Transmission Line Alignment Tables

[MJK]

I usually do not post announcements of updates to my website on forums. I figure that the people interested in the work are on my mailing list already. But since transmission lines have been discussed here several times in the past few weeks, I decided to add a quick note on this forum.

I have updated my transmission line alignment tables to include a more accurate method that covers a wider range of driver parameters. I have also added four case studies that look at the impact on the calculated response of the drivers T/S paramters, the driver offset position, the taper of the TL, and the amount of stuffing in the TL.

[Dave Jones]

That's great, Martin. Is this the link? http://www.quarter-wave.com/Theory/Alignment_Tables.pdf

[MJK]

Yes, the new document replced the older version.

[JC]

Thanks for posting that. I am starting to get the hang of your mathcad worksheets, but I am still learning what effect different changes make on the freq response so the design process is still a little random.

For example, I thought that stuffing and driver offset would have the greatest effect on the high freq ripples. Then I discovered that reducing the size of the port had a trememdous effect on this. Can you explain why this is?

As the port size tends toward zero, I am coming closer and closer to a closed box, but what is this doing to the sound waves? Also, I realize that flat frequency response is not everything. Are there any negatives to reducing port size like adversely effecting transient response or anything else?

[JC Fardo]

Martin,

I still can´t open your page. Would you mind email me a zipped file?

zfardo@terra.com.br

TIA
JC

[MJK]

JC,

I just e-mailed you a zipped version of the new transmission line alignment tables.

Now for your questions.

For example, I thought that stuffing and driver offset would have the greatest effect on the high freq ripples. Then I discovered that reducing the size of the port had a trememdous effect on this. Can you explain why this is?

Good question. I am not sure if I have a completely correct answer but I will take a shot. By making the open end more like a port you really have a combination bass reflex and transmission line enclosure. In a bass reflex design, above the box/port resonance, the port output becomes mass controlled and rolls off at 12 dB/octave. This allows the bass reflex enclosure to reinforce the bass at only the tuning frequency and in theory not contribute much above this point. Adding a port to a transmission line does the same thing but now you have the higher harmonics of the TL superimposed on the 12 dB/octave roll-off. So you still get a little bit of ripple but it is strongly attenuated by the mass of air in the port. You can see this hybrid bass reflex/transmission line behavior in the plots from the ML TQWT worksheet.

To read more about how and why this hybrid system works, take a look at the Lowther ML TL write-up on my website (Project #4) under the enclosure design description.

As the port size tends toward zero, I am coming closer and closer to a closed box, but what is this doing to the sound waves? Also, I realize that flat frequency response is not everything. Are there any negatives to reducing port size like adversely effecting transient response or anything else?

The sound waves in the closed box are half wavelength, so if you start with a ported TL and start to choke the port then the resonant frequencies will rise in frequency and eventually become half wavelength resonances. These resonances will still have a dramatic influence on the driver and cause deep sharp nulls to occur in the driver's SPL response.

If the port size is reduced too much you have the same problems as in a bass reflex design with respect to port noise commonly refered to as "chuffing". I have not had any problems with 3" or 4" diameter ports used with 8" drivers. I try and use the largest port diameter that produces a buildable design with the mass loading response properties.

In the end, driver position, port position, enclosure shape, and stuffing location and density are all variables that can be adjusted to achieve the desired response. Once you are really comfortable with the worksheets try the "Ported Box" worksheet which allows adjustment of the port position in the cabinet.

Hope that helps,

[Fabiano]

For the ones like myself and JCFardo, who cannot access Martin's website try accessing it through an annonimous server like:

http://www.the-cloak.com/login.html

It worked for me.

Best Regards,

Fabiano

[JC Fardo]

Thanks Fabiano, it really works !

Valeu, amigo. Obrigado e um grande abraço

JC Fardo

[JC Fardo]

OK, from now on I´m "JC Fardo".

Anyway the reply to "JC" himself should vey helpfull to me too.

Thanks to GM and MJK for zipped files. Fabiano´s hint works fine

JC Fardo