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Adventures In Transmission Line Loudspeakers

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Graham Slee View Drop Down
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    Posted: 30 Dec 2016 at 8:24am
A long time ago I remember there were a few loudspeaker companies advertising acoustic labyrinth loudspeakers. Since then I haven't heard much about them. It seems it is much easier and cost effective for them to now make bass reflex or sealed enclosures.

The trouble with bass reflexes is they are tuned to one particular frequency so you get what's called "one note bass". This might suit some but bass is actually more tuneful, and you can't have a one note tune (can you?).

Sealed cabinets are OK I guess, but it would seem are room position sensitive, in that in some placements the room contributes more (or less) bass than in others. Some like the LS35A are panel tuned to resonate and reinforce their bass output, or so it seems.

Then there are horns of all kinds of shapes and sizes, including the frugel horn, which some here have taken a liking to. Originally a horn was a compression driver plus a flare, and horns of different frequencies were arranged with magnets in the same plane so as not to produce allignment distortion. These were often seen in old cinemas which had the room to accomodate the long length of the bass horn.

There are also open baffle designs which to me are a little like brute force. Air pressures cancel out and to get any meaningful bass require large (12 - 18") bass drivers. This is OK if you have a big enough room (I don't).

The acoustic labyrinth or transmission line loudspeaker is a different approach. In a way it is like an open baffle because output from the rear of the driver is coupled to the open air. However, this is via a long pipe whose length is a quarter wavelength of the "boost frequency", and because it is quarter wavelength its output is 90 degrees out of phase with the cone front output, and so no cancellation occurs.

In addition other frequencies either side of the "boost frequency" are near to 90 degrees and so are not cancelled either, and that differentiates it from the bass reflex in that the bass isn't as one noted.

The "boost frequency" is usually set to the resonant frequency of the driver. It is far easier using an online calculator such as this: http://www.1728.org/freqwavf.htm, than getting the math wrong, and I punched in 70 hertz which gives me a wavelength in air at 20 degrees C of 193 inches (490 cm).

The quarter wavelength is therefore 193/4 which is 48.25 inches (1.225 metres). So the "pipe" needs to be that length.

This would be unmanageable - a 4 foot pipe on the back of a driver - so a cabinet will have to be made such that a labyrinth of dividers inside produces a folded pipe having similar cross sectional area to the area of the cone, and is 4 foot in overall length.

The "port" can emerge anywhere you want, but if facing forward might lead to objectionable results - two outputs 90 degrees out of phase with one another - and early experiments having porting to the rear sounded good to me.

I will offer more background on this next post.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote KJFAudio Quote  Post ReplyReply Direct Link To This Post Posted: 30 Dec 2016 at 9:26am
This is great stuff Graham, maybe worth pasting your plan to this page so that it doesn't get lost?

FWIW a BR can be made to not be one note, is a design choice really. Most modern designs are made to have a pronounced hump at the tuning frequency to give the impression of more bass but this does just lead to the one note sound you describe.

Also with noting that all speakers are just as much as a pain in the arse to get positioned correctly!

Edited by KJFAudio - 30 Dec 2016 at 10:15am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KJFAudio Quote  Post ReplyReply Direct Link To This Post Posted: 30 Dec 2016 at 10:14am
Oh, and if you can hang fire until mid week next week, I'll make you some prototype speaker boxes up.

Stefan

Edited by KJFAudio - 30 Dec 2016 at 10:14am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 30 Dec 2016 at 1:30pm
Originally posted by KJFAudio KJFAudio wrote:

Oh, and if you can hang fire until mid week next week, I'll make you some prototype speaker boxes up.

Stefan


I know you do the Hurricanes but can you make to my design (approx.)? I'd like to experience just how good or bad my own effort is, and it would be educational for this topic (please).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 30 Dec 2016 at 1:59pm
http://www.t-linespeakers.org/readme1st.html#TL

This site has links to most of the TL design masters.
Bruce
AT-14SA, Pickering XV-15/625, Hana EL, Technics SL-1600MK2, Reflex M or C, Lautus, Technics SH-8066, Eminence Beta 8A in custom cabs; Using Majestic DAC & Proprius
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KJFAudio Quote  Post ReplyReply Direct Link To This Post Posted: 30 Dec 2016 at 3:14pm
Originally posted by BAK BAK wrote:

http://www.t-linespeakers.org/readme1st.html#TL

This site has links to most of the TL design masters.


Yes, that's what I'll make for you. I'll just do it with MDF as a TL is well braced anyway.

Stefan
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 30 Dec 2016 at 5:16pm
Originally posted by Graham Slee Graham Slee wrote:

Originally posted by KJFAudio KJFAudio wrote:

Oh, and if you can hang fire until mid week next week, I'll make you some prototype speaker boxes up.

Stefan


I know you do the Hurricanes but can you make to my design (approx.)? I'd like to experience just how good or bad my own effort is, and it would be educational for this topic (please).


I believe Stefan was replying to Graham's quote above with...

Originally posted by KJFAudio KJFAudio wrote:

Yes, that's what I'll make for you. I'll just do it with MDF as a TL is well braced anyway.

Stefan

Stefan

Bruce
AT-14SA, Pickering XV-15/625, Hana EL, Technics SL-1600MK2, Reflex M or C, Lautus, Technics SH-8066, Eminence Beta 8A in custom cabs; Using Majestic DAC & Proprius
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