton stated, it is very difficult to build a good T/L design, let alone mass produce it. That is the main reason that they aren't that common. If properly done, transmission lines are about the only design that can rival the "tightness" of a properly designed sealed box. If you've got enough amp and the manufacturer has done their homework with good quality control, they can work excellently. Sean
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Thanks for the explanation! The medowlark.., posted on March 31, 1999 at 05:35:41
mikem


 
Heron has transmission line loading. It sounded very good.. but the room they were in was terrible.. I need to have another listen.. Thank you both the for the great explanations!
mike

 

Re: Thanks for the explanation! The medowlark.., posted on March 31, 1999 at 08:35:49
Audiophilander


 
If they sound very good in a poorly set-up listening environment, you should seriously give them another listen.

BTW, I was wondering if you had closed in the open side of your living room yet or are you considering adding on a dedicated listening room?

FWIW, the best of luck to you on your new speaker sojourn, I sincerely mean that. Keep us updated; I enjoy reading the kinder/gentler mikem posts.

Cheers,
AuPh


 

Re: Please explain: Transmission line bass loading., posted on March 31, 1999 at 09:33:09
Edp


 
>The benefits of the design are unparalleled neutrality, uncolored sound,moderate efficiency, and powerful extended low frequency support.

Umm, being kinda generous here. It is not a given that TL's will give neutrality or uncolored sound. IMO a good number of TL's are very colored and sorta bloated in their response. Some models of Meadowlark are just those examples.

Also there is something refered to as a terminated transmission line that has no exit or port, its purpose is to reduce the backwave transmission and have the same effect as "open TL's" on the impedance response. This approach is also used by some on tweeters also.

But the general jist of what you say is very much the purpose of a TL and excellent advice to mikem. It is not a buzzword.

Only mikem can tell, but would guess that change from VR 4's to Meadowlark Heron's would be a laterial move not a real upgrade.

 

Re: Thanks for the explanation! The medowlark.., posted on March 31, 1999 at 16:15:33
mikem


 
AP.. here you go again...

"If they sound very good in a poorly set-up listening environment, you should seriously give them another listen."

Great advice... we can't all have a wonderful enviornment like you have.. or such great equipment.. if I could only find a 10 year old HT receiver...

"BTW, I was wondering if you had closed in the open side of your living room yet or are you considering adding on a
dedicated listening room?"

Yeah.. I filled it all in.. I stuffed 10,000 pillows in the openning. Now we all enter the house thru the window. Works and sounds great.. Glad you mentioned it.

"FWIW, the best of luck to you on your new speaker sojourn, I sincerely mean that. Keep us updated; I enjoy reading
the kinder/gentler mikem posts."

Gee.. your sincerity is just too much to handle... I can't wait to hear some more good advice from you... Do me a favor please... Ignore my messages from now on.. I'll do the same for you.
mike

 

Re: Can you compare the Herons to Shearwaters?..., posted on April 1, 1999 at 16:32:34
RT


 
miken-

I'm saving for meadowlark shearwaters right now, but might decide to wait till later this year to buy meadowlark herons if they are significantly better. I'm sure that the bass was better in Herons, but did you perhaps notice anything else?

Thanks-

RT

 

Re: Can you compare the Herons to Shearwaters?..., posted on April 1, 1999 at 18:17:18
mikem


 
I thought the heron's sound was more "full". The imaging was great! I thought they had multi-channels on... The bass was full.. but I didn't hear enough of them to judge how natural it was. The dealer played alot of music with synthetic bass to show off the speaker.. The shearwater had a similar sound but not as dynamic. If you like lots of rock or dynamics say for HT.. then save up and get the herons...
mike

 

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