PMC Twenty.21 Bookshelf Speaker Review

[Soniclife]

What advantages does a true transmission line bring over other loadings?

 

[Archsam]

Somehow I missed that one. I confess I've never heard a Harbeth but get the feeling that the LS3/5a-sized ones are probably the best.

When I auditioned the P3esr a few years ago I also listened to the Compact 7ES-3 side by side for 2 hours. I found myself preferring the sealed box P3esr over the ported cabinet of the 7ES-3. Whilst there is less bass in the P3esr I actually prefer its bass quality. The bass coming out of the 7ES-3 is not as tight, maybe this is what it means to 'hear the box'?

I will love to see the P3esr (or other similar BBC LS3/5A variants) tested here to really get to the bottom of this.

 

[Sgt. Ear Ache]

Y'know, my Klipsch R-41MP may be really lame, but,
View attachment 71282
They're not 2 grand lame B)
View attachment 71283

Honestly, after all this time and all these reviews...I'm still pretty damn happy with my Micca RB42s.

 

[AudioSceptic]

Hmm. One thing that characterized the IMF (and the Radford/Bailey) was that there was a relatively large volume space directly behind the driver and before the entry to the line. Also, the entry to the line was at least as large in area as that of the driver. The line tapered smoothly from there. IMHO, none of PMCs cabinets are big enough to do that.
Yes and I built a few.
See above.

IMF TLS80 brochure http://p10hifi.net/planet10/IMF_TLS80_brochure.pdf. They would cost a fortune now, with all that internal partitioning.

 

[Verloc]

Thanks for that great description. I heard some PMCs briefly in the mid-90s (I think) and found the sound really odd, but I couldn't really say why. You've got it I think!

I don't think PMC actually know how a TL is supposed to work. Hear some IMFs or TDLs before you dismiss the concept forever.

Agreed but from the very little I know, a good TL has to be large so that precludes them for me anyway.

I understand why PMC and similar brands chase audiophile money with domestic products but I don’t understand why they don’t carry through good design principles. Voicing a product is fine but a badly designed product is not only unacceptable by could cause a negative perception of their professional range too.

 

[raindance]

@LarsS - those IMF's were very nice speakers. Especially on classical.

 

[AudioSceptic]

When I auditioned the P3esr a few years ago I also listened to the Compact 7ES-3 side by side for 2 hours. I found myself preferring the sealed box P3esr over the ported cabinet of the 7ES-3. Whilst there is less bass in the P3esr I actually prefer its bass quality. The bass coming out of the 7ES-3 is not as tight, maybe this is what it means to 'hear the box'?

I will love to see the P3esr (or other similar BBC LS3/5A variants) tested here to really get to the bottom of this.

My (limited) understanding is that ports (or passive radiators) can extend the response and improve efficiency, but you get a steeper drop-off at the bottom, and whatever comes out of the port is delayed compared with the signal, giving "slow" bass.

 

[thewas]

Yes, the B&W Nautilus was closed at the end, being long enough and tapered so that any possible reflection back up the line was so weak by the time it got to the top that it had insufficient energy to colour the sound.

Isn't the acoustic output of a TML the one that adds and increases sensitivity and/or bass depth? How would that work on a closed tube?

 

[AudioSceptic]

What advantages does a true transmission line bring over other loadings?

https://en.wikipedia.org/wiki/Transmission_line_loudspeaker

 

[KxDx]

I'd like to believe T+A built mine properly.

I'd love to also own a pair of Acoustic Zen Crescendo's one day but I doubt I'll ever get as lucky as I did again.

 

[Frank Dernie]

Isn't the Nautilus bass "snail" closed and continuously reducing its cross section for absorption?

Yes. That is the ideal, I believe, if the line is long enough to absorb all the energy from the back wave, but anybody correct me if that is wrong.

 

[Music1969]

An ATC SCM19 is here for review.

Just confirming you have the passive version in?

Some passive measurements:

http://www.avmentor.net/reviews/2016/atc_scm19_2.shtml

 

[xykreinov]

And most important they have more continuous directivity and lower distortions so they can be EQed easily to something significantly better.

This is true in practice. I eventually just decided to take the Klippel measurements from amirm and autoeq them with REW, as well as extend the bass response a bit.

Even though that's problematic, since I'm dealing with taming cabinet resonances in certain places among other issues difficult to fix fully with eq, there is a worlds of a difference for the better in my listening.
For bass extension, I had to manually find the sweet spot where it still was "fast" while not being non-existent. I'm surprised how much I could mess with it, even given my eq's preamp has to be set to -20dB at most in order to avoid distortion.
Though, I could barely keep all that tweaking under the 20 filter limit of REW.

 

[Frank Dernie]

IMF TLS80 brochure http://p10hifi.net/planet10/IMF_TLS80_brochure.pdf. They would cost a fortune now, with all that internal partitioning.

I used to go and listen in their room at every audio show!
I would have loved to own some but it was very many years later before I had made enough money to consider anything like them.
I did, however, include the B&W Nautilus in my auditioning for "the last speaker I will ever buy" search about 25 years ago. I so love the styling!

 

[sergeauckland]

Isn't the acoustic output of a TML the one that adds and increases sensitivity and/or bass depth? How would that work on a closed tube?

If it's a true Transmission Line, then there's NO output from the vent, so the vent doesn't need to be there. Therefore, whether there's a vent or not, doesn't matter as there should be no output.

However, in the real world, few loudspeakers are big enough to contain a true Transmission Line (the B&W Nautilus was one, the Radford-Bailey ones were another) so all commercial TLs are compromised and therefore not true TLs whatever the Marketing Department might think.

The real benefit of a TL over other forms of loading is that theoretically, it should have no resonances. A Bass reflex enclosure is deliberately tuned, a sealed box has an inherent resonance due to the compliance of the air, and horns have both passband and resonance issues unless enormously large and unfolded, totally impractical domestically for bass. Only a TL can be resonance-free, but at the price of size, weight, complexity and cost.

In today's market of small or tall, thin loudspeakers, a TL is a bit of a nonsense commercially, which is why they're so rare.

S.

 

[Soniclife]

https://en.wikipedia.org/wiki/Transmission_line_loudspeaker

That reads a lot like a PMC sales brochure sadly. I think the better way to ask my question is what proof is there that a well implemented real TL delivers something not achievable by other simpler methods.

 

[617]

Yes. That is the ideal, I believe, if the line is long enough to absorb all the energy from the back wave, but anybody correct me if that is wrong.

We're experiencing some confusion here. The nautilus has a long line to almost completely absorb the rear radiation of the drivers. This is a good thing, and it is an unusual design. Linkwitz's PLUTO or whatever he called the last iteration of the sewer pipe speakers work in a similar way. The principle is simple - if the rear radiation has to go through 3 feet of polyfill, it will eliminate that energy from bouncing back through the cone. Of course if you want to absorb bass, you need a long line.

The term 'transmission line' also refers to a certain class of woofer loading where the bass output of the speaker is augmented by using the back-wave of the speaker. The idea is very simple - you create a labyrinthine path which delays the rear wave of the speaker, and when it comes out the box, it reinforces the front radiation at a certain frequency set by the length of the path. This is called 'quarter wave' loading sometimes, because the rear radiation is delayed by 1/4 wavelength compared to the direct radiation. As I'm sure most know, if you delay something by 1/2 the wavelength, it will be out out phase; 1/4 wavelength and you get a bit of a boost.

There are other ways of using the rear radiation of a speaker to boost bass. The most common is a bass reflex, where you have a resonating mass of air which really likes vibrating at a certain frequency. A passive radiator does the same thing. This principle can also be combined with a transmission line, where you have a short transmission line which 'loads' a port. This is called a mass loaded transmission line, also called an MLTL. It allows you to get more bass in a smaller box than a transmission line. You will also notice that sometimes the line starts big and gets narrow, or starts narrow and gets big. The former is called a tapered quarter wave tube (TQWT). The latter is sometimes called a back loaded horn.

Quarter wavelength augmentation is not without problems. First of all, the claims that it has less group delay are somewhat misleading. Group delay is a function of the roll-off of the bass; it doesn't really matter what means you use to achieve it. A QL speaker will have a roll-off somewhere between a sealed and ported enclosure, and group delay will be somewhere in between. The idea that group delay is signficiant in bass indoors is sort of ridiculous anyway, we're talking about frequencies which bounce around the room a dozen times before they even reach one period.

The biggest problem with QL augmentation is that the line needs to be big, but for DIYers and expensive speakers this can be overcome.

There are some issues with resonances in the line; these manifest as a series of sharp peaks in the response in the midbass. This is where stuffing comes in. By adding acoustic absorption to the line, you can let the bass pass but kill all the other stuff. This is necessary for good performance.

In my opinion, the best thing about QL augmentation is not the bass but the handling of the back wave of the speaker.

PMC speakers aspire to use quarter wavelength augmentation. I have no idea what they think they are doing, but with their smaller speakers, the lines aren't long enough, and to make them as long as they are, the sectional area of the line is quite small. Both of these factors compromise the performance of the line.

Most DIY designers trying to make reasonably sized transmission lines will use the MLTL topology, which offers reasonable size, good back wave absorption, and good bass efficiency. See this link for an audioxpress article about a DIY speaker by Paul Kittinger which uses MLTL loading.

 

[BikeSmith60]

Hello all,
The results are surprising and worrisome. They do not justify the commercial success of PMC TL speakers which is undeniable. It should be noted that the 20.21 measured here is PMC's smallest design and that two generations have since superseded it (currently 25.XX and now the 25.XX i). Perhaps the quarter-wave implementation in this smallest model was insufficient and flawed. Further, given the commercial success of this very model and its successors, not to mention innumerable positive industry and owner reviews (not just from What HiFi) there has to be a missing element in the equation, most probably room acoustics and associated amplification characteristics.
For example, it is commonly accepted that Naim amplification marries well with PMC due to a complimentary synergy. Subjective appreciations cannot be measured and analyzed as they were here so some people can find them wonderful while others may detest them.
As for the ATC SCM19, I would hope @Amir could equally review these as well and as soon as possible! As fate would have it, I am hesitating between the PCM 25.22i (latest production, larger model) and the ATC SCM19. Goal is to home demo and see, feel and hear what my room says...Even if it is intellectually incoherent, I will prefer the speaker that sounds best in my room and with my amplification even if it has blatantly horrific measurements.
Just for the fun, subjective reviews of latest PMC 21i and 22i:
Brgds.

https://www.avforums.com/reviews/pmc-twenty5-21i-loudspeaker-review.17565

https://www.the-ear.net/review-hardware/pmc-twenty522i-standmount-loudspeaker

 

[sergeauckland]

That reads a lot like a PMC sales brochure sadly. I think the better way to ask my question is what proof is there that a well implemented real TL delivers something not achievable by other simpler methods.

My guess is that the answer to your question is none.

A TL does something no other enclosure does, in terms of non-resonance, but then so what? A sealed box if properly damped and using a drive unit with suitable T-S parameters does a pretty good job within its pass-band. I'm less convinced about reflex loading, it's always struck me a as a way of getting a quart out of a pint pot rather than it being in any way an 'idealised' enclosure. Horns have a lot going for them, but they really are impractical given that as soon as a horn is folded so you can get in the same room, it's hopelessly flawed, so why bother?

Over the past 50 years I've owned in order, Goodmans DIY bass reflex, Heco sealed box, IMF TLS50IIs, KEF Sealed Box (105.2), Meridian active BR, and my current B&W 801f sealed boxas well as numerous small 'speakers for study/office etc. For me, they all worked acceptably, but I do have a soft spot for the IMFs

S.

 

[maty]

Regarding transmission lines, xrk971 has designed speakers with the Purifi PTT6.5 woofer.

Simple Passive Harsch XO Using PTT6.5 and RS28F in a Waveguide
https://www.diyaudio.com/forums/mul...e-harsch-xo-using-ptt6-5-rs28f-waveguide.html