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Polk S30 Center Speaker Review

amirm

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This is a review and detailed measurements of the Polk S30 center speaker. It is on kind loan from a member and costs US $229 from Amazon including Prime shipping.

Polk has really nailed the luxury look of the S series despite the budget pricing:

Polk S30 Review Center Speaker.jpg


It actually looks better in person than the picture above. Even the back looks interesting:

Polk S30 Review Center Speaker back panel port.jpg


This is an "MTM" design which stands for Mid-woofer, Tweeter and Mid-woofer. As you will see later, this presents some interesting radiation pattern which you want to know about in center speakers. This review is also more detailed for that reason and another to come.

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 performed over 1000 measurement which resulted in error rate of less than 2%. Temperature was 78 degrees F.

Measurements are compliant with latest speaker research into what can predict the speaker preference and is standardized in CEA/CTA-2034 ANSI specifications. Likewise listening tests are performed per research that shows mono listening is much more revealing of differences between speakers than stereo or multichannel.

Reference axis was the tweeter center without the grill.

Polk S30 Measurements
Acoustic measurements can be grouped in a way that can be perceptually analyzed to determine how good a speaker is and how it can be used in a room. This so called spinorama shows us just about everything we need to know about the speaker with respect to tonality and some flaws:

Polk S30 Frequency Response Measurements Center Speaker.png


Ignoring that bump for a second, the on axis response looks pretty good! Indeed I could see that even in the in-room measurements I make for distortion. Very nice to see attention being paid to this important aspect of speaker design. Back to the bump around 940 Hz, a near field measurement by the port shows the exact reason:

Polk S30 driver and port Frequency Response Measurements Center Speaker.png


If that peak was a few hundred hertz lower, the woofer output would drawn it out but it is where crossover is acting on the woofer making the resonance be louder than its output. We will analyze this many ways through the rest of the measurements starting with our early window reflections:

Polk S30 Early Window Frequency Response Measurements Center Speaker.png


Let's highlight the rear reflections (sound coming from behind the speaker hitting the wall):
Polk S30 Rear Wall Frequency Response Measurements Center Speaker.png


So we have further proof that this is coming out the port. This is good news in a way that it will not be as strong hitting you and you can try to absorb it (but there is a better tool -- see listening section of the review).

Predicted in-room response shows that, some dipping due to directivity error and brightness:
Polk S30 Predicted In-room Frequency Response Measurements Center Speaker.png


We will qualify this through listening tests later.

As further evidence of the resonance coming out the rear, we can look at a "balloon" visualization of sound field of the speaker as we look at the side of it with tweeter pointing to the right:

Polk S30 Resonance Baloon Plot Measurements Center Speaker.png


The redder the color, the stronger the signal. We see the woofer on the right pumping out signal but so is the port in the back on the left. Ideally that would be in green to blue color.

As I mentioned in the intro, MTM configuration causes a problem in that the two drivers start to cancel each other out as the sound wave length approaches the distance between the two drivers and how far you are angled to left or right of the speaker. We clearly see this in the horizontal beam width narrowing:

Polk S30 horizontal beam width measurements Center Speaker.png


This is a problem for a center speaker because you want to have broad coverage of seating area. With just 40 degrees to play with, you better sit pretty far to get decent width with good frequency response.

Note how the rear port resonance shows up here just the same because it spits out sound to the rear so widens the coverage there.

Horizontal directivity shows the same problem again:

Polk S30 horizontal directivity measurements Center Speaker.png


But notice how much better vertical coverage is:

Polk S30 Vertical directivity measurements Center Speaker.png


Alas, we don't need vertical coverage. We need horizontal. As noted, if you can do it, you should use it vertically. We can see the effect of that on frequency response at 30 degrees:

Polk S30 vertical and horizontal response  Frequency Response Measurements Center Speaker.png


Do you want to have the blue response on the left or right? Left is using it as shipped in horizontal configuration. This is why you normally want a 3-way center speaker that would have a single mid-range producing this part of the spectrum, than dual woofers.

Dual woofers do have a benefit though in more power handling. We see that in very low distortion of the S30:

Polk S30 relative distortion vs Frequency Response Measurements Center Speaker.png


Polk S30 THD percentage distortion vs Frequency Response Measurements Center Speaker.png


Film sound mixers usually put even effects in the center channel to "anchor" them to the screen so that channel works very hard. As such you need good power handling and low distortion which the S30 delivers.

Our impedance graph shows our port resonance and others:

Polk S30 Impedance and Phase Measurements Center Speaker.png


It also says the "8 ohm" spec from the company is fantasy. In reality, it dips below 4 ohm. Fortunately efficiency is good so you shouldn't have a lot of problem driving it with mass market AVRs.

I usually don't post CSD/Waterfall graphs for passive speakers but to nail the coffin on resonances :), here it is:

Polk S30 CSD waterfall measurements Center Speaker.png


See how we can nail the story of a speaker from multiple angles and arrive at high accuracy conclusions?

Polk S30 Listening Tests and Equalization
I placed the S30 in my usual listening location in far field. I didn't need to play more than a clip or two to realize that on-axis, this is a very good sounding speaker. The nearly flat on-axis response is a great filter that separates good speakers from poor. Wanting to hear the effect of that resonance, I dialed in an EQ at its frequency:

Polk S30 equalization eq.png


The effect was subtle and most resulted in better clarity but was low enough that could be an error. Still, I tried it a few times blind and preferred removal of it. The overall tonality a bit bright on some tracks so I put in a quick shelving filter. As usual, you should use EQ to implement a sloping down target response for your system and adjust this to taste.

I placed the speaker on its side and enjoyed its high dynamic range. It has nice tuning of bass frequencies in that it doesn't reproduce sub-bass that makes other speakers distort easily. This may be the reason the highs stand out a bit more than they would otherwise.

Conclusions
The Polk S30 presents a good design with two clear flaws, one architectural and the other, implementation. The former being the classic MTM horizontal cancellation and the latter, the resonance in the enclosure escaping from the port. You can deal with the MTM issue by using only for a sweet spot or placing it vertically. The port resonance can easily be fixed with EQ.

Overall, I enjoyed my time with this speaker. It is a lovely looking one despite its budget price. You could buy it and just use it as mains on its side.

I am going to put the Polk S30 Center on my recommendation list. If you use it vertically with a bit of EQ, I would upgrade that recommendation to a strong one.

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

Appreciate any donations using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 

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pozz

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As further evidence of the resonance coming out the rear, we can look at a "balloon" visualization of sound field of the speaker as we look at the side of it with tweeter pointing to the right:

index.php


The redder the color, the stronger the signal. We see the woofer on the right pumping out signal but so is the port in the back on the left. Ideally that would be in green to blue color.
Nice way to demo the problem and show the relationship to on axis frequency response.
 

abdo123

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Sensible sensitivty with sensible price and performance.

btw for the nearfield component measurements, i would consider changing ‘port’ to ‘port/cabinet’ because these sort of measurements can’t really distinguish between port resonances and cabinet resonances.
 
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amirm

amirm

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btw for the nearfield component measurements, i would consider changing ‘port’ to ‘port/cabinet’ because these sort of measurements can’t really distinguish between port resonances and cabinet resonances.
Good suggestion. Will have to remember to change my template for future tests.
 

MZKM

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Spinorama 71.png
[/ATTACH]​
We haven’t measured too many center channels, I wonder if a trend will arise if them having a slightly hot treble as it has to cover a wide horizontal area, so slightly worse for the person in the system’s sweetspot to improve performance for those off-axis. I wonder if the same is done for surrounds as well.

__________
Also, this center with the S15 bookshelves seems like a good budget combo rec for others, though the Emotiva with its 3-way center will likely be better, but the looks for sure are not in the same class.
 

dfuller

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So my curiosity here is this: Considering the port doesn't really appear to be doing a particularly large amount in the low end (according to the nearfield), could it be worth plugging it with a piece of foam or similar?
 

thewas

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Surprisingly decent measurements for a low budget implementation of the per se flawed MTM configuration, a pair could be even used as a standing pseudo D'Appolito stereo setup giving added max SPL thanks to the 2nd midwoofer which can be seen also at the distorion plots:

By the way, wouldn't the red approximately -40 dB = 1% base line be better shifted on the right plot 10 dB higher?
 

Vict0r

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sam_adams

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I wonder how the LF response and distortion would differ on the S35 version given that it has four more 'woofers'. @amirm, is that cover over the rear port plastic or metal of some kind?
 

radio3

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Where would you guys set crossover to sub on this speaker? Never had a speaker that didn’t just cross at 80Hz before. Just curious. idea of having five of them with pair of subs for low budget, compact, low distortion, multichannel audio is intriguing.
 
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amirm

amirm

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Matias

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This resonance would go away by blocking the port. Of course bass would not extend as deeply, but then it would be high-passed by the AVR and a subwoofer would extend the bass anyway.
 

AlexanderM

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Really appreciate this review Amir! I have had a suspicion for a while now that the S60, the top speaker in this line, is worth a listen. It's one of the speakers on my list for when it comes time to buy.
 

aarons915

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Just a suggestion @amirm, for some of these MTM center speakers that some may want to use vertically, is it easy to flip the horizontal and vertical axis and produce the Spinorama in both orientations? I do agree that this speaker would be good as an LCR so it would be nice to see the Spin in that orientation.
 

MZKM

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Just a suggestion @amirm, for some of these MTM center speakers that some may want to use vertically, is it easy to flip the horizontal and vertical axis and produce the Spinorama in both orientations? I do agree that this speaker would be good as an LCR so it would be nice to see the Spin in that orientation.
Axes flipped:
Spinorama 72.png

Listening window much more similar to on-axis.
 

Helicopter

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Thanks Amir. Great to see such a nice looking speaker perform well at this price. This is one I can recommend to people who don't want to spend $300+ per channel. I like the sensitivity too. It should make many people happy.
 

enricoclaudio

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My son in law owns the S50s (Towers) and they sound OK for what he paid. I used to own Polk Audio in the past (RTi A and LSiM) but they were just too bright to my ears. Looks like the same brightness is showing up in these measurements as well.
 
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