P17 MTM TL help on MK's alignment tables

[led7]

My first post. I have 4 vifa P17WJ-00-08 (6.5" mid-bass) and 2 D27 tweeters which I'm hoping to use in a MTM TL similar to the Seas Thor kit (which I understand was the basis of TL7 model kit found here http://www.theloudspeakerkit.com/sho...&ProcessType=1 ).

I've run through Martin King's alignment tables using 2 of the P17 per enclusure but the results don't appear to be anywhere near what has been used for the TL7. I'm new to TLs so perhaps I'm not using the tables correctly. I chose 35 Hz as the system's tuning freq.

For 2 driver I've used cone area of 2x136=272 cm^2, and voice coil resistance divided by 2 as I'm planning wiring in parallel.

For asthetic reasons' (and high SAF) I'd like to stick to a narrow baffle with internal width around 200mm.

Plugging the numbers through I get L=2.438, Dz=34, Dr=0.1133 and So/SL = 2.97.

Questions:-
1) does the So/SL ratio mean that my closed end area should be 2.97 times my open end area ?

2) I gather the open end area should be Sd (272 cm^2). If so, the closed end would end up around 816 cm^2 which appears about twice the the area used in the TL7?

3) The TL7 appears to have used Sd/2 for the open end area and Sd/2*3 for the closed end area. ie the ratio still around 3. Does this different areas alter the performance compared to what I've arived at in 2 above from use of the alignment tables?

BTW, I arrived at 2.34 m line length but understand the TL7 uses 1.8m (70 inches).

Grateful for anyone giving feedback or steering me in the right direction.

led7

[led7]

anyone?

[andrew01]

The TL6 appears to be based on the old Augspurger tables NOT on MK's............

quote from their website :


"The TL6 is a transmission line speaker. The alignment of the TL6 is based upon the tapered, stuffed folded transmission line technique described by G.L. Augspurger.
Compared to older folded horn and infinite labyrinth type transmission line designs common in the 1970’s...."

[andrew01]

There is a similar design to what you're doing here :

http://www.geocities.com/etude317/TLconstruction.html

By an Aussie too !!!

[MJK]

led7,

I don't know anything about the TL7 design so I cannot comment on how it works or its predicted response.

To use the alignment tables for an enclosure with two drivers, assumed wired in parallel, first you have to calculate an equivalent single driver.

Sd' = 2 x Sd
Re' = Re / 2
BL' = BL

I think you have done this correctly but wanted to restate the relationships. The equivalent driver is located midway between the two drivers.

You have not provided enough info to totally check you math but it would appear that you are designing a straight TL, S0/SL = 1. For a straight TL the open and closed ends will have the same area. I believe the final result you calculated should be S0/Sd = 2.97 (I don't know what So/SL = 2.97 means for a straight TL) which is not out of the range of possibilities. If I am correct this means your TL will have the following geometry.

S0 = 2.97 Sd
SL = 2.97 Sd
L_effective = 2.439 m (96 in)

L_actual = L_effective - 0.6 x (SL / pi)^0.5

Again, I cannot double check your work without you laying out the calculations and input values in detail.

The reason the TL7 is shorter is that it is tapered, a tapered TL will be shorter. Look at Table 1 for SL/S0 < 1.

Hope that helps,

[led7]

Martin

Thanks for the response which has clarified my misunderstanding. It appears I incorrectly assumed from the sample design problem in the alignment tables that one should start with a straight pipe and somehow the best geometry would be established.

I've since read through the 'Clarity on Seas Thor Kit' thread (all 40+ pages) through which I've learned a good deal through the exchange of Scott, Dave and yourself. Also re-read you alignment tables paper and a page from http://www.t-linespeakers.org/design...ies/index.html by Bjorn Johannesen.

I'm still not sure about determining the best geometry. Is it an arbitrary choice and just dependent on what size enclosure I can live with ?

The drivers I'm going to use (2 per box) have the following T/S parameters:-

Fd=37 Hz
Vad=34.7 ltr
Qtd=0.35
Qed=0.45
Qmd=0.95
Re=5.8 ohm ----> 2.9 ohm for 2 drivers in parallel
Sd=136 cm^2 ---> 272 cm^2 for two drivers in parallel
Mmd=14 g
Le=0.55 mH
Bl=6.5 Tm

Using the alignment tables I've tried TL with Sl/So=0.1 ; 1 and 10 resultant line length or areas are pretty large as shown below:-

1) Sl/So=0.1
Le (uncorrected) --> 56"
So = 5.3 x Sd = 1441.6 cm^2

2)Sl/So=1
Le (uncorrected) --> 91.2"
So = 2.83 x Sd = 769.7 cm^2

3) Sl/So=10
Le (uncorrected) --> 129.6"
So = 0.634 x Sd = 172.4 cm^2 but that means Sl = 1724 cm^2

Would mass loading by restricting Sl using a classic tapered box (ie option 1 above) permit a smaller enclosure like the Small Thor?

If so, any pointers on how I to go about determining the correct opening size, line length and closed area ?

led7

[led7]

andrew01

Thanks, I had seen the "Etude" on the link you provided. It is very similar to the TL7 so I assume the Etude was also based on Augspurger's models. BTW, I couldn't locate the reference to Augspurger on theloudspeakerkit's website

At first I though I'd run through the design process as an excercise and learning tool and noted TL7 box alignment doesn't appear to be anywhere near what is suggested using Martin's model for the drivers it uses(Vifa P17WJ-00-08). After reading the "Clarity on Seas Thor Kit" thread on this forum it seems the Seas Thor was also designed using Augspurger work. The "Thor" has apparently been improved in terms of bass response by some of the forum's members by the use of Martin's model.

led7

[MJK]

led7,

I'm still not sure about determining the best geometry. Is it an arbitrary choice and just dependent on what size enclosure I can live with ?

I am not sure how to define best geometry. There are a number of trade-offs that can be made between enclosure size and bass performance. The alignment tables tend to go for bass performance resulting in a larger size. The only way I know how to improve on the design in the tables is to go to the next step and do computer simulations. With computer simulations you can optimize for size ot bass performance or something in between.

Would mass loading by restricting Sl using a classic tapered box (ie option 1 above) permit a smaller enclosure like the Small Thor?

Probably.

If so, any pointers on how I to go about determining the correct opening size, line length and closed area ?

The only way I know how to do this is by using a computer simulations and iterating the design. I have not run as many of these designs as other people on this forum so somebody else may have come up with a rule of thumb or a curve fit which will provide an initial alignment.