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Old 2nd October 2013, 06:42 PM   #11
sreten is offline sreten  United Kingdom
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Hi,

As others have said its just driver offset and near 1/3 along the
line was a rule of thumb years before MJK got involved in the act.

An old school TL side on would have 3 sections, gap at the front
gap at the rear and exit at the top. Placing the driver near the
top at the front placed it about 1/3 along, usually a bit less.

Old school was also 2 sections, gap at the top, exit at the
bottom, expanding tapered driver on the front (or rear),
usually a truncated narrow section to get nearer 1/3.

Then Bud Fried came along with a load of unverified nonsense,
but he made it sound good, which it isn't, and was very influential.

MJK's stuff is far more rigourous, and far nearer the truth.

rgds, sreten.
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Last edited by sreten; 2nd October 2013 at 06:45 PM.
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Old 2nd October 2013, 07:20 PM   #12
Onur is offline Onur  Belgium
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There are many options to tune a TL. Reactive filter approach for me the most efficient one, if for example compared to the approach in which the designer fills the cross-section of the line with damping material or even worse, adding a mass loading at the exit of the line.

The reactive filters are solid structures and they do not change properties with increased volume levels. In the case of filling material, they start to move with relative to the air particles, therefore the friction gets smaller and they loose functionality. In the case of mass loading, the friction around the sudden cross-section change also introduces a non-linear behavior, which also changes with the increased volume velocity.

Reactive filters, like resonators, band pass filters or placing the loudspeaker at a certain ratio of the line is a robust design. One can create a low-pass filter in the line to eliminate the unwanted resonances above the fundamental or just kill the second harmonic in high accuracy. Some even tries to kill the third one, because once it is achieved, also the second harmonic gets attenuated. Yet, although the names differ, the application of these can be seen from the very early days of the TL designs. Even in the text of Beraneck he talks about in line low-pass filters and notch filters.
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Old 2nd October 2013, 08:20 PM   #13
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I believe the Germans like to implement Helmholtz tuned cavities to null out the main peaks at 3rd and 5ths. An offset between these points serves a similar purpose.
Much like Visatons kit 'vib130tl' which inspired my own TQWT based on MJKs alignment tables and honed with PeteMcks helpful mathcad checking of my initial work. Except in that i omitted any resonant chambers and used an offset instead.
A real TL still has its charm to me, aside from the sealed box comparisons, the reduced pressure box effect and tamer impedance plot are almost unique to conventional dynamic speaker systems.
How great the advantage depends on perspective though, i guess.

P.s.

The bose offset looks like a ballpark 3rd line length, if you ask me, and id bet it was between 1/3 and 1/5 and following the old school rule.
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Last edited by mondogenerator; 2nd October 2013 at 08:22 PM. Reason: spell check changing Bose to nose. brilliant. the only time wp8 made me laugh.
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Old 3rd October 2013, 02:47 AM   #14
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Quote:
Originally Posted by mondogenerator View Post
The bose offset looks like a ballpark 3rd line length, if you ask me, and id bet it was between 1/3 and 1/5
Very good guess, in fact the offset is 1/4 exactly. Certainly driver offset effects standing waves, but here that is not what is intended. Here, the offset is to modify the amplitude and phase relationship of the two combined pressure waves, one from direct radiation from the driver, and the other from the transmission line terminus. The null effect is caused by the "adding" of pressure waves that are in phase opposition. The OP is on the right track, and there is more than one way to skin this cat.
refer to: U.S. Pat. 7426280

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Old 3rd October 2013, 06:51 AM   #15
DrBoar is offline DrBoar  Sweden
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In the thread "oundpressure inside quarter wave pipe" it appears that 1/4 is better than 1/3 placement in surpressing the 5th harmonic at the expense of less effective supression of the 3rd harmonic.
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Old 3rd October 2013, 07:55 AM   #16
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Yep.if i recall, the offset i used was 0.228.

So nearer a 5th than a 3rd, but much much closer to a quarter. Doing so damps the fundamental a little (i think that's correct), which isn't the aim, so much as damping the 3rd and 5th.
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Old 3rd October 2013, 06:15 PM   #17
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I can recommend MJKs models.

From MJKs excellent work you can model different geometries, volume, tuning frequency, stuffing, driver offset – all the parameters that has an impact on the final result.

My own rules of thumbs are:

Driver offset 1/5.
Tuning 10 to 20 Hz below driver Fs depending on Qts.
For driver with Qts below 0.30 set the tuning higher than driver Fs.
Stuffing upper 2/3 of the line.
Use a straight line, with the last / part having a smaller cross section.

Hi from
Bjrn
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Old 4th October 2013, 01:02 AM   #18
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Yes, Martin King's work is great, and it is cool to see people doing simulations too.

The reason I posted here is that I seen a topic that I have not seen discussed. That is, the interference nulls created with transmission line type speakers. It is the same effect that causes the combing with a center mounted driver on a circular open baffle. Bose worked on this problem and has at least two patents that deal with it each differently.
U.S. Pat. 7426280 and 6771787.

One of the patents uses a driver offset to create a "1/4 wave stub filter" that eliminates the offending frequency(s) from radiating from the terminus.
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Old 4th October 2013, 03:18 AM   #19
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Onken tech resonant-tuned bottom slot ported MLTL "Onken-on-the-floor"

Q: Could Onken type narrow multi-slot ports on the bottom of a TL be used to remove MLTL upper resonant harmonics, as a technology solution to reduce cabinet volume and stuffing requirements? Narrow, tuned, Onken slot ports designed to attenuate the quarter-wave second, fourth, eight harmonics in order to reduce T-line size but keep efficiency. Locating the driver slightly less than 33% down from the cabinet top reduces third, fifth, seventh odd harmonics to complement the Onken slot ports.

Gene Higara Altec 604 appears to use narrow tuned Onken type slot ports on the bottom of the quarter-wave bass reflex cabinet to reduce midrange resonances without excessive MLTL stuffing that also reduces SPL.

Jean Hiagra on his 604 cabinet design:
"The chosen solution here accepts a load of much smaller speaker volume (around 190 liters, instead of the usual 350 liters required for a 15 in. woofer) without experiencing a sharp decrease in sensitivity over the low frequencies, none of the usual sound colorations when a large woofer is associated with a load of small volume. This goal has been reached successfully thanks to the merging of several means : no back-chamber/throat (decompression) but a tuned load (BR) thanks to a laminar vent situated under the speaker itself, to adequate filters and, finally, proper internal padding on wooden walls made of an optimized sandwich of wool plies.
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Old 4th October 2013, 04:25 AM   #20
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Quote:
Originally Posted by LineSource View Post
Q: Could Onken type narrow multi-slot ports on the bottom of a TL be used
I don't know if it does what you suggest, but we have implemented a number of Dr. Lindgren ML-TLs with such vents. They worked out really well.

dave
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