PMC Twenty.21 Bookshelf Speaker Review

[Kal Rubinson]

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

You're telling me!?! I built my first one based on John Wright's diagrams. Cutting and fitting the partitions and installing the long fiber wool was a bitch. I cheated by using an external (and, later, active) crossover so that I didn't have to snake wires and was able to tune the crossover after assembly. A major project.

 

[maty]

The day before this review I commented on a PMC and how bad it measured. PMC twenty5.24

https://www.diyaudio.com/forums/mul...using-ptt6-5-rs28f-waveguide-post6257577.html

 

[Frank Dernie]

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 It should be noted that the 20.21 measured here is PMC's smallest design and that two generations have since superseded it (currently 25.XX and now the 25.XX i).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

The tweeter level being the best part of 5dB too hot and the large amount of distortion are both probable reasons why I didn't like the PMCs I listen ed to.
I would be amazed if the ATC has either of these problems so I will be interested to hear your conclusions.

 

[sergeauckland]

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.

It may be called a Transmission Line, but that's wrong. The term Transmission Line comes from Communications theory, where a wave is carried along a line, and if the termination matches the characteristic impedance of the line, there is no reflection, all the energy goes into the termination. Any mismatch in impedance and some energy is reflected, which in the case of a gross mismatch sets up standing waves in the line.

The way the theory translates to loudspeakers is that the energy entering the line at the back of the driver is progressively absorbed by the line's stuffing, which has to be exactly correct to match the acoustic impedance as seen by the driver. If the line is too heavily stuffed, especially near the driver, then there will be back-pressure and energy reflected. Too loose and energy is still substantial at the end of the line and is reflected by the sudden change in impedance due to the vent or end cap if not vented. Clearly, it's a lot easier to get the stuffing density right if the line is long, but that makes for a large loudspeaker box, so in practice, most so-called TLs have less than ideal stuffing and a vent, which also helps keep the bass level up. Calling these very compromised loudspeakers 'transmission lines' is like calling a Porsche a Sports Car!

S.

 

[Pluto]

The real benefit of a TL over other forms of loading is that theoretically, it should have no resonances

Pete Thomas always said that the real benefit of a transmission line was not the amount of bass or the super-low frequency that could be achieved, but the improved LF distortion over other design approaches.

 

[617]

It may be called a Transmission Line, but that's wrong. The term Transmission Line comes from Communications theory, where a wave is carried along a line, and if the termination matches the characteristic impedance of the line, there is no reflection, all the energy goes into the termination. Any mismatch in impedance and some energy is reflected, which in the case of a gross mismatch sets up standing waves in the line.

The way the theory translates to loudspeakers is that the energy entering the line at the back of the driver is progressively absorbed by the line's stuffing, which has to be exactly correct to match the acoustic impedance as seen by the driver. If the line is too heavily stuffed, especially near the driver, then there will be back-pressure and energy reflected. Too loose and energy is still substantial at the end of the line and is reflected by the sudden change in impedance due to the vent or end cap if not vented. Clearly, it's a lot easier to get the stuffing density right if the line is long, but that makes for a large loudspeaker box, so in practice, most so-called TLs have less than ideal stuffing and a vent, which also helps keep the bass level up. Calling these very compromised loudspeakers 'transmission lines' is like calling a Porsche a Sports Car!

S.

I agree that the conventional terminology is misleading.

 

[Archsam]

Silly question, but looking at a lot of TL speaker cross section (since this morning!) I noticed a lot of the designs uses acoustic foam to line the 'passage' to the point where there is more 'solid' than 'void' within the speaker. It looks as if there is little air for the soundwave to transmit through the passage at all. Does this have an effect on the sound, or affect the effectiveness of the TL design?

I thought you might as well forget about construction the internal partitions inside the speaker, and just get a linear slab of foam, folded it up onto itself, and stuff the entire cabinet enclosure with it?

 

[AudioSceptic]

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!

I heard them a few times in the 70s but also couldn't afford them at the time.

I've never heard them, but if I had the "right" sort of modernist house, the Nautilus would look the part. Did the sound live up to the looks?

 

[JaapDeventer]

I had the PMC 20.21 briefly. It had no body at all compared to my Penaudio Cenya's. Sold it within a few weeks.

 

[sergeauckland]

Silly question, but looking at a lot of TL speaker cross section (since this morning!) I noticed a lot of the designs uses acoustic foam to line the 'passage' to the point where there is more 'solid' than 'void' within the speaker. It looks as if there is little air for the soundwave to transmit through the passage at all. Does this have an effect on the sound, or affect the effectiveness of the TL design?

I thought you might as well forget about construction the internal partitions inside the speaker, and just get a linear slab of foam, folded it up onto itself, and stuff the entire cabinet enclosure with it?

If the foam is of the right sort, then it's not a bad solution. Foam passes sound quite well, but does attenuate it, so by choosing the right type of foam, the back-wave from the driver can be absorbed correctly, without reflections. 'Thin foam near the driver, more dense foam away from the driver. It's also a lot easier to install blocks of foam than long-haired wool, and wool packs down and makes shipping difficult whilst foam stays put.

The important thing isn't so much the use of foam or wool, but that the line should be long enough and tapered correctly thus absorbing the back-wave without reflections back to the driver. My IMFs in 1974 or thereabouts had foam stuffing and it looked like different grades of foam were used in different places down the line.

S.

 

[Archsam]

If the foam is of the right sort, then it's not a bad solution. Foam passes sound quite well, but does attenuate it, so by choosing the right type of foam, the back-wave from the driver can be absorbed correctly, without reflections. 'Thin foam near the driver, more dense foam away from the driver. It's also a lot easier to install blocks of foam than long-haired wool, and wool packs down and makes shipping difficult whilst foam stays put.

The important thing isn't so much the use of foam or wool, but that the line should be long enough and tapered correctly thus absorbing the back-wave without reflections back to the driver. My IMFs in 1974 or thereabouts had foam stuffing and it looked like different grades of foam were used in different places down the line.

S.

I think we're on to something here - if the problem with TL design is creating sufficient length due to the difficulty in forming long enough 'tunnels' using rigid wall construction, why not use a soft material that folds and can pack tightly into the cabinet?

 

[AudioSceptic]

The day before this review I commented on a PMC and how bad it measured. PMC twenty5.24

https://www.diyaudio.com/forums/mul...using-ptt6-5-rs28f-waveguide-post6257577.html
View attachment 71370

That is not high fidelity!

 

[AudioSceptic]

If the foam is of the right sort, then it's not a bad solution. Foam passes sound quite well, but does attenuate it, so by choosing the right type of foam, the back-wave from the driver can be absorbed correctly, without reflections. 'Thin foam near the driver, more dense foam away from the driver. It's also a lot easier to install blocks of foam than long-haired wool, and wool packs down and makes shipping difficult whilst foam stays put.

The important thing isn't so much the use of foam or wool, but that the line should be long enough and tapered correctly thus absorbing the back-wave without reflections back to the driver. My IMFs in 1974 or thereabouts had foam stuffing and it looked like different grades of foam were used in different places down the line.

S.

Not mentioned yet, but a TL has the useful side effect of making the whole enclosure very rigid because the partitions are also braces.

 

[AudioSceptic]

I think we're on to something here - if the problem with TL design is creating sufficient length due to the difficulty in forming long enough 'tunnels' using rigid wall construction, why not use a soft material that folds and can pack tightly into the cabinet?

That is, in effect, what most "infinite baffle" (sealed box) speakers do. You stop the back radiation from the drivers interfering with the front, wanted, radiation by containing it all in the box, but you have to absorb it or it bounces around and comes out through the driver anyway, and also excites resonances in the panels.

 

[Frank Dernie]

I've never heard them, but if I had the "right" sort of modernist house, the Nautilus would look the part. Did the sound live up to the looks?

I didn't buy them!
They were last on my short list with the Goldmund Epilog. I did not prefer them and the cost, they need 8 channels of amplification and back then there were no well priced amps like the nCores, made them not worth the difference in price just for the looks.
I am still enjoying the Epilogs (I have 1&2) 25 years later.

 

[BikeSmith60]

Forgot to mention another point of humour...PMC think so highly of themselves and their new "i" series that they will raise the prices by 300£ or 371 USD on July 15th. They are already silly expensive...

 

[Archsam]

That is, in effect, what most "infinite baffle" (sealed box) speakers do. You stop the back radiation from the drivers interfering with the front, wanted, radiation by containing it all in the box, but you have to absorb it or it bounces around and comes out through the driver anyway, and also excites resonances in the panels.

There is a difference though - if the absorber is layered in a series of folds you will drastically increase the surface area of absorsion , which can only be a good thing if the whole principle of TL design is to reduce / elminiate resonance frequencies inside the cabinet?

I will call this the 'Lasanga' speaker design.

 

[AudioSceptic]

There is a difference though - if the absorber is layered in a series of folds you will drastically increase the surface area of absorsion , which can only be a good thing if the whole principle of TL design is to reduce / elminiate resonance frequencies inside the cabinet?

I will call this the 'Lasanga' speaker design.

Well, yes, and you will find all sorts of variations in the materials (bituminous pads, foam sheets, fibre, wool) used in different IB designs to get that effect. If a single, long, folded sheet worked as well, then it would have been used as it would be cheaper and simpler to make the speaker that way.

 

[DSJR]

When I started in a higher end audio store in 1974 (as a Saturday boy originally), Crown driven IMF speakers were the dogs danglies (excluding the Crown/Amcron electrostatic hybrids, JBL L200's and AR LST's)and we had master tape copies to play into and through them, as 'vinyl' wasn't judged any good (well, most direct drives fed back due to bad siting and springy belt drives suffered wow quite often back before the Linn LP12 took the UK over ). An IC150- D-150 driven pair of IMF pro Monitor III's could sound awesome with a good source and they did an excellent job with a tape from Angus McKenzie (a notorious reviewer of the era) of Hendon Brass band playing Stars & Stripes Forever

We bought the final batch of III 'Improved' speakers (half a dozen pairs in around 1981 I think) and driven by 'bolt up' Naim and an LP12 source, they were horrible (the amp had a higher output impedance and Lord knows what an LP12 was doing to the bass back then, but we didn't care in our ignorance with the bass light boxes we started to sell and almost brain washing at the time - the master tapes had been 'mislaid' over the intervening years, although we sold Revox and other large-spool tape recorders including the fancy Technics model with oval head-block.

In a smaller room, the very low bass of the IMF could give uncomfortable 'pressure waves' on vinyl sources of the times and the RSPM mk4 model from 1976 offered considerably tautened low bass with lf filter options too. I'd love to hear a pair today driven from a decent modern amp and digital source (to be fair, the ancient Crowns I still use were measured by ken Rockwell and still offer a balanced solid result even today I think).

PMC make several ranges of products all aimed at a slightly different market. The 'hot' ehf is common to most I've heard I think and a definite design intention going back to the LB1 original, which sounded great on a Quad 606 amp but horrible on the then market leading Naim 250 (why my then employer turned the brand down as 'we' were so Naim centric back then). The top domestic model Fact Fenestria are awesome sounding boxes though, but totally overpowering in a smaller space and a chat with Pete Thomas, whom I've known a little bit since the late 70's when he was at the BBC, he indicated that prospective purchasers should look to DSP to blend the speaker to the room better.. I think in fairness the domestic PMC's have been improved over the nearly? ten years since the speaker reviewed here was introduced. The market for these doesn't look so much at price and Naim is routinely used with them locally...


As for an impending ATC review. I'm dreading it myself as I've held a brightly lit torch for the brand for thirty years now and owned and loved a few of their products, only giving up some much cherished 100A's for love and marriage (not saying more on that one ).. Hopefully the 19's are current issue with the latest tweeter design. Listening to so many bling boxes in recent times, I return to ATC's (11's are fine in smaller UK rooms) with a huge sigh of relief! I also fell in love with the Kii Three's as well and going against pre-conceived ideas of Class D amp modules, they sounded so clear and sweet in the dem I sat in on.

Forgive the lengthy post and hope some of the above experiences from the mid 70's bring back memories for some of you.

 

[amirm]

Amir, just curious but do you draw the trend line by eye or use a Least Squares fit?

By eye usually anchored at 200 hz.