TL Sub VS Ported Sub

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dis

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#1
I'm thinking of building a sub of the summer, the sub will be meant for music over HT.

Two types of subwoofer enclosures that caught my eye are:

1) A TL sub similar to the pmcloudspeaker TLE1 (http://www.pmcloudspeaker.com/tle1.html) consisting of 2x Peerless CSX 176 H, 6.5" drivers.

2) A ported enclosure much like the REL Storm III which consists of a 10" Volt driver (yet to be determined)

Cost between the two is no real issue, which type of enclosure would you forum veterans suggest and why?

Thanks!
 
#3
I have heard 8" Focal kevlar TL subs, and played with some test TL sub boxes myself and found them to be extremely good for maximum bass extension and bass quality. For example a Focal 8" would normally model about 35hz f3 in ported box compared to about less than 20hz in TL. Check the DIY TL website, google search for info. Not all drivers are suited to TL.

One design that works well is a parallel TL length (not tapered). Assume cone Sd area is continued from behind driver to throat exit for about 4m TL length but 1/2 to 2/3 of the 4m is OK. Stuffing is not needed.

TL boxes will be much larger than ported. For example, an 8" driver can be folded 6 times into something like a (18mm MDF) 26xm wide x 70cm high x 70cm deep box (90L).

I have tried old Kef B139 (10"), and others that worked very well. 10" gives a very big TL box of around 70h x 35w x 52d cm 18mm MDF with compromised 4 TL folds of 2.8m line length but still great bass.
 
#4
What's your intended F3? A TL will work quite well as a sub if you have room for it, but here are your restraints: Line length of 1/4 wavelength is required, so for 25 Hz that's 11 feet. Driver Fs must be equal to or less than the line Fo, so the Peerless is out of the running unless you want to settle for a 38 Hz Fo; Qts should be .5 to 1.0 for best result, so the Peerless is OK; Line total volume of Vas x .6 comes in at about 1.2 cu ft for two of them; Xmax of 5.5 mm also means that you can forget about much of an SPL below 40 Hz.

Result: Using the Peerless in an MTM 2 way TL good down to the mid 30's is fine, but as a real sub to go to 25 Hz or less they won't cut it. If you do decide to go TL with the Peerless they'll work OK in the Seas Thor box; get plans for it from Seas or Madisound.
 
#5
BillFitzmaurice said:
Line length of 1/4 wavelength is required, so for 25 Hz that's 11 feet.
Click to expand...

This is only true for a non-tapered pipe. If the line gets bigger near the terminus (ie TQWT) the line needs to be longer to support the same quarter-wave resonance, if it gets narrower near the terminus then the line gets shorter.

dave
 
#6
According to the traditional methods of TL calcing that's true, but Augsperger rendered those to the dustbin four years ago. The final determination of length for a given driver is predecated on the driver Fs and Qts, and once the Fs/Fp ratio based on Qts determines the potential Fp of the line the length will be a quarter wavelength irrespective of the geometry. As for a TL that expands in volume towards the exit that isn't a TL, it's a horn.
 
#7
Bill,

I'm looking for an F3 around 30 hz if possible. The pmcloudspeaker site suggests that it uses a line length of 9.5 feet and yet its supposedly able to play down to 22hz, using 2x 6.5" drivers.

What I'd like to is clone the TLE1 as much as possible, it dimensions are relatively small, unfortunately I don't know what driver is used by them.

Maybe I should be looking at another driver? Any suggestions?

Such as one of these...?
http://www.d-s-t.com/scs/data/18w_8545k00d.htm
http://www.d-s-t.com/scs/data/18w_8546-00d.htm
 
#8
The final determination of length for a given driver is predecated on the driver Fs and Qts, and once the Fs/Fp ratio based on Qts determines the potential Fp of the line the length will be a quarter wavelength irrespective of the geometry.
Click to expand...

I think that what you will find in Augspurger's alignment tables is that he has set up TL geometries based on optimization of a particular variable which produced Fp as a function of Fs and Qts. It is not based on some mathematical closed form solution but was most likely derived by a series of simulations. There are other choices of variable that could be optimized which would produce a different alignment table. This is just like the different alignment tables, based on filter theory, for bass reflex designs.

There is no single correct answer for a TL alignment table, there are only different compromises. I optimized my alignment tables on a different variable/goal and normalized the cross-sectional areas against Sd. I would not consider Augspurger's or my alignment tables as gospel, I would consider them as two valid options that the TL designer can use. The TL designer could use something entirely different as well.

As for a TL that expands in volume towards the exit that isn't a TL, it's a horn.
Click to expand...

So any line that expands in volume is a horn? If that is what you are really saying I cannot agree. My definition of a horn would be based on the expansion being large enough, and the mouth being large enough, so that the resonances or standing waves are so well damped that the output from the mouth is smooth. If you see a series of peaks in a "horn's" SPL and impedance response, I would conclude it is closer to behavio generally associated with a TL. You have discrete quarter wave resonances that cause peaking in the output from the mouth/terminus.
 
#9
As I noted at another thread there is a great similarity between the driver requirements for a TL and those for a VB, and simply put what will work in a VB will probably work in a TL, and if modeling in a VB won't get you to the desired F3 then you won't get it from a TL either. For an F3 at 30 Hz depending on Qts you'll need a driver Fs of somewhere between 20 and 25 Hz or so. The line volume (Vp) is dependant on Vas, and the lower the Qts the larger the Vp will have to be. For the most part what you can do is to model the driver in WinISD, and the TL F3 will be about 10-20% higher than the VB, while the Vp (total line volume) will run about the same percentage smaller than that of the VB.

The question arises is that if the TL has a higher F3 for the same driver than the VB then what is the benefit to the TL? The answer is that the TL rolls off below F3 at only 12dB instead of the 24dB of the VB, so despite the higher F3 the usable frequency range is actually lower, and the TL doesn't exhibit the sometimes nasty low end problems that ports exhiibit, having the sound of a sealed box. That, plus the bass lift of the TL, make it a good choice.

MJK: a line that expands in volume by definition is a horn, though of course not necessarily a good one., and you certainly wouldn't stuff a horn. The pitfalls you describe are de riguer for any horn and overcoming them is what makes a good one. As for compromises, they are present in every box, and there's always another way of doing it.
 
#10
Bill,

Most of my work contradicts what you have said above. I am afraid I cannot agree with your summary of TL behavior as far as f3 is concerned or the roll-off rate. I also do not agree with your definition of a horn.

I really don't want to get into a long discussion or arguement over these topics so I will remain silent. I will let my documentation and software present my understanding of TL behavior.
 
#11
Martin & George's work have both shown us that the pigeon-holed names TL, TQWT, Horn, are really just small subsets of a much larger quarter-wave space of designs... i use the terms somewhat interchangably now -- i certinaly consider a TQWT to be just another flavour of TL.

dave
 
#12
Well. if you look at Voigt's original 1934(?) artwork it's clear that he wasn't really sure what was what either and pretty much tried to encompass every possible configuration in his claims. Must have been the Amar Bose of his day.

As to MLK not getting a 12dB rolloff below Fb I can't comment on his speakers as I have not measured them, but as to his reflex/TL hybrids I wouldn't be surprised if they were 4th order boxes, as once you start using a mass-loaded concept, ie, a tuned duct, then you're no longer dealing with a non-resonant single impedance peak device. The boxes done by Bailey, Augsperger and D'Appolito with pure TLs that are totally stuffed and thus rendered non-resonant do exhibit the single peak impedance and 2nd order rolloff typical of sealed enclosures, and this I can verify from personal experience in measuring my own TL.
 
#15
Yes, he does, and he also notes that to get a single impedance peak and the resulting 2nd order alignment the stuffing density must be adequate, which also serves to eliminate resonance nodes above Fp. No where that I can find in his work does he state that he recommends either understuffing or settling for the reduction in performance that understuffing results in, but the next time I talk to him I'll be sure to get a clarification.
 
#16
He does say that geometry can be used to reduce the need for stuffing to kill the ripple & therefore get greater bass output... Martin has explored the limits of this recomendation and it usually results in a 4th order roll-off.

One of the beauties of a TL is its flexibility.

dave
 
#17
dis said:
Bill,

I'm looking for an F3 around 30 hz if possible. The pmcloudspeaker site suggests that it uses a line length of 9.5 feet and yet its supposedly able to play down to 22hz, using 2x 6.5" drivers.

What I'd like to is clone the TLE1 as much as possible, it dimensions are relatively small, unfortunately I don't know what driver is used by them.

Maybe I should be looking at another driver? Any suggestions?

Such as one of these...?
http://www.d-s-t.com/scs/data/18w_8545k00d.htm
http://www.d-s-t.com/scs/data/18w_8546-00d.htm
Click to expand...


Both of these drivers don't have the Fs/Qts ratio you need for a 30 Hz TL. A TL works best with Qts of at least .5, and even then you need an Fs that is below the intended F3. For a tapered line a Qts of about .3 will result in a Fs/F3 ratio around 1.7, so to get a 30 Hz F3 you'd need around a 18 Hz Fs. But with a .5 Qts the Fs/F3 ratio is about 1.2, so a 25 Hz Fs would get a 30 Hz F3.

I don't know if you can get them where you are but a driver that has close to those specs is the Dayton Titanic 12, with Fs of 22Hz, Qts .44. That would get an F3 around 30 Hz. Below that response will drop at 12dB, but if your maximum room dimension is no more than 37 feet (1 wavelength at 30 Hz) you'll get room pressurization and room lift that conceiveably would get you flat down to 15 Hz or so.
 
#18
MJK said:
Bill,

I really don't want to get into a long discussion or arguement over these topics so I will remain silent. I will let my documentation and software present my understanding of TL behavior.
Click to expand...


I, personally, appreciate long discussions and (yes) arguments over this subject. Everytime I see this being discussed in a civil forum such as this one, the level of my vocabulary is raised, and with it, my understanding of TL-construction. In truth, morose technicality yields freedom in the creative process.

Thanks to both Bill and MJK for taking up the topic once again. And special thanks to MJK for the continual publication of his work for consideration.

Dave
 
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