Can anyone tell me more about PMC's on their Advanced Transmission Line technology vs. traditional vented designs? How does it compare?
A good transmission line speaker increases the bass performance by having a longer transmission duct. When properly done by such companies as PMC, it is often much more effective than a vented design.
Thanks for the input gstarr.
This is from the PMC site and is obviously better than I can explain:
. How do transmissions lines differ from ported and vented speakers?
Transmission line, ported and vented designs are three different concepts on how to load the bass driver in a speaker enclosure. Transmission lines and sealed boxes have a 6dB per octave roll off after the resonant frequency, while a vented box has a 24bB per octave roll off. Ported speaker are the most common as they are cheap to build and easy to design, though the quality of the bass reproduction is questionable in many designs and such a steep roll off can have knock on problems further up the frequency range.
Sealed boxes have a similar roll off to transmission lines, yet can suffer from an ‘oil can” effect due to high pressures behind the bass cone, interrupting its movement. Both sealed and vented designs can suffer from rear radiating sound bouncing off the cabinet walls, and passing through the bass driver causing what can be described as a boxy sound.
Transmission line by contrast sound very natural because there is no build up of pressure behind the bass cone, with the rear radiating air being forced through an internal labyrinth to reinforce the bottom end of the frequency band. This also means no rear sound is re-radiated through the bass driver. The other advantage is that the air in the transmission line loads the bass driver and lowers its resonant frequency. This allows for the extended low end response and keeps the bass driver well damped, requiring less excursion than sealed or ported speakers to produce the same output.
PMC has painstakingly developed their own advanced transmission Line (ATL™) and has taken loudspeaker design to new levels, by using a cabinet construction and highly specified drive unit and crossover components. The PMC ATL™ design has enormous benefits including Improved resolution & reduced distortion, Even frequency response and Deeper, faster and better defined bass.
Interesting that more speaker companies are not taking advantage of transmission line designs - based on the description provided by gstarr, transmission line sounds like a superior design to vented or ported designs that most speaker companies are using today. Wonder why that is?
The complexity of transmission line cabinets adds to material and construction cost and requires skilled design to get just right.
There are also issues of size. To have a transmission line work properly down to the lower frequencies they must be of significant length.
Castle attempted to overcome this limitation with their 'quarter wave' loading but it's nowhere near as satisfactory below, say, 45Hz, as a proper transmission line.
From Positive Feedback online.
Peter Thomas – PMC Owner and Chief Designer
Baker's Dozen Questions with PMC Designer Peter Thomas
I caught up with Peter Thomas on the phone from London, with a 'baker's dozen' questions for him—actually a rather chintzy baker, as there were exactly twelve. Here's an edited transcript.
Sasha Matson First of all Peter, I just want to thank you for doing good work. I've really enjoyed living with the product of that for the past several years… Maybe you've already primped for this exam, as I sent these questions on. If it's o.k. with you we'll just rip through them. Number one on my list—the 'transmission line.' When and how did you first explore this design approach?
Peter Thomas In the early 70's I bought a pair of speakers over here that used a transmission line and I really liked them. The only other speakers I liked at the time were electrostatics. I didn't realize why I liked both of them, but it was because of the low distortion bass… I ended up working at the BBC where I was doing some speaker work. There was a requirement to build a really hi-fi speaker and I thought I'd use a transmission line, as that's what I had in my home. The trouble is there was nothing really written about them. No theory. And what was written was really wrong basically. So it was like rediscovering it by doing certain measurements.
SM Right now in 2010, transmission line design is obviously not a majority viewpoint. Why do you think that is?
PT I think people are frightened of transmission lines. And I think the reason why is that if you get a transmission line wrong, boy does it sound bad—really dreadful! But that doesn't mean we should just give up on them. It's much easier with current technology. With the computer, the design of a reflex or an infinite baffle is pretty well sorted out.
SM The current FB1i model… What a difference a tweeter can make. What goes on with SEAS? Is it like some Harry Potter vault where all these designers are working in secret for different clients?
PT (laughter) I love it… Well SEAS has been around since the 70's. And one of the useful things as you grow as a speaker designer is you either make dedicated drive units that you exactly specify, or you can collaborate with people who manufacture vast quantities of drive units and get them to produce a special edition which works for you. I like to cross tweeters over really low… that's a big thing for me. To develop a tweeter that would go suitably low—and I wanted to open up the dispersion that gives you that more three-dimensional feeling.
SM You will see my notes in due course, but I must say this is all getting scarily holographic… With this new tweeter, did you then also re-voice the overall?
PT It sounds funny, but we always use 'voice' to voice systems. A recording of a male voice. You get the character of the speakers across a range… I was taught when I worked for the BBC in London that you design a speaker, you give it to the guys who are going to use it, and they have it for a few months and use it, and then you get the comments back. You modify it, you measure it, and give another version. Measure, adjust, listen…Why we use speech, is that our brains have evolved over millions of years—we are very focused on communication. If something is not right with speech, you will know it. I use mono speech—I spend 90% of the time on that, then I go to stereo speech, then to music. And for matching of speakers it should sound like the width of a head—not six feet wide. You listen to some speakers and it sounds as if it's coming from some huge monster. (laughter)
SM You have many children now. Do you love them all equally?
PT We really do feel that as you go up the range with our products that you get a discernable value for the money—that the performance does improve. One of the things that worry me about our industry is that this is not always the case. Our customers are not stupid.
SM The low-end response of the FB1i, and all of your speakers, is very special. How does this behave, how did you achieve this?
P.T One of the nice things about transmission lines is that they don't roll off very steeply… It gives a nice smooth transition as it rolls away. With a 'room lift' you can get a very nice flat response in-room. The roll-off is very very gentle. You can use a two-way with a relatively small bass speaker and really get that bass extension. It's like having a sub-woofer built in.
SM Do you view the 'consumer' and the 'pro' audio worlds as inherently different, or are they getting more similar in 2010?
PT I think they are converging. Large studio use is dropping, there is more 'home studio' use—that is changing. There are different demands in the two industries—often they have quite different acoustics. For me, I'm just trying to make the most accurate speaker I can make—I don't change the speaker design to suit where it is going to be used.
SM So that older world of harsh studio monitors, that 'reveal' everything—that's not so much where it's at in your view?
PT Well, it isn't for us… There are a lot of studio monitors that are very in-your-face and distorted and loud. That is how we got into the industry, there were so many of those speakers that produced that really distorted sound. And the BBC wanted a speaker that could play at a high level that wasn't distorted. There are more (better) ones out there now than when we started twenty years ago, but there are still some pretty bad ones out there. (laughter)
SM Have you visited some of the film and mixing facilities in Los Angeles that use your gear?
PT Yes I have. The top three film scoring engineers use PMC. A lot of the composers use PMC as well—James Newton Howard and others. It's a big part of our business over there.
SM The FB1's are not one of these refrigerator behemoths that may be o.k. in the studio setting, but less so for many people where they live.
PT That's where we changed transmission lines. They used to have to be big huge cabinets to get that kind of bass, but what we've worked on, what we call 'Advanced Transmission Line,' or ATL, gets that extended response, but in a much smaller cabinet. Nobody wants a great huge cabinet in their front room, or not many people anyway.
SM On your website you list many musicians that use PMC gear. Tell us a story.
PT Probably the nicest time I had was with Stevie Wonder. He came to the U.K. for an award… and rang me up at the factory and asked for a pair of speakers. Six months later I was in Los Angeles and was invited to his studio. Stevie had just remastered all his albums on our BB5 speakers. And he was so pleased with them he said, "Pick any album.." I picked Talking Book and he played me that. And to stand there and listen toTalking Book on my speakers with Stevie Wonder standing next to me and singing along with it—unbelievable! That was pretty special.
SM And that 'BB5,' your first product, still is in production?
PT Twentieth anniversary this year.
SM To those skeptics out there, even including some experienced high end audio journalists, what can you say to clear up any lingering doubts they may have about the point of transmission line designs?
PT I think the easiest thing to say is, just listen to them. (laughter) They work. How many reflex speakers produce that kind of quality at the low end? That's the easiest answer—that low bass extension, in a real world room—you can hear it. The technical answer I suppose is two things. First of all, it's very difficult to measure a transmission line, because the vent is producing low frequencies at some distance from the bass unit; to combine those two responses—it is difficult to get a proper measurement. The second thing I'd say is that 'transmission line' is a very misleading term. A lot of people believe that a transmission line is an infinitely long tunnel that absorbs everything from behind the bass unit—which is the true technical meaning of 'transmission line', that there is no reflection, no return of the sound coming out of the back of the driver. But in the practical world it couldn't be infinitely long. So we have an open vent at the bottom, which actually allows bass reinforcement. And that's why you get a very similar impedance measurement to a reflex design. So at the very low frequency it doesn't work like the technical idea of a transmission line. But that's not to say they work like a reflex—and you can hear that, and measure it. And that's why I love them—because the bass distortion is so low you get this lovely open mid-range.
SM Right. You are not screwing everything up higher up the food chain.
PT That's it. Because that's what happens with a lot of these bass units in cabinets—the distortion ripples up into the mid-range, and masks it.
Be excellent to each other.
cool interview! thanks mv!