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PMC Twenty.21 Bookshelf Speaker Review

amirm

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#1
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:

PMC Twenty.21 stand mount bookshelf speaker Review.jpg


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:

PMC Twenty.21 stand mount bookshelf speaker back panel binding post biwire Review.jpg


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:

PMC Twenty.21 stand mount bookshelf speaker Spinorama CEA-2034 frequency response measurements.png


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:

PMC Twenty.21 stand mount bookshelf speaker Spinorama CEA-2034 early reflections frequency res...png


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

PMC Twenty.21 stand mount bookshelf speaker Spinorama CEA-2034 Predicted In-room frequency res...png


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:

PMC Twenty.21 stand mount bookshelf speaker driver components port woofer tweeter measurements.png


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:

PMC Twenty.21 stand mount bookshelf speaker Relative distortion measurements.png


Here is the breakdown per component:

PMC Twenty.21 stand mount bookshelf speaker driver components port woofer tweeter distortion m...png


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:

PMC Twenty.21 stand mount bookshelf speaker Absolute distortion measurements.png


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

Impedance is nicely high for a bookshelf:

PMC Twenty.21 stand mount bookshelf speaker impedance and phase measurements.png


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

PMC Twenty.21 stand mount bookshelf speaker beamwidth measurements.png


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

PMC Twenty.21 stand mount bookshelf speaker Horizontal directivity measurements.png


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

PMC Twenty.21 stand mount bookshelf speaker Vertical directivity measurements.png


Finally, here is our waterfall:

PMC Twenty.21 stand mount bookshelf speaker CSD waterfall measurements.png


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/
 

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thewas_

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#6
Thank your for the measurements which unfortunately are in line with few PMC monitor measurements of the German magazine Sound & Recording like http://www.musikmedia.de/mstore.php?SKU=PCM0005071-000 and also to my limited experience with few Twenty-something models in some local audio shows where I immediately noticed their weird tonality (exaggerated mid-bass and treble) which was actually liked by some audiophiles there, as it really sounds different than most loudspeakers, thus a typical voiced "highend" product.
 

Archsam

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#7
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
 

MZKM

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#8
Hi-Fi News also reviewed/measured them:
Screen Shot 2020-06-29 at 7.01.12 PM.png


Screen Shot 2020-06-29 at 7.02.27 PM.png

Not sure how they calculate Sensitivity, looks like they probably include >20kHz for them to get almost 87dB.
Their -6dB frequency is near identical to mine, which is good to see.
A pair matching of +/-0.9dB is too high for my liking, especially at this price point ($2000 USD).
 
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Soniclife

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#10
Nothing in this review surprised me, I've not heard this specific model, but the others in the 20 series seemed just as bad to me.

Is there any chance of measuring the phase of the port and driver?
 
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amirm

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Thread Starter #11
Or is this "monitor" speaker broken? Do you have a second one to remeasure distortion?
I do but not the motivation to do it over. :)
 
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amirm

amirm

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Thread Starter #12
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amirm

amirm

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Not sure how the calculate Sensitivity, looks like they probably include >20kHz for them to get almost 87dB.
Must be as I had to boost my levels 2 to 3 dB relative to Revel M106 to get the same level at 1 kHz.
 

617

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#15
The use of the undersized transmission line as a brand differentiator really tells you all you need to know about PMC. Embarrassing performance, even for a small two way without a waveguide.

Tweeter appears to be SEAS Prestige 27TBFC/G

The hifi news FR graph is pretty hilarious, looks even worse than what Amir measured.
 

MZKM

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#16
Here is how their $12,000 PMC Fact.8 [signature] measured in the Stereophile review (review from 2020, but released around 2010):


Keep in mind this is a 30-degree horizontal response, so less jagged than on-axis. is also using a close-mic response for the bass which boosts it by like 5dB, so it is more bass-shy than the graphs leads on.
 
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Kal Rubinson

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#17

MZKM

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#18
The fact.8 was originally released in 2010 but we reviewed new version, the fact.8 Signature: https://www.stereophile.com/content/pmc-fact8-signature-loudspeaker
Fixed.
Weird how John Atkinson got a sensitivity close to spec yet you stated:
I noticed that my volume control settings were about 6dB higher with the fact.8s than with my usual speakers, even though the specs indicate just a 1dB difference in sensitivity.
Which is inline with the discrepancy with this speaker. Maybe he needs to re-evaluate how he does this (though that would cause issues with previous measurements).

Nice to see your hearing is still good:
I needed more richness in the upper and midbass.
I liked roundabout way of addressing this in the conclusion:
I've heard similar sound from other wellknown speakers that are prized for their clarity and detail.
 

pozz

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#20
They ask how much for tihs? Ridiculous.
Look up their real "small" monitors, like the IB2S-AII. $19k per pair. The reputation is incredible.
 

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