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:
It actually looks better in person than the picture above. Even the back looks interesting:
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:
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:
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:
Let's highlight the rear reflections (sound coming from behind the speaker hitting the wall):
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:
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:
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:
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:
But notice how much better vertical coverage is:
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:
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:
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:
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:
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:
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.
------------
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/
Polk has really nailed the luxury look of the S series despite the budget pricing:
It actually looks better in person than the picture above. Even the back looks interesting:
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:
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:
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:
Let's highlight the rear reflections (sound coming from behind the speaker hitting the wall):
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:
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:
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:
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:
But notice how much better vertical coverage is:
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:
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:
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:
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: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:
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.
------------
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/
