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PMC Twenty.21 Bookshelf Speaker Review

Frank Dernie

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Let me attempt to defend... or rather... rationalize this price/performance

- PMC was formed in 1990 by Peter Thomas (previously of the BBC) and Adrian Loader (previously of FWO Bauch, an audio distributor)
- PMC sold studio monitors to BBC

These speakers were probably born out of requirements back then, and probably had good performance back then. And hence also commanded good price back then.

However speaker technology progressed. By early 2000s we already have brands moving away from boxy edges, and even a few experimenting with rudimentary waveguides. Oh and ports became the standard.

But consider the situation. We have a big studio with a long history (BBC and etc.) using speakers with a long history. (PMC, and even Rogers...) It is just not possible to tell people to change speakers, especially when the newer speakers sound so different.
What would a person in this situation logically do? Reject the newer sounds as inferior products, embrace the "British sound", and give an Emmy Award, of course.
These speakers, the Twenty.2* range, are new not something from "back then".
This performance is nothing even vaguely similar to what the BBC was producing or using.
I did not know PMC had supplied the BBC, or which department if they had, but I am 100% sure they never used anything like these.
Rogers produced several designs to BBC specs for years. They were nothing like this in design or performance.
@Pluto maybe knows where PMC may have been used in the BBC.
All BBC designs were ported except the LS3/5 and have been for over 50 years, nowt new there.
 

Archsam

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Been waiting for one of these. It gives me the fear though, as an ATC owner.

I'm not at all surprised with the PMC's performance. I demoed a pair of their floorstanders against KEF R7s and ATC SCM40s, and strongly recall feeling the PMC's upper range sounded detached and artificial. It's striking to see that impression borne out so clearly in measurements.

I did, however, come away with the ATCs, and feel Amir's SCM19 measurements might reveal less than SOTA performance...

Even more relevant:

http://www.avmentor.net/reviews/2014/atc_scm40_2.shtml
 

Verloc

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I must say that this is no surprise to me. I listened to a pair a few years ago and the first failure of the speakers was to have no relationship between the bass and rest of the music, they sounded like the the mids upwards were from a transistor radio and the bass from a car audio system parked outside the shop.

It was recently mentioned to me that PMC speakers have crossovers with s***loads of components to try to correct for their shortcomings, dunno if this is the case or whether it has a bearing on the measurements.

If I see transmission line port on speakers now my ears have a Pavlovian response and fold over.
 

sergeauckland

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These speakers, the Twenty.2* range, are new not something from "back then".
This performance is nothing even vaguely similar to what the BBC was producing or using.
I did not know PMC had supplied the BBC, or which department if they had, but I am 100% sure they never used anything like these.
Rogers produced several designs to BBC specs for years. They were nothing like this in design or performance.
@Pluto maybe knows where PMC may have been used in the BBC.
All BBC designs were ported except the LS3/5 and have been for over 50 years, nowt new there.
I first heard PMCs at the BBC Maida Vale studios, (BB5 I think they were, with XBD bass extenders) where they had been installed to replace the KEF KM1s. I subsequently heard the larger PMCs at other studios, and always thought them very nice indeed.

On the other hand, I've not been a fan of their consumer models, always found them a bit 'boom&tizz'.

The larger PMCs use 'proper' Transmission Line loading, by which I mean they have to have long line, and therefore are big. Small TLs just don't work, Those of us of a certain age may remember IMF loudspeakers, which also used TL bass loading. The smallest which had a 'proper' TL was the TLS50, the smaller ALS40 had what IMF called an 'active' line by putting a driven cone where the vent would be (not totally successfully) as the box was too small for a full TL.

A proper TL absorbs ALL the rear radiation so doesn't need a vent, but needs to be infinitely long, so all practical TLs are a compromise, the larger the box the less the compromise. B&W's Nautilus used a spiral TL, which was long enough and tapered enough not to need a vent, but that was very large and very expensive.

S.
 

thewas

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By the way the owner of that review page owns himself a pair ATC SCM50 PSL to which he compares all loudspeakers, although he admitted to like the Kii Three more than his ATC http://www.avmentor.gr/reviews/2019/kii_audio_three_0.htm

Generally the ATC drivers are nice and were long time reference (like their 3" mid dome) and their general loudspeaker engineering is as good as it gets without using waveguides, so more comparable to Harbeth than PMC (which although also uses the nice 3" Volt mid dome on their larger models).
 

Marou

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On the strength of enthusiastic reviews i bought a pair of PMC 20/23s a few years ago to replace Harbeth P3ESRs which I loved but lacked bass (I thought). All the failings described above - disengaged bass, uninspired treble - were immediately present but, dummy that i was, I persisted with them and only got rid of them after I'd convinced myself it was my room at fault. I reinstated the Harbeths which had been relegated to another room and thought the few hundred quid dropped was a suitable price for a lesson learned.
 

Frank Dernie

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By the way the owner of that review page owns himself a pair ATC SCM50 PSL to which he compares all loudspeakers, although he admitted to like the Kii Three more than his ATC http://www.avmentor.gr/reviews/2019/kii_audio_three_0.htm

Generally the ATC drivers are nice and were long time reference (like their 3" mid dome) and their general loudspeaker engineering is as good as it gets without using waveguides, so more comparable to Harbeth than PMC (which although also uses the nice 3" Volt mid dome on their larger models).
I remember deciding the ATC SCM50 was so well regarded I should buy some, about 30 years ago. I found my local dealer and organised a demo.
I was not particularly overwhelmed by them, I had Yamaha NS1000Ms at the time (still have them in fact) so did not buy. The bass and mid units have been around a l-o-n-g time, I do have a pair of Proac EBS speakers which use the same ATC mid and bass units in my study.
 

wwenze

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If I see transmission line port on speakers now my ears have a Pavlovian response and fold over.

No need to close the mind completely, but a saying is relevant here:

"If something really works, it would be copied by the Chinese already"
 

AudioSceptic

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I'm shocked as PMC has a great reputation here in the UK. It was on my shortlist of speakers to audition a few years ago, but never got around to listen to them due to my encounter with the Harbeth P3esr, which I now own.

Looks like I dodged a bullet then lol
Yes, a shocker! I'd love to see the Harbeth tested here, along with a few other "BBC derivatives" such as the Spendor SA1 and D1. The recent Revel review has set a benchmark at this price level and it would be good to see some competition.

I did hear some PMCs briefly a long time ago when they were new and the current rave and was not impressed, but this is just ****ing awful. I think you could do better at 1/10 of the price.
 

Frank Dernie

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No need to close the mind completely, but a saying is relevant here:

"If something really works, it would be copied by the Chinese already"
Real transmission line speakers have been around for 50+ years. The first I heard were IMFs.
The problem isn't that they don't work but that they are much more expensive to make. The B&W Nautilus has one and the top Vivid models all do.
I guess PMC have a "transmission line" in this model is that they have rather hung their hat on the transmission line is better peg whereas only their big pro monitors probably have a proper one.
DB5_0001.jpg
 

tuga

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Yes, a shocker! I'd love to see the Harbeth tested here, along with a few other "BBC derivatives" such as the Spendor SA1 and D1. The recent Revel review has set a benchmark at this price level and it would be good to see some competition.

I did hear some PMCs briefly a long time ago when they were new and the current rave and was not impressed, but this is just ****ing awful. I think you could do better at 1/10 of the price.

One Harbeth has been tested: https://www.audiosciencereview.com/forum/index.php?threads/harbeth-monitor-30-speaker-review.11108/
 

Verloc

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No need to close the mind completely, but a saying is relevant here:

"If something really works, it would be copied by the Chinese already"

My conscious mind isn’t closed but if I get stung by wasps once or twice a year I need a really good reason to trust a black and yellow flying insect.
 

AudioSceptic

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This is a review and detailed measurements of the PMC Twenty.21 Stand-mount Speaker. It is kindly loaned to me by a member and costs US $2,000 but member purchased it used for much less. Professional Monitor Company (PMC) is a UK company so keep that in mind when looking at pricing. Their mainline business is to provide monitors for professional world both in music and cinema. So my expectations going into this review is that we see neutral response in the design.

The overall build of the speaker is excellent:

View attachment 71231

Solid, glossy finish gives a feeling of high quality. A transmission line exiting out of the front port is one of the differentiators in the design.

The back panel shows the very nice speaker terminals:

View attachment 71233

Measurements that you are about to see were performed using the Klippel Near-field Scanner (NFS). This is a robotic measurement system that analyzes the speaker all around and is able (using advanced mathematics and dual scan) to subtract room reflections (so where I measure it doesn't matter). It also measures the speaker at close distance ("near-field") which sharply reduces the impact of room noise. Both of these factors enable testing in ordinary rooms yet results that can be more accurate than an anechoic chamber. In a nutshell, the measurements show the actual sound coming out of the speaker independent of the room.

I used over 800 measurement point which was sufficient to compute the sound field of the speaker.

Spinorama Audio Measurements
Acoustic measurements can be grouped in a way that can be perceptually analyzed to determine how good a speaker can be used. This so called spinorama shows us just about everything we need to know about the speaker with respect to tonality and some flaws:

View attachment 71235

Oh, boy! Nothing like having every one of your expectations about the company dashed in one graph! What kind of response is this? Did different people design different parts of the speaker and never talked to each other? Or is this PMC's idea of what a neutral response looks like?

On top of on-axis not being flat, we have directivity error where the woofer starts to "beam" (its response narrows) as it gets to the crossover frequency. This is normal and the solution is typically to put a waveguide around the tweeter since it would start its job with a very wide beam. None is here so we see the "DI" graph show a rather severe dip. This translates to off-axis response not being similar to on-axis. Reflections will be tonally different than on-axis making the speaker very room sensitive:

View attachment 71236

Putting all of these together we can try to predict the tonality you may get in your room:

View attachment 71237

The flattish trend line means the speaker will sound bright. And the two peaks in bass mean there will be strong bass emphasis at certain notes/frequencies. This tonality will be overlaid on everything you play. Not a good thing.

I split the terminals and drove the tweeter separately from woofer+port in nearfield. Here is how that looks:

View attachment 71238

Looks like the port tuning is wrong as it is responsible for that bump at 100 Hz where the woofer is already peaking some. It should have been tuned to higher frequency so that it would fill in the two peaks in the woofer.

We also have a lot of distortion:

View attachment 71239

Here is the breakdown per component:

View attachment 71240

The large peak in the port distortion at 200 Hz is interesting. As is spikes all the way through the spectrum.

In absolute terms this is the distortion profile:

View attachment 71241

Fair bit of distortion exceeds my threshold of -50 dB. This is at 96 dB SPL though.

Impedance is nicely high for a bookshelf:

View attachment 71242

Beamwidth is chewed up as we expect based on directivity plot earlier:

View attachment 71244

It is wider than normal because of lack of waveguide. But gives up smoothness which you can also see in our 3-D plot:

View attachment 71245

I forgot to mention that this speaker tilts back. That seems to have shifted the center line up some relative to my microphone placement:

View attachment 71246

Finally, here is our waterfall:

View attachment 71247

We see the same distortion peaks around 400 Hz.

Speaker Subjective Listening Tests
My first "5 second" reaction was: "this speaker is screechy bright." It was so bad it set off my tinnitus. Female vocals didn't sound terrible but became lispy and after a bit, annoying.

The bass was there but strange. It was not full and it sounded kind of tubby.

I put in a low pass filter and pulled the highs down above 5 kHz. Pushed up the dip in mid-frequencies and knocked off the two bass peaks. Sorry, lost the EQ setting so nothing to show you. But I am sure you can guess them from the frequency response. Once there, the PMC Twenty.21 sounded "OK." The harshness was mostly gone, bass was less but more natural. And there was more detail.

Conclusions
The PMC Twenty.21 is a failure in design. It does not follow much of what we know that results in speakers that are neutral and as such, can garner listener preference. I have no idea how a company with a tradition and core business of pro speakers would make a hifi speaker this bad. I suspect it was all "tuned by ear" which makes me shed a tear for what hearing they must have to produce something like this!

Needless to say, I cannot recommend the PMC Twenty.21. If you have it, use manual or automatic EQ to salvage what is there.

------------
As always, questions, comments, recommendations, etc. are welcome.

It is almost 4:00pm and I still have not had lunch! Will have to rummage through my coupons to see if I can find a cheap sandwich. If you want me to eat on time and better, please donate what you can using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
Amir, just curious but do you draw the trend line by eye or use a Least Squares fit?
 

AudioSceptic

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Tks

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Conclusions
The PMC Twenty.21 is a failure in design. It does not follow much of what we know that results in speakers that are neutral and as such, can garner listener preference. I have no idea how a company with a tradition and core business of pro speakers would make a hifi speaker this bad. I suspect it was all "tuned by ear" which makes me shed a tear for what hearing they must have to produce something like this!

Simple, as you said, it was tuned by ear, or simply not tuned at all.

Again, as always, I just don't understand why the professional market is in on this whole atmosphere of not knowing what they're buying.

Really surprising to see not a single sound engineering or mastering engineer ever chime in on this forum about verification on ANYTHING their studios are outfitted with. The pro-market is nothing better than the consumer market it seems (in reality, it's worse because it's slow with upgrades so who knows the sort of distortion ridden nonsense is in studios and home setups), the only thing perhaps the pro market gets, is warranty (which is why enterprise or industry products cost a ton in the first place in almost every sector, it's not really that the products are so much better, it's the fact that you get almost a dedicated line to support and communications).

Then again, it's not surprising considering many of these folks are pretty up-there in their age. So all these supposed issues are probably inaudible to them, or they just don't care possibly.
 

thewas

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Real transmission line speakers have been around for 50+ years. The first I heard were IMFs.
The problem isn't that they don't work but that they are much more expensive to make. The B&W Nautilus has one and the top Vivid models all do.
Isn't the Nautilus bass "snail" closed and continuously reducing its cross section for absorption?

Also Vivid Audio calls their constructions "Exponentially Tapered Tube enhanced bass reflex", in the end often mixed bass enclosure design form are used like also the tapered quarter-wave pipe.
 
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