PMC TLE1 style sub

[Dr_EM]

I'm keen on making a sub similar in design to the PMC TLE1:

http://www.pmcloudspeaker.com/tle1.html

I really like the design both aesthetically and in it's nature. The idea of deep extending, low distortion bass is very appealing .

I am looking at the Morel HU631 for drivers. They are the right dimensions, a reasonable price and have specs (just about) suitable for a TL design. They are also readily available to me :

http://www.morelhifi.com/products/pdf/hu 631.pdf

The relatively low QMS might be less than optimal though . Qes and (presumably most critically) Qts fall into the suggested range though.

I found this calculator to help design the TL with. I'm not 100% on the tuning though; would you tune it to the lowest frequency you need, or does it need to relate to the drivers resonant frequency somehow? I'm also unsure on the exact meaning of some of the terms.

http://www.mhsoft.nl/spk_calc.asp#transmissionline

Width ca. is presumably the width of the cabinet? With 2 drivers the total SD is 238CM, which gives a reasonable value of 23.76cm assuming that is indeed what it is.

I'm not sure on the meaning of "Opening behind driver"?

The TML opening = the total driver SD and just refers to the size of opening at the end of the line?

Is that calculator especially relevant to folded line designs? I assume it'll still behave similarly or do the folds slow it down and thus make the required line shorter?

Any opinions? Would love to try it if it's likely to work properly

 

[MJL21193]

This is an attractive looking sub. Using two 6.5" drivers makes for a very slim box. The issue I have is that they clain to have a true TM line of 3 metres inside. That would make the line very narrow. Martin J. King ( TML Guru) advises the start of the line should be equal to SD and taper inwards to the end for best results.
Having built TML speakers before based on his research, I have to agree.
With that said I am sure that the sub sounds good. I just have to question is there is an easier way to achieve the same results. IMO yes.
http://www.quarter-wave.com/

 

[Dr_EM]

Thanks for your reply

Yes, I am arriving at numbers around 3 metres with that calculator too (which makes sense with my tuning being 22hz and the driver SD by nature being almost identical to the PMC design). So you are suggesting a shorter line may yeild the same results? That is an interesting idea. I don't mind making the sub slightly deeper to fit the full line in if it is worth doing though.

The start of the line should have a cross section of 238CM for those drivers then? The final opening should also be 238CM? Does the line need to remain at 238CM cross section throughout or not, if it tapers inwards then it'll somehow need to go out again for the opening to be the correct size; or should it be a bit smaller?

The articles on that page are a bit high level for me . How would I decide the tuning? Can it be anything (like the 22hz of the PMC design), or does it somehow need to relate to the Fs (though 22hz is actually half the driver Fs in this case).

 

[MJL21193]

Sorry, by tapers I mean narrows. Cross section area should reduce to half SD( around 100-140 sq. cm) I was not saying that the whole line will fit in the box, the box is too small. The definition of TL is blurry with several different interpretations floating around. I, and thousand of others, subscribe to Martin King's definition. He has produced scientific models for TL speakers that have been proven accurate.

 

[planet10]

That calculator is useless. It is based on obselete classical TL methodology.

1st off Sd has nothing to do with a TL (unfortunately Martin chose it as a variable unit of area which confuses things). The parameter that is important -- just like in every other kind of box -- is the Vas.And as soon as you taper the line it has to get shorter.

And why, with 2 drivers, would you not mount the drivers push-push? The benefts are so large that it seems totally a waste.

An externally hosted image should be here but it was not working when we last tested it.

I'll alert Scott to have a look at your app.

dave

 

[MJL21193]

Quote:"That calculator is useless. It is based on obselete classical TL methodology."
Dave:
You are refering to the MHSOFT link in post#1? Not Martin King's mathcad models?
I worked with those models for 3 months to design the speakers that I am currently using. The drivers are the Scanspeak D2905/9300 and the Vifa PL18WO0908. I have smooth bass response down to ~28Hz. Nothing like a vented box.
I also buildt a TL with an old MAX Pentivent 12" with an impossibly high VAS (338) that in every other box configuration sounds like crap. In the 120" TL, it found a home, impersonating a 15"
As you can tell, im a firm believer.

 

[planet10]

ou are refering to the MHSOFT link in post#1? Not Martin King's mathcad models?

Yes. Martin's models (seconded by Augspurger's) have clearly shown the pitfalls associated with using classical TL design methodology (as in the link in the 1st post). If you look at the most successful classic TLs, it is clear they trial & erred themselves to designs somewhat similar to what we would generate today using MJK. There are also a lot of less successful examples of TLs that are really little more than overly complex aperiodic boxes (not to diss aperiodic boxes which can be VERY good)

dave

 

[Scottmoose]

I'll have a look at those drivers & see if we can come up with something in MathCad. Specs look OK for TL loading. Just remember, it's not going to be particularly small! As Dave says, forget that TL calculator. Doesn't work. Especially inaccurate for tapered lines BTW. Tuning frequency of a TL / QWR is dependant upon both length and taper, not just length.

 

[Dr_EM]

Thanks for the replies

That would be ace if you could figure out some dimensions for those drivers in TL configuration as I'm starting to think I can't very well on my own . I already bought some of the electrical parts for this since there was a special offer at my supplier today, so hopefully I will end up with a useable design.

I don't mind if the cabinet is a little taller or deeper than the PMC model but i'd like to keep it narrow. Thier design is fairly large but has unusual dimensions to give it a smaller appearance (and footprint).

Push pull mounting is an interesting point. I guess the design aspect was considered quite highly so they stuck with having both drivers visable (which looks nicer imo). If there are serious advantages to using push pull configurations in this TL design I might think about it more (especially if it makes it easier to achieve the same results).

 

[planet10]

ush pull mounting is an interesting point. I guess the design aspect was considered quite highly so they stuck with having both drivers visable (which looks nicer imo). If there are serious advantages to using push pull configurations in this TL design I might think about it more (especially if it makes it easier to achieve the same results).

Not push-pull. Push-push. Yes the advantages -- espicially in a woofer -- are so large that one has to think twice about building a woofer with only one driver. Also a push-push arrangement usually allows for the least width.

The morel has a nice basket for push-push but the rear pole-piece vent (a necessary evil in many cases) means you have to allow clearance for that -- you could probably get away with a box 160 mm (6 1/4") wide, which would be less than if front mounted. I'd probably go a bit wider thou or you end up with more issues with the TL fold(s).

And if it is push-push you could get away with 12 or 15 mm plywood which would further shrink the box size.

dave

 

[Dr_EM]

So in that arrangement is one driver placed behind the other, both forward firing but with only one visable? So one will be placed on a baffle inside the cabinet? Would the TL start behind the internal driver or between the two if that is the case?

Still prefer to have both drivers on show like the PMC model mind . Actually does that also cause problems with the drivers having differing effective line lenghths

I plan to use MDF for construction, get it cut at the shop

 

[Scottmoose]

No. What you've described is an isobaric arrangement. Push-push is bipolar. Imagine a sealed box, for sake of example, with one unit mounted on the front panel, firing forward, and one on the rear directly behind it, firing backward. That's what Dave's talking about, though in this case, it would have to be with one driver mounted on each side.

Having the drivers at slightly different positions along the line doesn't really affect things much. One excites some resonant modes more than the other does, that's all.

You can get fairly acceptable results out of a 100in line, 3:1 taper, So=3Sd, Sl=1Sd, centre-point between the drivers 10.5in down from the internal top. Graph attached. It doesn't look pretty, but this is a 1/2 space graph. The LF roll-off should actually match room-gain quite well well & end up being fairly flat to the mid 20Hz regions. XO to your main units at ~80Hz or so and you should be in business (I wouldn't try crossing over higher up -over 80Hz, sound becomes increasingly directional, so you'd need two subs to preserve stereo imaging).

 

[planet10]

So in that arrangement is one driver placed behind the other,

No. Configured as you want the box to be, drivers would be placed on the sides.

The arrangment (in a over the top implementation) is like this.

I plan to use MDF for construction, get it cut at the shop

You are spending all this money on drivers, and spending the time building them, why degrade the box performance to save a bit of money by using MDF?

dave

 

[planet10]

Here is a real-world example of a push-push woofer (with room gain should get into the low 20s)

Implemented in 18mm BB it is 247x544x510mm

dave

 

[Scottmoose]

Suitably menacing Dave. What were the woofers in those?

 

[MJL21193]

Quote:"...degrade the box performance to save a bit of money by using MDF?"

Dave, how does MDF degrade the box performance? Acoustically? Mechanically? Aesthetically?

 

[planet10]

Each has 2 Adire Extremis 6,8. Actively XOed at 106 hz to the sealed FE167. Woofers driven with Bryston 4B, sats with a Red Light District. Sealed FE167 centre & 3 FE127 diyRef monopoles built into the walls & ceiling for surround. Centre + surrounds use another 4 channels of RLD. All Fostex are modified. FE167 phase plugs match the veneer & the bases on the RLD.

XO is not yet published but is a gyrator based tube XO.

dave

 

[planet10]

ow does MDF degrade the box performance? Acoustically? Mechanically? Aesthetically?

Acoustically, because it is not as stiff as plywood, has higher energy storage, and panel resonances have lower Q so -- coupled with lower panel resonance frequency -- are more likely to be excited. The biggest thing in a sub would be energy storage issues. It also won't be as mechanically strong. 12mm ply will outperform 19mm MDF. For a sub 15 mm is probably the most practical.

On top of that push-push with its active newtonian motion cancellation will transfer far less energy to the box (which is why you could make it with 12 or 15mm ply.

dave

 

[MJL21193]

Mechanically yes, I don't dispute you could build a stronger box with bb plywood, but since most of us are not constructing stage equipment, I don't thing it's that much of an advantage.
As I am not from the tenth planet , I have to admit that I cannot hear the difference between the two materials. Everything resonates and stores energy, but how much? My drivers drown that out..
Absolutely true about the opposite motion of push-push mounted drivers cancelling out vibration, especially if they are identical, with very similar T/S specs.

 

[Scottmoose]

Oh, you can hear the difference easily enough in A/B comparisons. The MDF box has considerably more colouration / smearing in the time domain due to it's abrupt break-up mode. Remember, down low, around the cabinet tuning frequency, driver output is minimal -it's the box doing all the work. And things only get worse as the box gets bigger, with larger panel areas to vibrate.