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Old 15th October 2005, 07:49 AM   #121
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Quote:
Originally posted by catapult


The original crossover is already compensating for the baffle step as long as you keep the baffle width the same. The Seas anechoic measurement is remarkably flat above 200 Hz and Bjorn's close-mic'd measurement clearly shows the baffle-step compensation. The bass is the only thing that needs to be fixed.

Err, that's my point. Below 100Hz the original crossover isn't going to compensate enough to retain that slight rise in the response of the stright pipe I posted. The result will be a nominally flat response down to F3.
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Old 15th October 2005, 07:56 AM   #122
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Quote:
Originally posted by Scottmoose
I couldn't resist. Sorry Dave! Final thoughts: a straight pipe is at last possibly on. So & Sm = 3.5Sd. 45" pipe length. Tweeter centred at 0.35% pipe length from top. Vent is circular and 3"x6" (WxL). Close to a BR box, but not quite -it's still a TL. Stuffing density 0.5lbs to the cubic foot. Stuff from the top to the half-way mark, or just beyond.
And then there was ShortThor
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Old 15th October 2005, 09:37 AM   #123
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Old 15th October 2005, 10:21 AM   #124
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Aha! That would be letting on!
Ahem, sorry about that, don't know what came over me!

Quick and dirty version; the line stretches from the top to the bottom of the cabinet.

OK, I know this is confusing, it looks like a bass-reflex box doesn't it, so I'll try to explain. I'm afraid we're getting into theory here, so bare with me.

People often confuse the shape of a cabinet with what it does. The name 'Transmission Line' is actually inaccurate -it's an electrical term, not an acoustic one at all. The proper moniker should be 'Quarter-Wave Resonator' -that's what the cabinet of what we generally call a 'Transmission line' does: it is designed to resonate strongly (using the quarter-waves -now there's a surprise!) at a particular frequency set by the designer, and there are a number of ways to achieve this.

The 'traditional' way of designing a TL cabinet is / was usually to create something wide at the top of the line, and narrow at the bottom of the line, which is often open. That's the cabinet most people expect to see when talking TLs. Thing is, it's not the only way of loading those quarter waves, there are others.

You probably know the TQWT, sometimes known as the Voigt Pipe. That's a TL too, but it reverses the 'traditional' TL taper: it's narrow at the sealed top of the line, and wide at the bottom, which is open, or has a vent or port of some description in it (I'll come to that bit later).

The final way of building a QWR or pipe is the least well known one: the straight, or untapered TL as shown in Dave's most excellent drawing of the ShortThor above. It looks like a vented or bass-reflex box externally, but its still a quarter wave resonator or transmission line, because it's designed to utilise these, unlike a normal vented reflex enclosure.

A few things about TL cabs. in general you might be interested in. They produce resonances at multiples of the pipe fundamental frequency (the length it's tuned to in other words), and that's why stuffing or lining is employed: to damp these down. The less required, the better the pipe / cabinet. The best shouldn't need much stuffing / lining at all, around 0.3 or at the most 0.5 lbs to the cubic foot should be sufficient to supress these higher resonances without affecting the fundamental. If you go too far, bass begins to sag away, because the stuffing has started to damp the fundamental resonance the cabinet generates. Its a ballancing act really.

When designing a TL, you can also employ the mass-loading technique Martin developed. The Fat and ShortThor suggestions I have made both employ this. Mass-loading involves sealing the end of the TL, apart from a small[ish] port. This can be a narrow slot or vent, or a round port like those usually employed on a bass-reflex cabinet. The air mass in the port will provide an additional load to the quarter-waves in the cabinet.

So, in answering your original question, the line of the ShortThor is straight and untapered, and runs from the top to the base of the cabinet. If the end of the cabinet (the end of the line) were open, it would cut off at around 70Hz, give or take. Being a mass-loaded design, this places pretty much halves this cut-off point down to slightly below 40Hz.

For more information if you're interested, have a look at Bob's Theory and Design articles here:
http://www.geocities.com/rbrines1/Articles.html
and then go to Martin's site at www.quarter-wave.com for some heavy math and theory.

Best
Scott
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Old 15th October 2005, 07:00 PM   #125
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Err, that's my point. Below 100Hz the original crossover isn't going to compensate enough to retain that slight rise in the response of the stright pipe I posted. The result will be a nominally flat response down to F3.
Not to put too fine a point on it but the "baffle step", i.e. the change from full space to half space due to baffle width, happens at a much higher frequency. For the Thor, with a 9" wide baffle, it's centered at about 600 Hz and extends about an octave each way. Perhaps you are referring to the boundary loading of the floor, which usually transitions from half space to full space somewhere in the 100-200Hz range?
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Old 15th October 2005, 10:15 PM   #126
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Bingo. I know I shouldn't have had that Anlnwick rum earlier... ;-)
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Old 17th October 2005, 09:00 AM   #127
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Quote:
Originally posted by tamosius
BTW - what do you use? AutoCAD? Thanks for metric measurements!
Shudder... AutoCAD -- no way. I use VectorWorks. IMO better than AC with enuff change left over to buy the computer it runs on (Mac or PC -- better on a Mac)

the metric/english is just a choice of dimensioning set.

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Old 17th October 2005, 03:00 PM   #128
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Old 17th October 2005, 03:26 PM   #129
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Default TL design discussion

Good for you Scott that was a very well written discussion on the basics of TL design. Additional discussion os of course available from Martin Kings web page.

Nice drawing Dave, here are some additonal thoughts: In order to get the tweeter to target 36 - 38" @ 8 - 10 ft the cabinet could either be laid back in a paralelagram or raised on legs.
We have done both my preference is the laid back paralelagram. I have done this two ways calculated the angle and drawn full scale on the floor of my shop to confirm. It makes for a very interesting non typical cabinet presentation.
Legs are pretty well self evident the vent can be located in the base with legs or placed on the front or rear of the enclosure. I like the base as it keeps the cat from crawling in! The rear is nice as it allows a bit of room tuning to the rear wall. I would try and avoid the front as it does not look as nice to me.
Boy this is a big speaker.
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Old 17th October 2005, 04:28 PM   #130
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Scott,

I agree with SCD, good summary.

But I will add two minor corrections.

Quote:
They produce resonances at multiples of the pipe fundamental frequency (the length it's tuned to in other words)
This is only true for straight TL's. If the TL is tapered or expanding then the frequencies of the harmonics will not be nice even multiples of the fundamental. The equation f = n x c / (4 x L), where n = 1, 3, 5, 7,... , only works for straight TL's.

Quote:
you can also employ the mass-loading technique Martin developed
I would take credit for the "ML" label which I applied to my TQWT and later to my TL projects, I don't think it had been used previously (pure marketing on my part). I can also claim that I attempted to provide an explanation for the effect of restricting the end if a TL, thus mass loading. But I cannot take credit for developing this style of TL design, it was being used long before I even considered building a TL speaker for myself.
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