TL quandry (post is not the project )

Madmike2

Member
2005-04-24 11:55 pm
Toronto
Ok i am questioning if this is wrong forum or not but here foes the question. If its wrong place forgive me but you guys are all about the TL here.

This is where my problem came up.....
http://www.diyaudio.com/forums/showthread.php?s=&threadid=56630

is the original post

I thought i had this TL thing licked math wise but apparently i was wrong ? I am putting this picture here and question so i can "visually' understand what was said to me in the above linked post. I dont understand how 70 odd inches applies to any driver as i said i n that post so if one of you can pick this picture apart and tell me where the math doesnt apply. I would appreciate it.

I have exhausted myself reading on the web on TL's and the theorys vary. I love a good problem, but not one with no consistent answer.

SO the picture example is like using two 8 inch woofers with a front firing port. I used the drivers combined Sd and 35 hz as a set point. Internal dimensions are 22 inches high 28 inches deep and 10 wide. You can move the inner walls in your head where your math takes you. But i am not getting 72 inches for the drivers Sd times the Frequency for a wave length (rough math, still like +/- 5 inches ).

Please help, i have half cut wood lying around for my morel 10 " and Planar project. :xeye:
 

Attachments

  • example1.jpg
    example1.jpg
    98.3 KB · Views: 375

Madmike2

Member
2005-04-24 11:55 pm
Toronto
Wait. 2 x Sd = line width minimum ? with 2 drivers. I am getting near 96 inches line length for 35 hz plus the Sd minus the Bass reflex box tuning by volume which = 84 inches with the numbers in my head.

Tell me its not this easy. Is it just a box long enough to fit 96 inches of line length in it ? Regardless of Sd and Bass reflex volume ?

Have i complicated it too much ?
 
purplepeople said:


Note that driver Fs translates into the TL length. Sd is needed for the volume of the line.


:)ensen.


ooooooooo here i am trying to 'tune' the box. I think i get what i was missing before. The driver you use, the Fs of that driver is what the box is tuned to ?

Somewhere around the web i read that the Sd of the driver was to be removed from the length of the line. And then i found that that was the cross sectional area minimum of the line.

> A TL gives much more bass than most people expect...do you really need two drivers? <

Thats was an example i popped off in my head. My actual project is a 10 " Morel and a BM Planar.

> 84" sounds a little long for a 35Hz TL tuning. I get a figure closer to 77" long. <


Sound divided by the wave length divided by 4 multiplied by the number 12 inches in a foot = 96 inches for 35 hertz ? ? ?

MJK > I have tried to read your thread, it seems to ramble in different directions. I have no idea what your question is or how you determined the lengths. You need to simplify your question.

Ya i am stupid like that. I sort of have a stream of consiousness way of typing or expressing myself. Drives people nuts. See ? there i went again. My question was ... changed now. Ill ask it the way it is in my head right this moment.

Single fold multi-fold , Straight pipe. They all dont work out the same even though it is all 1/4 wave length.

Why are no two design formulas the same for designing a TL enclosure ?
 
Single fold multi-fold , Straight pipe. They all dont work out the same even though it is all 1/4 wave length.

Why? If the cross-sectional area is constant they should all be approximately the same length.

Why are no two design formulas the same for designing a TL enclosure ?

Because everybody else is wrong, just ask any TL "expert".

Now a serious answer.

The newer computer modeling methods for designing a TL are all consistent. The computer models use the same assumptions with respect to how the stuffing acts and how the line geometry effects the tuning frequency. They also produce essentially the same modeled responses for the same driver and geometry.

Pre 1999/2000, most of the TL design methods were based on voodoo and black magic. The biggest error was assuming that the fibers moved and dramatically reduced the speed of sound in the TL. This was just plain wrong and was the basis of almost all of the TL design rules of thumb since the 1960's. Anybody telling you that the speed of sound in a TL is dramatically reduced by the fibers moving is totally out to lunch. There are a lot of people still preaching this theory as a fact so you need to be very careful when reading TL documents on the web or in the literature. Pick a source you have confidence in and follow his/her methods.
 
Thank you MJK :D

Last thing here and i will be set. In this picture (that you should assume is cross sectional area throughtout regardless of what it looks like) where does my line start? The green 'x' or the red one?

And you use straight pipe as an example and later explain that low frequency air pressure creates a mass for resistance. In reality will that straight pipe actually play ? or is a 'pressure chamber' nessicary like the one i keep drawing in my bad art work ?

I have taken up enough of your brain power already. Short answers are good for me. Your website drew the picture i needed in my head, basically i am now just asking which brush you used on the canvas. Thanks a lot :)
 

Attachments

  • example2.jpg
    example2.jpg
    57.9 KB · Views: 168
After I made my first TL it became very apparent to me that the stuffing did move inside the line because I didnt have a screen on the end of the line at first and a bit would fly out every time it played a low note. But I never really thought that the speed of sound was any slower. It seems to me more like the stuffing adds mass to the air inside the line.
 
Never mind my picture up there. I now know that it wont work but Dave's site has pictures that resolve the design quandry i had in my head. Also i need a link to buy the working software for TL design. This math cad thing. I DL'd something but they are just files of some kind no Executables. Thanks.
 
Mike,

A rough sketch to scale of your (weirdly folded) box yeilds a line length of about 75".


Originally posted by purplepeople
Note that driver Fs translates into the TL length. Sd is needed for the volume of the line.


Fs/4 translated into a length gives you the nominal length of a fixed cross-section line.

Sd actually has nothing to do with line cross-section -- Vas does. Sd was used in classic rules-of-thumb to determine cross-section -- and one of the reasons its success rate was low. MJKs use of the term i'm sure is rooted in those same rules-of-thimb, but is really only another unit of area like the cm^2 or in^2. Perhaps in retrospect a misfortune, because even using Martin's sheets people still think in terms of Sd controlling line area.

dave
 
Also i need a link to buy the working software for TL design. This math cad thing. I DL'd something but they are just files of some kind no Executables.

There is a freeware version of MathCad 8 that can be downloaded from my site. Good news is it is free, bad news is you cannot save your work. You can print or copy and paste the results.

If you want to buy MathCad I recommend E-Bay (search under MathCad) and pick up version 2000 or 11 since they are the most stable of the recent releases. I strongly recommend MathCad 2000 over all other releases, should be very reasonably priced.
 
-Can you calculate me the pipe(I calculated myself,but I think,that my calculations are wrong)?

My alignment tables contain line lengths as functions of frequency and shape. That is the best source of this type of info that I know of on the web.

-If I heavy stuff the pipe,what is the incrased lenght(in %)?

The lowest speed of sound that I use in my calculations is 320 m/sec for heavy stuffing densities of Dacron or polyester fiber. This is based on test data. I also have test data for wool and it is essentially the same. So percentage wise :

(344 m/sec) / (320 m/sec) = 1.075 as an upper limit

This value is probably less then the margin of error in the entire method so I recommend that you use the free air speed of sound to calculate the required transmisison line length.