Transmission Line Monitors - Obsolete?

[amanise]

I just saw an audio student page describing some different internal monitor formats. (Closed, Transmission Line, Ported, Backloaded, Janis Principle, Bandpass).

I'm interested in the relative merits of transmission line boxes. What are/were they best at, and are they still made - or have they become outdated?

Cheers.

[SafeandSound Mastering]

In professional circumstance transmission line are very much not outdated, it's quite an old technique of extending low end in monitoring.

Still plenty of models using their version of transmission line. PMC in the main use a version of the technique.

https://pmc-speakers.com/studio/main-monitors

In room my PMC IB1S, big 3 ways, but not gigantic reproduce 25Hz as well as any frequency above. Very vibrant deep sub bass, but in a very well treated room with a lot of bass trapping the bottom end is amazing and clear, it's clear and vibrant bass not a muddy indistinct, ill defined mess.

I really enjoy working with them daily.

The down side is they, like reflex ports suffer from some time domain issues unlike a sealed box. (waterfall plots will show longer decay times in room than any sealed boxes) But with a sealed box you need very large drivers to produce similar levels of bass which have their own practical issues (size and driver expense for large high end drivers), or a sub which is another set up headache and there is just something about subs, a pain, and another variable.

Comfortably reproducing 25Hz in a speaker this size is great, absolutely no need to consider a sub.

There are more modern speakers, typically sealed or with passive radiators and with the digital room tuning DSP and I am certain technically better speakers as you spend £12,000 and up.

Ultimately you have your room and the monitors and get them as two sides of the same coin, sounding and measuring right and then you listen and work. And that work produces a reference and that reference is valuable. I personally don't like the idea of changing things too much once I know how everything sounds. That really creates problems, problems I don't need.

They cost a bit more to make so not so many of them at the budget end where reflex loading dominates, with the occasional sealed box design for those who wish to hear more accurate but quite significantly less bass extension.

I like them, I would like to hear some of the newer DSP designs but most of my work is approved first time and I don't want to rock the boat, as a reliable boat takes quite a while to build.

There is not much I do not hear on these PMC's, I like the speakers a lot. (1B1S) I am still very happy with them, they tell me what I need to know to hit the mark most of the time, not much else to to do with them really.

Other than just enjoy music playing through them. Puts the hairs up on my arms and legs regularly with the right tracks.

As far as how they sound here, if I close my eyes which I do a lot when working, they could be 1 point source driver. This is in big part due to the distance from the speakers and having them set up right.

I have nothing in between my ears and the 3 drivers which are not equilateral triangle set up as many speakers are suggested to set up. PMC IB1S ideal set up are toed in just a tad for ideal dispersion. I am about 2.3 M away from them so the drivers have time to integrate at the listening position. No desks in front, nothing to produce reflections or comb filtering, I operate at the side and swivel in chair over and again.

Trouble is good transmission line cost quite a bit of money otherwise they might be a bit more common.

Hugh probably knows the details on TL and I think he has owned PMC IB1S.

[Hugh Robjohns]

I'm interested in the relative merits of transmission line boxes. What are/were they best at, and are they still made - or have they become outdated?

Definitely not outdated! It's a very clever idea that tries to combine the best elements of sealed box and ported cabinet concepts. The name is rather misleading, though; the way they work has little to do with the way electrical transmission lines work!

Transmission line speaker cabinets are complex and difficult to design well, and relatively expensive to construct compared to sealed box and ported cabinets. And that's why very few manufacturers attempt to make them.

The best known manufacturer of transmission line speakers today is, undoubtedly, PMC and all speakers in their consumer and professional ranges are transmission lines of varying sophistication.

What transmission line speakers are best at doing is delivering deep, clean bass from a small cabinet — much better than a ported design and way better than a sealed box design of the same size. Subjectively, the low-end balance also remains consistent as the level is reduced, which isn't always the case in other speaker designs. PMCs speakers also have very clean midrange and high end so they are very revealing but not in a forward or fatiguing way.

It's true that the vent (not port) introduces a delay and so the time domain performance isn't as tight as a good closed box design should be (but often isn't!)... but this only affects the very lowest frequencies. The way the line loading works means that the upper bass and midrange are effectively working in a sealed cabinet environment with all the usual benefits.

PMC have spent 30+ years researching, developing and fine-tuning their understanding of transmission line design — the only company in the world to have done so — and that has paid off handsomely in the quality of their designs.

I'm a big fan and have owned several PMC speakers over the years and they are widely installed in recording studios, mastering houses, and broadcasters all over the world.

Like Barry, I too own IB1s as my main reference monitor speakers and I absolutely adore them (driven by a Bryston 4B SST2 amp)....

...but they are an old design now and PMCs current pro speakers have significantly better resolution from (slightly) smaller cabinets — thanks to further improvements in both the line implementation and the driver designs. Technology never stops...

[amanise]

Wonderful! Thanks very much both! I haven't been lucky enough to experience the PMC end of things, but I have a pair of these which have always seemed to have a very rich and powerful bottom end to me.

http://www.tdl-loudspeakers.co.uk/kv1.htm

I don't think they are in business any more (hence the thought that they might be an obsolete design). Different market than PMC, of course, but at least I now know that it should be PMC I aim at if my premium bonds every come in

[Hugh Robjohns]

I think a company called IMF was one of the first, and TDL was well known for TL speakers too, but there were actually quite a lot of long defunct companies that dabbled with them for a while in the 60s and 70s.

But the complexity and cost of building TL speakers persuaded most to stick with easier reflex designs instead.

I remember there was at least one DIY design in one of the diy electronics magazines in the UK, too.

These days, the availability of much stronger magnets and long-throw drivers with vastly enhanced linearity, and sophisticated DSP, allow bass extensions from smaller (ported or sealed) cabinets that was undreamed of 20 years ago.... but use that same tech in PMC's ATL cabinets and the results are genuinely awesome!

These things, for example totally kick the IB1s arse... and most other monitor grade speakers too!

https://www.soundonsound.com/reviews/pmc6-2

[Drew Stephenson]

These things, for example totally kick the IB1s arse... and most other monitor grade speakers too!

https://www.soundonsound.com/reviews/pmc6-2

They had some of these (with the extension cabinets) at GearSpace a couple of years ago and they were very, very impressive (even compared to high-end offerings from Neumann, Genelec and Amphion at the same show).

[SafeandSound Mastering]

PMC easily hang with those names, even excel them. Never seen many of those brands in mastering studios.

Down to 33Hz without the sub so they don't go quite as low as the IB1S in standard config (25Hz) which is testament to the 10 inch Nomex single driver and the overall large but still not giant cabinet size. A few Hertz does not sound like much, but when you consider what needs to be added to get extend them down to 25Hz.

A pair of subs brings the 6-2 to 25Hz and the price to £18,000 - £20.000.00

https://pmc-speakers.com/studio/pmc-6-2/

Sub bass response is not everything, but it is important at a certain level of work. I feel as a mastering engineer that hearing the reality of the bottom end is required.It would be short changing not to know what is going on.

Especially working with any music destined for the amazing PA systems that exist these days. Some of them given a chance will blow your head clean off.

I can EQ the deepest sub bass with confidence.

I will make peace with owning the IB1S for some time yet. With the Hypex 1KW amp if I "open them up" to say 90dB they literally sound like they will just get louder and remain crystal clear forever.

There is always something better than what one uses, this is a fact.

However, with all changes to a known reference point you can also lose something at least for a while, that must be bourne in mind and how I placate my wallet.

[Nazard]

I think a company called IMF was one of the first,

Yes, in 1978, for example, IMF had three transmission line monitors for sale, the TLS 50 II, TLS 80 II and Reference Professional Monitor Mk IV. The latter cost £803.68 per pair, inc Vat which works out at £4330 when inflation is taken into account.

PMC certainly have winners on their hands with their newish Prodigy 1 and 5 speakers, I'm looking closely at the 5s!

[Hugh Robjohns]

Down to 33Hz without the sub so they don't go quite as low as the IB1S in standard config (25Hz)...

Got to make sure you're comparing like with like specs wise.

The 6-2 LF corner frequency is 33Hz at -3dB (ref 1kHz). The IB1 figure is 25Hz at-6dB if I recall correctly... but it might be a -10dB figure which is what most manufacturers quote. I can't find properly specified figures at the moment.

In practice, I don't think you'd find the 6-2s lacking in low-end extension. I certainly didn't!

A pair of subs brings the 6-2 to 25Hz and the price to £18,000 - £20.000.00

True... but the price does include all amps and DSP control!

In a situation like yours I dont believe the sub would be required, so a pair of 6-2s (with amps included) costs roughly £10,300 compared to £9k for the IB1s without amps. So comparable.

The IB1s look nicer though....

I will make peace with owning the IB1S for some time yet.

Don't get me wrong, the IB1 is a brilliant speaker, and I discovered yesterday that I've had mine for 24 years. That alone is a testament to its reference quality...

All I'm saying is that good as they are, there is still room for improvement.

For example, the IB2s with the (originally ATC and now in-house) 75mm dome midrange is a major step up in terms of mid range clarity and resolution, for example. Sadly, I can't justify them here, but once heard never forgotten.

The 6-2s up the game further ... as they should with all that R&D and DSP tweakery behind them.

But quality and technology always comes at a cost, as we all know, and we all have to cut our cloth accordingly. And a well-known reference has a value over learning something new.

[SafeandSound Mastering]

In room the last time I measured 25Hz was at the same level as most frequencies up to about 80-90Hz at listening position.

I was quite surprised, but pleased.

Below is why it important to know what is going on. Today I am working on this...



It's just a static screenshot of a graph of the low end. Of course frequency bins absolutely do not tell everything. In reality this is a dynamic, shifting bass line in conjunction with a kick drum as well on another stem.

In that bin there is a fundamental of a bass note, as it happens it is about 31Hz. (likely B0)

Ultimately I use ears and at significant volume when I EQ the bottom. I was working with this this morning. It's the bass line stem from a track I am working with.

There is no way I would be wanting to work with less than IB1S and these issues are a daily experience. For their size, not to mention the price I paid for them they seem difficult to beat without spending vast sums of money. In audio, physical reality often dictates ultimate choices. Spec sheets always have to meet physical reality.

It is testament that they are working as intended and work goes out approved first time the very vast majority of times.

As mentioned there is always better even if you own £20,000.00 of monitoring. Concerning about what you do not have, does not help you complete audio work.

The better your monitoring system the less compromises and errors.

Spec sheets are interesting, maybe even slightly useful. (If your studio is an anechoic chamber). For myself results are the most important. And this will be the same for anyone, results.

Something extraordinary would have to be happening in the current state of play, for a main monitor upgrade here.

[Hugh Robjohns]

In room the last time I measured 25Hz was at the same level as most frequencies up to about 80-90Hz at listening position.

Sure... but again, you've got to compare specs with specs.

The manufacturer's bandwidth specs are, supposedly, anechoic measurements.

As soon as you put a speaker in a room. You get all manner of boundary boosts and room mode effects which will enhance (or disrupt) the low-end response. That's why we take such care in (a) treating the room acoustics, (b) finding the optimum speaker location w.r.t room boundaries, and (c) possibly EQ the speaker response...

In room, I would not be surprised to be able to achieve an even lower LF extension with the 6-2s because of their greater LF driver headroom capability and DSP tweakability.

There is no way I would be wanting to work with less than what I have and these issues are a daily experience.

Absolutely. Genuine full range monitoring in a properly treated room is an absolute and incontrovertible requirement for a mastering room. The IB1s are perfectly capable of meeting that requirement.

[Drew Stephenson]

Even for less-than-mastering scenarios, I've been surprised at how much I'm missing my sub whilst it's on repair.
Obviously it also means I've lost my room correction as well, but I'm definitely missing that confidence that I was hearing everything across the frequency range (if not to the detail level from your PMCs).

[SafeandSound Mastering]

In room the last time I measured 25Hz was at the same level as most frequencies up to about 80-90Hz at listening position.

Sure... but again, you've got to compare specs with specs.

I also agree, but it is no greater oversight to consider PMC would use the same reference point for a cut off measurement than to suggest the IB1S is down 3,6 or 10dB at 25Hz.

The spec (if it can be called that at this point) I can actually recall was "in room" and that was written on the PMC website at one point. I am not sure they ever published a turn over/cut off.

That is not a very good reference point.

If it was -10dB at 25Hz which I would be very surprised as I have never once seen any speaker manufacturers state -10dB as a cut off. It bears no relation to what is heard here either. (though that is in room and that is all I have to go by in reality, which is where I work.)

For IB2 same bass/treble drivers/dimensions - no longer state any cut off reference...

https://pmc-speakers.com/studio/passive/ib2s/

Shifting off topic a bit.....

If you have a well integrated trusted sub Drew I am sure it will be missed. I like the fact I don't have to worry about a sub, wherever simplicity is working and producing consistent results, that is the route I take here. (you can easily disappear up you own backside with audio and be worse off.) It is a fine balance, like everything audio is, especially reference related. Reference demands so much more focus than I find most give it.

I have been considering arguably / technically "better" near fields, a swap to my existing nearfields (a trusted yet very much secondary check speaker) but I cannot justify them from an operational point of view. I consider, as far as I can, I may well lose as much as I may gain (and not just some money).

As such I am planning another possible additional approach to my nearfield monitoring but need to 'sit with it' and consider if it is likely an improvement or a detriment. Once you get to a certain point of quality you can become unsure if some additions are stepping backwards from where you are already.

Always worth considering options though.

[Hugh Robjohns]

...I have never once seen any speaker manufacturers state -10dB as a cut off...

The manufacturers that are likely to resort to such skullduggery to make their speakers' performance look good are unlikely to quote any reference levels at all... which, sadly, is the norm amongst semi-pro brands and even some 'pro' ones.

But the reality is that many speakers bass responses are measured at a -10dB attenuation level on the basis that in-room boundary gain will improve things to make the given corner frequency seem more plausible.

And, as you say, at the end of the day it is the in-room performance that matters because that's what we hear.

But anyway...the only point I was trying to make was that your earlier negative comment about the 6-2's LF response being poorer than the IB1s was based on non-comparable figures and/or ill-defined specs — something we all need to be very careful about when evaluating gear on paper.

[SafeandSound Mastering]

Sure and paper evaluation is the worst evaluation method possible. Well I did not know that I learnt something -10dB cut off is a thing.

[Hugh Robjohns]

...although I would say comparing specs — assuming they are meaningful and directly comparable — is useful as a first level of evaluation. If you're looking for full bandwidth speakers there's no point auditioning something with a specified bandwidth to 100Hz (at -xdB)....

[forumuser840717]

For example, the IB2s with the (originally ATC and now in-house) 75mm dome midrange is a major step up in terms of mid range clarity and resolution, for example. Sadly, I can't justify them here, but once heard never forgotten.

Sad but true. Though, if you try hard, you can at least partially flush the memory and replace it with thoughts of something more listenable - like chalk on a blackboard or wet polystyrene sliding down a window pane

Of course, the real problem with IB1s (and IB2s) is that while they will hold even quite a heavy door open really well, they're on the point of being big enough to get in the way all the time so not really useful for that either. Maybe if one replaces the drivers with glass they might make novelty fish tanks - could use the amps as heaters for keeping tropical fish. Or just leave the driver holes open and use them as over-large play spaces for kittens.

[Hugh Robjohns]

I was planning on recycling mine as coffins when the time comes....

[SafeandSound Mastering]

The mid range on the IB1S is amazing, and that is courtesy of being 3 ways and the soft dome mid. Vocals especially well presented in mixes, very easy to hear what is wrong, if anything.

Music reproduction is a system. Speakers, their setup in space, DAC (Crane Song Solaris), amplification (Hypex) and room.

As mentioned these speakers could be a single point source of sound when I close my eyes, as they are set up. Whoever tuned them, if done by ear really knew what they were doing.

It would be ridiculous to say they are the best speakers that exist but I go with my daily working results in the specific room they are in. (very deeply treated).

One reason I have not changed them in 12 years of ownership is there is not much I am missing, they are a tad lean in the lower mids, but as this is known it is easily worked around. (and double checked).

[SafeandSound Mastering]

Speak of monitors had me checking the specs of the Dynaudio BM6P I use as the secondary set (reflex loaded) -3dB at 43Hz, that is very good for 6.5inch that is close to 20 years old. Again some quality speakers stand up very well.

These are what I would replace if I was going to. It may or may not happen, I know them like the back of my hand so they have a reference value beyond their size. Not a money issue, just wondering if true benefits would be realized or not. It would be difficult to improve on for what they are used for.
I will contemplate this for some time.

I can also recall there was more 30Hz coming from those than I ever could have imagined. Which brings me onto another aspect which is not always immediate when reading a cut off point.

Scale.

The BM6P is a great speaker IMO, otherwise I would not have held onto them however next to the IB1S they evidently lack scale. Totally. The IB1S make the Dynaudio sound like Multimedia speakers when I flick the switch.

When you see the numbers, the reality of listening to them, the difference is gargantuan. In purely subjective estimated "flowery" terms the PMC's I hear 10 x more and they sound 10 x better than the Dynaudio's. Which was reflected in the cost as well.

I like the big speaker sound, it gets me closer to what this is going to sound like on something like a Funktion1 PA system and half my work is with dance and/or "modern" music so it is very important.