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Old 14th October 2008, 05:52 PM   #1
Dr_EM is offline Dr_EM  United Kingdom
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Default Closed end TL designs

Hello all . I am interested in closed end TL designs, I don't think the topic has been covered a great deal with ported variants being discussed more.

As far as I understand it, the closed end TL is the "traditional" type, the type originally concieved. Features are a flattened impedence (mainly due to stuffing?) and as far as I've heard this can also improve the midrange response. I also believed the closed end TL provided bass reinforcement, beyond what say a sealed box might give, through quarter wave resonances?

A design that I find interesting is the B&W Nautilus:

http://www.bowers-wilkins.co.uk/disp...rid=1583&sc=hf

whose specs quote a -6db point at 10hz . Is this due to the use of a "heavy" sub type driver in the curled up closed end TL, or are they perhaps quoting an EQ applied response with bass compensation which may be integrated into the active X-over?

Assuming its not the latter, what kind of bass response can one expect from a closed TL? Playing with the MJK sheets I find that virtually no matter what you do, you end up with a response practically identical to an IB response. Is this the norm for such a design or should there be more bass reinforcement when one is designed ideally?

I attach the response of the Hi-Vi M8a driver in a closed TL of 83inches, So=3, Sl=1.2, stuffing 0.5lb ft3, created with MJK worksheets. Would you say it were possible to achieve a "better" (ie, more bass reinforced) response in a closed TL than this? Would you perhaps say that is a good response when coupled with room gain? I'm just looking at various enclosure options for this driver and the closed TL has interested me for a while

Ultimately, why might you choose a closed end TL over a sealed box? Improved midrange performance? Lower group delay? Flatter impedence resulting in easier passive crossover design? Is there any bass extention advantages?
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File Type: gif m8a closed tl response.gif (5.4 KB, 474 views)
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Old 15th October 2008, 02:23 AM   #2
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Default Re: Closed end TL designs

Quote:
Originally posted by Dr.EM
Hello all . . . . . Features are a flattened impedence (mainly due to stuffing?) and as far as I've heard this can also improve the midrange response. I also believed the closed end TL provided bass reinforcement, beyond what say a sealed box might give, through quarter wave resonances?

Ultimately, why might you choose a closed end TL over a sealed box? Improved midrange performance? Lower group delay? Flatter impedence resulting in easier passive crossover design? Is there any bass extention advantages?

Impedance is flattened not by stuffing, but by calculating the length to cause a reflection from the tube end back to the driver that is 180 degrees out of phase when it arrives back at the cone, causing the driver to be extremely damped (controlled) at cone resonance. Wrong length could be bad news, as there is no port to help adjust tuning. There are practical limits to how much stuffing can do to correct the fundamental length. Stuffing is more useful to skew, and higher up, absorb, higher resonances of the tube - knock them off from being strong multiples as in a hollow pipe.

With adequate stuffing the Midrange (off a bass driver, at least) will effectively be in an infinite baffle condition - little of the reflection from the tube end survives the round trip back to the cone. (I dunno what they do for the short-tube on the midrange of the Nautilus....must just stuff the heck out of it)

That's some of the theory. MJK and others can better comment on why it's more popular to go open TL. I suspect that a well-tuned, well-damped open TL performs as well or better, possibly with more efficient deep bass for a given line length.
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Old 15th October 2008, 03:11 AM   #3
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The point of a 1/2 wave TL is to completely absorb the rear wave before it can reflect and pass back thru the cone. Response should be expected to be the same as a similar size sealed box.

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Old 15th October 2008, 12:31 PM   #4
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Default Re: Closed end TL designs

Quote:
Originally posted by Dr.EM


A design that I find interesting is the B&W Nautilus:

http://www.bowers-wilkins.co.uk/disp...rid=1583&sc=hf

whose specs quote a -6db point at 10hz . Is this due to the use of a "heavy" sub type driver in the curled up closed end TL, or are they perhaps quoting an EQ applied response with bass compensation which may be integrated into the active X-over?

Assuming its not the latter, what kind of bass response can one expect from a closed TL? Playing with the MJK sheets I find that virtually no matter what you do, you end up with a response practically identical to an IB response. Is this the norm for such a design or should there be more bass reinforcement when one is designed ideally?


Hi,

I would assume it is the latter, i.e. active EQ.
As I understand it sealed TL cannot reinforce bass.

/sreten.
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Old 15th October 2008, 12:59 PM   #5
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Their main use is in providing the amplifier with an almost flat impedance curve. Other gains will be minimal.
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Old 15th October 2008, 01:16 PM   #6
Dr_EM is offline Dr_EM  United Kingdom
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Thanks for your answers, that helps a lot

So in an application with the driver playing up to about 800-900hz I would likely be better off using an open end TL or more likely a MLTL to give bass extention. Using an active crossover and hence the bass drivers being connected directly to a high damping factor amp, the impedence flattening of a closed TL will have no advantage?

Makes me wonder why B&W chose to use one in thier design. The driver only plays up to 220hz so the advantages of possibly cleaner mids are fairly irrelevant and it is also actively driven.

Does the properly designed closed end TL possibly limit cone excursion better/differently to a typical sealed box making it more viable to EQ?

What is the design criteria for defining the length of a closed 1/2 wave TL; the driver resonance? Does fs change in the box, my 83 inch line corresponds to the 33hz resonance I think, but mabye I'm thinking of quarter wave
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Old 15th October 2008, 02:07 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

The resonant frequency of a closed line is half that of an open ended line.

/sreten.
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Old 15th October 2008, 03:37 PM   #8
Dr_EM is offline Dr_EM  United Kingdom
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Ok, thought I'd try another sim. Made line length half wavelength of driver fs, which is a massive 204.7inches . S0 and Sl are both 1.4sd. It seems to "work" like this looking at the impedence plots (created with MJK worksheets):

Undamped:

Click the image to open in full size.

Damped:

Click the image to open in full size.

I think the impedence could still be flatter, is it possible to get it totally flat, as in 10 ohms max? This still rises to 20 ohms.

The other "big" problem is the size! If I built that it'd be enourmous yes? I could use a ratio in S0 to Sl which in an open TL can allow a decreased line length, but in this closed end sim it does't seem to have the same effect. The line dimensions at the start would need to be a certain size or the driver would have a wall right behind it, which really won't help!

Also, does using 2 drivers in the line make any difference to the parameters I'm using, should I double sd and vas driver parameters?
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Old 15th October 2008, 04:43 PM   #9
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Quote:
Originally posted by Dr.EM

Makes me wonder why B&W chose to use one in thier design. The driver only plays up to 220hz so the advantages of possibly cleaner mids are fairly irrelevant and it is also actively driven.
FYI, only straight tapered pipes are TLs and the Nautilus is an exponentially? expanding horn with a very high BW throat, so with the right driver specs in its mouth it will have a highly damped response in its pass band with a ~flat acoustic and electrical phase response as well as a critically damped alignment, i.e. all the 'perks' of a FLH, an excellent reason for using it IMO.

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Old 15th October 2008, 04:56 PM   #10
Dr_EM is offline Dr_EM  United Kingdom
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I am intrigued

So, the Nautilus is not a tapered closed end TL, but rather a sort of horn in "reverse"? Does this concept only work with this ideal style of construction (ie, with all dimensions expanding) or could a simplified version still work made of wood (so curves possible, but flat sides for instance, as in most BLH designs)?

Can this style of enclosure be simulated with any MJK worksheet? Assuming it is possible to implement without needing fibreglass etc. It may be the enclosure I was seeking really, but I didn't know what it was . Any DIY projects doing this design, I know of Nautilus clones but anybody know any made with wood or with specs/calculations published?
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