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Old 18th October 2013, 09:36 PM   #31
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Hi,

I cant say I completely understand all the technical issues. But my first speaker build I plan on being a T.L.

I'm I right in saying, that you are describing the same technique used in PMC speakers, with this back pipe?

http://www.tnt-audio.com/jpg/pmc_peter.jpg

http://www.6moons.com/audioreviews/pmc4/02.jpg

http://www.positive-feedback.com/Iss...ges/pmccut.jpg
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Old 18th October 2013, 10:09 PM   #32
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I hope not. It's all one line (or it should be) -the driver just happens to be offset / tapped into it a x distance from the throat. 99.9% of the time, it should be offset for the most balanced response (there are exceptions). None of this is new, it's been done for years. Augspurger, King et al all go into detail & there's a wealth of information & quality designs out there for all kinds of QW lines, be they straight or tapered TLs, MLTLs, or whatever & the vast majority have the driver offset. TBH, I'm startled the OP hadn't noticed since it's rare nowadays to find a properly designed QW / TL where the driver is not offset.
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Old 19th October 2013, 09:13 PM   #33
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Quote:
Originally Posted by Patrick Bateman View Post
we see the dip is occurring at a frequency that's equal to the length of the transmission line.
Or more precisely: at a frequency whose wavelength is equal to the TL line.

I would also imagine most folks have seen TL driver offset, but I wonder how many know why it is used. I think most would say that it effects harmonic modes. But, I think that less folks know that it also effects destructive interference. And, that is what the OP was referring to.

Fig.2 refers to what the OP was talking about. Fig.3 is what I referred to in post #25

Regards,
Coch
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Old 20th October 2013, 02:35 AM   #34
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Quote:
Originally Posted by cochleus View Post
I would also imagine most folks have seen TL driver offset, but I wonder how many know why it is used. I think most would say that it effects harmonic modes. But, I think that less folks know that it also effects destructive interference. And, that is what the OP was referring to.
I would think that this is well understood by anyone who has heard of Augspurger and King.

Bob
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Old 20th October 2013, 03:36 AM   #35
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Well it is very basic stuff. I knew about the effect as a young man working for an antenna inventor, we called it the 1/4 wave stub filter.
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Old 18th November 2013, 02:49 AM   #36
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how did u simulate it in Hornresp can you please attach the horn resp file..
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Old 13th May 2014, 09:43 PM   #37
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Default 1-wavelength waveguide cancellation

Quote:
Originally Posted by Patrick Bateman View Post
Interesting!

Like most people do, I typically take a 'trial and error' approach to hornresp, and generally optimize for flat low frequency response.

The thing that was intriguing about the Acoustimass is that the woofer offset is quite large!

Click the image to open in full size.

Here's a clone of a TH-Mini tapped horn, along with the Acoustimass; the TH-Mini has nearly zero offset.

Interestingly, it appears that the TH-Spud uses a lot of offset, along with a slow taper, similar to the acoustimass. I'll have to run some sims on the TH-Spud and see if that offset is tuned to the first dip. Perhaps that explains why the TH-Spud has relatively low efficiency, but wide bandwidth? Whereas the TH-Mini has high efficiency and *low* bandwidth?

(Just thinking out loud here.)
Hello Patrick,

You seemed to stop short in this thread. Did you pursue your transmission line or quarter wave pipe with the offset solution to the 1-wL cancellation? Any actual results?

I believe that the total line length, including your "stub" sets the quarter wave line length, so the "stub" length is not wasted, but part of the total. The driver is just moved up the waveguide by approximately a quarter of the length.

Curious if you realized positive results, or stopped the thread due to a lack of success.

All the best,

- James
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Old 13th May 2014, 09:54 PM   #38
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Quote:
Originally Posted by JamesCroft View Post
Hello Patrick,

You seemed to stop short in this thread. Did you pursue your transmission line or quarter wave pipe with the offset solution to the 1-wL cancellation? Any actual results?

I believe that the total line length, including your "stub" sets the quarter wave line length, so the "stub" length is not wasted, but part of the total. The driver is just moved up the waveguide by approximately a quarter of the length.

Curious if you realized positive results, or stopped the thread due to a lack of success.

All the best,

- James
For the most part I was just exploring how Bose did it.
Considering that they sold bandpass boxes for many years, I found it interesting that they switched to transmission lines, so I was curious to learn why.

At the moment I am running the following arsenal of subs:

At home I have three subs. One is vented, on is a tapped horn, one is bandpass. All are distributed a la Geddes.

In my car everything is horn loaded. The sub is a front loaded horn.

The threads related to these projects are Tapped Horn for the Lazy and Impatient , Car Audio | DiyMobileAudio.com | Car Stereo Forum - View Single Post - Sealed Box versus Infinite Baffle

The car sub is particularly effective. It's a horn built into a sonotube. Picture a horn sliced up into pieces like slices of pie, and you get the general idea. Works very well. The horn is huge, something like six cubic feet, but it weighs less than 40lbs. The use of sonotube allows for a sub that's light enough to remove from the car when necessary, but is still strong and rigid. Recommended!
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Old 31st May 2014, 12:25 PM   #39
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Even if I don't like to be considered a friend of MJK, I have to completely agree with Bob Brines.
Even if the MJK mathematics and physics could be largely improved (using that "academic" knowledge that he carefully dislikes -private communication), the calculations provided by his Mathcad sheets are far more predictive than the majority of the models (unfortunately) still in use.
Let me guess that a better handling of the WHE or the equations governing the stuffing could improve his calculations by a 20%. Let me say that his 80% accuracy is "good enough".
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Old 31st May 2014, 02:51 PM   #40
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Quite. Martin's got the pipe physics down. Hardly surprising given what he does for a living. Damping is more of a variable & I'm not sure there's much he can do about that. There's a baseline for hollow-fibre material which he used -fine of course & flexible enough for most conditions. But the fine details vary with the type of that (not all created equal, or intended to be so), and shift somewhat more if other materials are used also. Still good enough though, especially if you've done some testing of your own & can adjust for those differences.
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