DIY ?uestion


Greetings people,
     What are the arguments, pro and con, for transmission line speakers that: have channels with parallel walls, and those that progressively get wider, as in a horn?  I want to do a build with plans that show parallel walls, where I can alter them to create an ever widening channel within the same size container.  I am a newbie in this area of DIY speakers, and am sure there are opposing views.
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@trelja said: "True transmission lines (TL) handily outperform both bass reflex / ported and sealed and aperiodic bass loading configurations."

I agree that good transmission lines sound wonderful, and have greater potential than either reflex or sealed or aperiodic boxes. As a manufacturer, with enclosure cost being the dominant cost, I believe that I can offer more bang for the buck with a good reflex box.

Going back to my DIY days, one issue that I ran into with transmission lines is the one-wavelength cancellation dip. At the frequency where the path length is equal to one wavelength, the backwave energy emerging from the open terminus is 180 degrees out-of-phase with the energy coming from the front of the cone. The two are physically close enough that significant cancellation occurs. I could hear the dip before I figured out what was causing it. You can see it in anechoic measurements of transmission line speakers:

https://www.soundstage.com/index.php?option=com_content&view=article&id=775:nrc-measurements...

https://www.soundstage.com/measurements/pmc_gb1/

The dip is probably not as bad in the power response (summed omnidirectional response) but it will still be present. Fortunately dips look much worse on paper than they actually sound - the ear is pretty good at ignoring them.

There are techniques for mitigating the dip (positioning the woofer partway down the line, using very dense stuffing, building a Helmholtz absorber into the line), but like practically everything else in speaker design, they involve tradeoffs.

From your experience, do you have any comments on the one-wavelength dip? Did Bud Fried do anything in particular to mitigate it?

Trelja again: "As great as TL is for bass, it’s all the more advantageous for midrange loading."

Again, I agree. I think the transmission line acts like a "trap" for the backwave midrange energy, so that very little of it reflects back into the cone.

I had an article published in SpeakerBuilder magazine back in 1986 that used transmission line loading for the two 7" midwoofers and for the 30" tall ribbon they were mated with. Over the years several people have told me they tried various enclosure designs for the same ribbon and my "W-shaped" transmission line geometry worked the best.

I still think about doing a transmission line "satellite" speaker that would be augmented south of 80 Hz by a distributed multisub system, since I believe the room effects are the biggest issue in the bottom two octaves. This would let me use a smaller transmission line box (less costly) and would take advantage of its superior midrange potential (which is imo its biggest benefit). Maybe one of these days.

What I have found with reflex boxes, and this is somewhat counter-intuitive, is that placing the ports right smack behind the woofer cone results in better midrange than from a sealed box. I think this is because the (flared) ports act like traps and remove at least some of the midrange energy that would otherwise reflect back into the cone. So this is perhaps a crude first approximation of one of the benefits of a good transmission line. On these speakers I include a reduced-level rear-firing tweeter to combine with the port-escape midrange energy, to correct the spectral balance of the reflections.

Duke

Wow!  Excellent info!  (especially for the uninformed).  Thanks, people.
I was intending to use an 8" full-spectrum speaker of quality (and cost).  I will stick to a known plan for the box until I get more knowledge.
@sound22card, I sent you a PM so I can forward you the Fried D subwoofer plans.  This TL cabinet is 36H X 15.75X X 15D, and used an 8" driver.  Personally, I find it a much bigger step back from my O subs with their 10" driver than I or most would have expected, but I can't find the paperwork on that right now.  The benefit of the D subwoofer is that it mates perfectly (in width) with the C (pyramidal) Series satellites that sit on them to form the Valhalla System.  Think along the lines of the Wilson WATT/Puppy aesthetic.  But it will at least give you a look at one of the more highly regarded TL cabinets produced.  Fried sold a pretty good number of D subs, and very few O subs.  The IMF Electronics site used to carry a wealth of information on many of the IMF / Fried products, including these, but the content doesn't seem to come up right now.  Hopefully, it's not lost, as there is a treasure trove of documentation there.

@areed622 "I like 8" drivers that can handle at least 100W continuous and have SPLs at least 92dB and around 0.3 Qts."

You included an incredibly important detail when mentioning the Qts.  Bud Fried considered it critical importance to select a driver with as low a Qts specification as possible.  Over the years, I've learned that's exactly opposite of most opinions I find in the internet discussions, which recommend the opposite.  Bud even replaced the 10" drivers in his subs, and when he handed me a cardboard box, he explained these were the original drivers, but he was able to have Gefco make him a special pair with an even lower Qts, and those were the ones in the cabinets.  I can tell you from my Fried O subwoofers, they really bring the thunder, and apart from being a bit larger than most folks feel comfortable with, have no real weaknesses.  As I said, even when I know what's coming, for whatever reason my body will still (involuntarily) flinch when certain notes get played.

@audiokinesis, my hat's off to you, Duke.  You REALLY understand TL!

Thank you very much, @trelja. I made a lot of sawdust trying out different transmission line ideas in the late 70’s and 80’s.

You wrote:

"Bud Fried considered it critical importance to select a driver with as low a Qts specification as possible. Over the years, I’ve learned that’s exactly opposite of most opinions I find in the internet discussions, which recommend the opposite."

Must admit that I had better results with medium to high Qts woofers in transmission lines, but then I’m no Bud Fried!

Can you reveal anything else about what parameters Bud looked for? I used KEF B-139’s in most of my sixty-something transmission lines, but I’m sure the over-the-counter version’s parameters were not ideal. It’s x-lim was rather modest, as I found out the hard way.

I would love to know how to use a low Qts woofer in a transmission line. Low Qts = high motor strength = good midrange articulation, in general.

If you don’t think it would violate any confidentialities, I would be very interested in the plans for the D that you mentioned. I’d love to peek under the hood and see what the master did.

Very best wishes,

Duke

For TL theory that is backed up by measurements consider looking at the articles on my site.

www.quarter-wave.com

There is more bad information about TL's on the Internet than accurate information. The methods on my site have been used to design many TLs (100's probably) that have performed as predicted. The methods have been used by Salk Sound, Dennis Murphy, and a few other commercial TL manufacturers.