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:
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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:
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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:
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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:
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Putting all of these together we can try to predict the tonality you may get in your room:
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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:
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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:
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Here is the breakdown per component:
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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:
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Fair bit of distortion exceeds my threshold of -50 dB. This is at 96 dB SPL though.
Impedance is nicely high for a bookshelf:
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Beamwidth is chewed up as we expect based on directivity plot earlier:
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It is wider than normal because of lack of waveguide. But gives up smoothness which you can also see in our 3-D plot:
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I forgot to mention that this speaker tilts back. That seems to have shifted the center line up some relative to my microphone placement:
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Finally, here is our waterfall:
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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.
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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
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