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11th September 2007, 06:17 PM  #1 
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Join Date: Aug 2007

Transmission Line Question
Hi,
I'm new to all this but am quite interested in building a t/l 3 way speaker with the Volt RV3143 as the bass driver. I've read some of MJ King's stuff but find it mind bogglingly complicated. I'm only on a Mac computer so I haven't been able to use his worksheets yet unfortunately. Anyway.... One of the first things that I'm wondering is how do I know what the cross setional area of the line should be? I've read some stuff that gives an equation to work it out and I've read other stuff saying that it should be the same as the Sd of the driver. Can anyone shed any light on this? I'd also be very interested to hear from anyone who's made a t/l with this driver. Thanks, and I'm sure that this will be the first of many questions! 
12th September 2007, 12:03 AM  #2 
diyAudio Member

Here is one possible design.
b 1(1) 
12th September 2007, 03:26 AM  #3  
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Join Date: Mar 2007

Re: Transmission Line Question
Quote:
Hi, You can work out TL cross section and length on paper. I explained it here: http://www.diyaudio.com/forums/showt...hreadid=107889 Quote from the linked thread: "DrEM, To make up for my previous blunder I'll try to help out here. The drivers Fs is 31, use that and the lines taper ratio to get the overall line length. Using Martin's alignment tables, I get 89 inches with a 3:1 taper for this frequency. To determine the closed end (So) cross section involves some math. Using the drivers Qts to get variable Dr from table 3: Dr= .1313 Using Fs and taper ratio to get variable Dz from table 2: Dz= 41.73. Driver Bl is 13.83 Driver Sd is .0363 (m^2) Driver Re is 6.1 Formula: So/Sd=p*c*Sd*Dz*Dr*Re/(Bl)^2 So/Sd=2.62 So=951 cm^2 That's about 12" x 12" for the cross section line start. Driver should be somewhere around 1/5 of the way down the line from the start. The open end will be 317 cm^2. That's about 12" x 4" Increasing the taper to 4:1 or higher will reduce the line length but could cause vent noise, as the open end gets smaller." A little math won't kill you! 

12th September 2007, 10:44 AM  #4 
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Join Date: Aug 2007

Hi,
Thanks for the reply. In fact I had read your post already I was just a little confused because I had also read elsewhere that the line area could just be the same as Sd. Is there any truth to that? From that equation, I've calculated So to be 795 cm2. I believe this driver is used in a PMC MB2 and from the dimensions of their cabinet, either the line area is way smaller (more like Sd) or the line length is way shorter than MJK sugests. Does anybody know much about how they achieved this? Does anybody happen to know what the internal geometry of this speaker is? (Doubtful I know!). The reason that I'm asking is because I'm keen to keep the cabinet as small as possible. With the size that it's looking at the moment, I may look at other possible drivers. 
12th September 2007, 10:44 AM  #5 
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Thaks very much bjorno. So I'm guessing that these are the calculations that I would be able to achieve with MJK's worksheets?
In this arrangement how did you arrive at the cross sectional arrangement I see that Sl is equal to the driver Sd. Also, am I right in thinking that the red line in the first chart is the overall theoretical system frequency response? 
12th September 2007, 11:05 AM  #6 
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DrEM did another sub based on a PMC design a few months ago with good results.
The calculations for line cross section and length give the best size. If this is too big, you can adjust several things, such as taper ratio to get shorter line length. Reduction in cross section is possible without causing too much of a reduction in response. Typically, the line start should be 2 to 3 x Sd but you can make it smaller. It's all about compromise. One of the great things about TL geometry is that it is very tolerant of changes (within limits). 
12th September 2007, 01:57 PM  #7 
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Join Date: Aug 2007

Bjorno,
Is there any way that you could run the following parameters through the worksheet? No problem if you don't have the time!! Line length =2.109 m Sl = 0.03633 m2 So = 0.07266 m2 Driver position ratio = 0.206 I have used MJK's worksheets to calculate the length and Sl/So although I have reduced Sl/So by about 10% to make the enclosure a little smaller. With John's comments in mind, I guess this shouldn't compromise the design too much. Thanks a lot for your help. 
12th September 2007, 03:29 PM  #8  
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Quote:
Quote:
Quote:
b 1(2) 

12th September 2007, 03:32 PM  #9 
diyAudio Member

2(2)

12th September 2007, 05:23 PM  #10 
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Join Date: Aug 2007

Thanks Bjorno that's really very kind of you.
So on paper at least, the bass response looks pretty good right?! I've attached the initial design that I've tried to work out based upon the parameters I was talking about above. This is about as big as I would like the cabinet to be from a practical point of view. I had to lengthen the line just a fraction in order to make enough room on the front baffle for the mid/treble units so it's not exactly the right length about 5% longer I believe. I'm looking at using the Wilmslow Audio Prestige driver/crossover kit to build this speaker (Which uses the ATC SM75150 mid and ScanSpeak R2505/9900 treble). The question for me is... will this give me a better end result than spending the equivalent amount (about 1.8k £GB) on a genuine PMC or similar set of speakers obviously much smaller! I suppose this question is impossible to answer until these things are built! Basically though, what I'm looking for is extreme clarity and very low bass extension I'm not too fussed about them being hugely loud though! I don't like the idea of seperate subs as I have always found it awkward to set them up properly and get a good coherent sound. Whenever I have heard any of the large PMC's they sound absolutely incredible and that's what I'm after but I haven't got £6k for a pair of MB1's! 
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