Polk T15 Bookshelf Speaker Review

Loudspeaker manufacturing giant Polk Audio has a range of choices available in low cost home-audio speakers, from $100/pair bookshelf speakers to $4,000/pair floor-standing behemoths. In this review, we look at the $100/pair bookshelf speakers, the T15, to see what Polk can do for those among us on a particularly tight budget. The T15s are a medium-sized bookshelf speaker intended for a variety of applications. A large manufacturer like Polk certainly has the resources and engineering prowess to make a good, affordable speaker, so let’s take a close look at what they have delivered at this very modest price…

First Appearance

     

The T15 speakers arrived well packed for transit; they were cushioned between two large foam pieces within the box and were covered in soft plastic bags. With grilles on, they don’t look bad. The slight curvature of the grille and the Polk emblem do add a dash of style, and even these small touches make a difference. The black wood-grain side panels are also a bit classier than the norm for this price point, which is usually just textured vinyl veneer. With the grilles off, the speaker emerges; the front baffle uses a semi-lustrous plastic with rounded edges. The tweeter dome is surrounded by a blue plate with the Polk logo. The woofer cone has a creamy gray surface with a glossy dustcap. One significant visual feature is the front-mounted port, which is just a gaping black tube set between the tweeter and woofer. Ports are usually placed underneath the woofer when they mounted on the front baffle of a speaker like this, so Polk probably used this arrangement to save space on the height dimension. Overall, the Polk T15 speakers look fine for the price. They are not exactly flashy, but they don’t look shoddy either.

Design Overview

    

The Polk T15s are not tremendously complex speakers, but few bookshelf speakers in this price range are. That is OK, because complexity doesn’t guarantee good results, and simplicity does not forbid good results either. The Polk T15s are vented, 2-way bookshelf speakers using a 5.25” woofer and 0.75” tweeter. As mentioned, the unflared port is mounted on the front of the speaker. The front-mounted 3.5cm port combined with a keyhole mount on the rear make the T15s easy to mount directly on a wall. The cabinet construction is in line with what would be expected at this price point, with ½” MDF paneling except for the front baffle which uses ¾”-thick MDF. There is wedge-bracing along the internal edges of the cabinet and a layer of stuffing at the back of the cabinet. Five-way binding posts are used to connect to the speaker wire. The T15 comes with rubber pad stickers that the user can apply as the speaker’s feet if they wish which Polk has thankfully made optional.

    

The tweeter uses a silk/polymer composite dome that is mounted in a shallow waveguide. Being a 0.75” diameter dome, it should have wide dispersion (or as wide as the waveguide will allow) but might not have a whole lot of dynamic range in lower treble bands compared to a full 1” dome. The 5.25” woofer uses a polymer-composite cone. The cone of the woofer and dome of the tweeter takes advantage of Polk’s “Dynamic Balance” technology that combines materials of highly different resonant behavior so that they damp each other’s resonances for an overall low-resonant composite. The woofer’s motor section is covered in an aluminum pot, so I can’t really discern anything about it, except that it is relatively heavy, weighing in at 3 lbs. Strangely, there is no venting for the motor that I can see, and the aluminum pot covers the entire motor. Aluminum has good thermal conduction and diffusivity for cooling, but this driver is seemingly relying entirely on aluminum’s thermal properties for cooling. It’s a strange design choice, but perhaps a company with the R&D resources like Polk has found it to be a better solution than traditional ventilated designs. The crossover circuit is merely a capacitor and small inductor mounted to the back of the terminal cup, so we can only get 1st order slope filters on the woofer and tweeter. That is just a step above the bare minimum for speakers in this price range, which mostly only use just a capacitor to protect the tweeter. 

The grille is held to the speaker with some thick grille guides which do not look like they would break easily. They use an acoustically-transparent black fabric draped over a diagonally crisscrossing plastic grid. Grilles do not normally improve the sound, and nothing about this grille suggests that it would, so for optimum sound, I would leave it off.

The design and construction of the Polk T15 looks slightly above average for a speaker of this price point. Aside from a couple peculiarities, nothing seems outwardly amiss. Let’s now see if they can live up to their sensible design by doing some listening...

Listening Tests

When I first sat down with the Polk T15 speakers and started listening, I sensed something was ‘off’ right away. Switching between them and another set of speakers on the B speaker outputs of my AVR confirmed this feeling. There was a veiled aspect to the sound, as though, when compared to other speakers, a blanket had been draped over the Polk speakers that masked parts of voice and instruments. I decided to put them aside and not listen to them again until I ran some tests to see what was going on. Later on, after testing and examining the measurements, I could see the problem. For reasons that we will get into in the ‘Measurements and Analysis’ section later in this review, the T15 speakers do not behave quite like normal bookshelf speakers. They are best heard at a placement above or below the listening position rather than at a placement with the speakers level with the listener’s ears, which is the case with most bookshelf speakers. When I first started listening with them, I was using them facing directly at me on a height level with my ears, and this had a deleterious effect.

Armed with the knowledge of their unusual ‘altitudinal’ behavior, I later set them up so that the tweeters were at a 20-degree height with respect to my ears. Measurements suggested that this would make for a much improved sound. So, with a 5-foot height and an 8-foot distance from my listening position, I gave them another chance. Listening was done using a Pioneer SC-55 receiver, along with various subwoofers alternating between a 100 Hz and 80 Hz crossover frequency. Subwoofers were used for most listening since the T15s do not have much deep bass capability.

Music Listening

As always, I try to begin listening to something with heavy emphasis on vocals, because a clear reproduction of the human voice has to be one of the most important duties of a loudspeaker, if not the most important duty. For this purpose, I listened to Dead Can Dance’s classic live album “Toward the Within.” The music from this album takes its influences from many different directions, from middle-eastern styles, Gaelic music, psychedelic rock, and medieval chants, among a multiplicity of sources. The recording of the performance is first-rate which makes this album a common choice as demo material for audiophiles. Listening to this recording on the T15, I was first impressed by the full sound that these budget speakers put out. The sound was perhaps a bit soft in high treble but overall did not lack anything substantial, and this wasn’t the case when I tried to listen to them where their tweeter was level with my listening position. Indeed, simply standing up which puts my ears near level with the tweeters makes for a significant and adverse change in the sound. Sitting at a lower angle to these speakers does raise the soundstage so that the center image is a bit higher than dead ahead, but this isn’t all that bad. For one thing, in most performances, the audience is typically lower in elevation than the performers anyway, so a raised soundstage makes for a more realistic staging in a live album like this. Also, most people mount their television sets a bit high anyway, so those who use the T15s as left and right fronts will get imaged sound from direct television’s location. Speaking of imaging, the T15s imaged remarkably well for a speaker of such modest price. Instrumentation on “Toward the Within” cover a wide angle over the stage, and the T15s managed to capture much of that pretty well, even projected an image at angles farther out than the angle between the speakers. While their imaging might not have had the pinpoint precision of other speakers I have heard, the soundstage was wide and distinct, albeit a tad higher than normal.

On the orchestral side, I turned to a recording of Symphony No. 5 by Dmitri Shostakovich performed by the Pittsburgh Symphony and conducted by Manfred Honeck. I had not heard this album before, but this double-Grammy-winning album received one of its awards for “Best Engineered Album, Classical,” so I figured this must be a top-notch recording. I was not disappointed: this album sounds fantastic. The album’s full title is “Shostakovich: Symphony No. 5/Barber: Adagio” and it beautifully demonstrates the range of expression in classical music, from playful to thundering to gentle and everything in between. Listening to “Symphony No. 5” on the Polk T15s, I did not detect anything off at first, except that, as with other recordings I had listened to on these speakers, the upper treble was a somewhat soft. In fact, the treble softness reminded me of the sound of older classical music releases on analog recordings. These speakers do not have the sparkle or ‘air’ of many other speakers, but they aren’t completely incapable of high-treble; they were just upper-treble shy. This isn’t an altogether bad thing, as many speakers artificially boost the treble in this range to exaggerate detail in the sound. Those who are sensitive to high-frequencies might prefer this laid-back sound. Certain instruments such as strings and brass had a more heightened presence, which I think was an attribute of the speaker rather than the recordings. The Polk T15s imaged well on “Symphony No. 5” with a nice broad soundstage, although, again, it does not achieve the pinpoint precision that some other speakers are able to project, probably due to their recessed upper treble.

On the opposite end of the musical spectrum, I turned to a very aggressive Drum’N’Bass album from 2001 by Panacea and Cativo called “The Hardest Tour on Planet Earth” to see how the T15s can handle something abusive at loud levels. This ferocious electronic music album uses all the dynamic range it can get and rides the digital full-scale of the sound mix from beginning to end. It is mostly bass-heavy, high-tempo breakbeat music layered with distortion filters and bizarre dialogue and vocal samples. I cranked this album pretty hard on the T15 speakers, and I can report they withstood the abuse. I can’t really say if they were distorting heavily, since the music has so much deliberate distortion in it already, however I did not notice any significant changes in timbre that would accompany a large rise in harmonic distortion when I cranked the volume from moderate to very loud. These speakers have good dynamic range capability and can get surprisingly loud for their cost. The use of subwoofers would give them a boost in dynamic range capability, as was the case in my system, especially if used at a higher crossover frequency such as 100 Hz or so. Subwoofers are absolutely needed with these speakers for music like this, since the T15s do not have a great deal of low frequency extension. The Polk T15s are able to reproduce the harmonics of a bass line but they won’t catch the fundamental. They have traded bass extension for dynamic range which is not an unreasonable trade when it is assumed subwoofers will be in use.

While listening to the Polk T15s, I did some A/B comparisons with another bookshelf speaker that I was actually using as a speaker stand for the T15s in order to obtain their higher-than-normal elevation. One difference that was immediately clear was that the T15s have a less open and more ‘cupped’ sound. It’s not too obvious when listening to these on their own without an external reference, although I still did get a sense of it on some recordings that I was familiar with. The T15s seem to have a kind of midrange congestion that produces this cupped effect. It is a flaw but not so severe as to ruin the sound, however, relative to some other speakers, it is certainly present. The sound of the Polks was smoother but also less detailed as well. However, simply boosting the treble via equalization does bring back some detail.

Movie and Television Watching     

Continuing where I left off in my Monoprice MP-65RT and  Dayton Audio MK402 Bookshelf Speaker Reviews, I watched episodes five and six of the second season of True Detective. If you enjoy dark crime dramas and hate cheerfulness, this is the show for you. As I said in the last two reviews, I think “True Detective” is a good trial for dialogue intelligibility because of how abridged and elliptical the dialogue can be at times even though it is all well-recorded. Music is also beautifully recorded; it is generally foreboding and atmospheric but has some folk music performances in a grim and sad style that compliments the show. This is a big-budget, well-produced television series, and it sounds like it. I watched this show in stereo, so only the T15s and subwoofers were generating sound with no center speaker used in order to get a better idea of dialogue intelligibility from the Polk speakers. One reason for this is there is no corresponding center speaker in the Polk T series line-up. Dialogue and lyrics came through the T15 speakers with no problem. I was worried that the cupped effect I heard with some music might impact dialogue intelligibility, but it was too slight to be noticeable. Music and effects all came through fine. Whatever shortcomings these speakers exhibited when I was critically listening on music never became an obstacle to my enjoyment on TV listening. On True Detective season 2, it wasn’t long before I was lost in the show. That, to me, means the speakers are doing their job. Those who are looking for an inexpensive upgrade for their TV speakers will find the Polk T15s to be a major improvement over the sound of their built-in television speakers.

One movie that I watched using the Polk T15s speakers was Tim Burton’s overwrought gothic horror film “Sleepy Hollow”- on the extinct HD DVD format no less! The main reason that I decided on this star-studded opus was the magnificent orchestral score by Danny Elfman (I also wanted to make sure my HD DVD player still worked). While “Sleepy Hollow” has a good mix for dialogue and effects (especially the headless horseman’s thunderous galloping), Elfman’s score is the real reason this movie should be played back on a capable sound system: it is, in turns, cataclysmic, delicate, heartfelt, frightening, and whimsical. As is usual for Elfman, the music beautifully blends in choral singing with traditional orchestral elements. The T15 speakers handled “Sleepy Hollow” well enough, although I do think they missed some of the sparkle of the upper harmonics of certain instruments like chimes and violins. They did provide the violent moments with more punch than any of under-$100 speaker I have heard, however. Again, they exhibited exceptional dynamic range for their price and size. I didn’t sense any problems with dialogue intelligibility either.

Polk T15 Bookshelf Speaker Measurements and Analysis

 

The Polk T15 speakers were measured in free-air at a height of 7.5 feet at a 1-meter distance from the microphone, and the measurements were gated at a 8.5 millisecond delay. In this time window, some resolution is lost below 250 Hz and accuracy is completely lost below 110 Hz. All curves have been smoothed to a 1/24 octave resolution.

Normally, in our speaker reviews we start posting our measurements using a graph of curves in a manner that Harman International has termed the ‘spin-o-rama’ curve set. In this review, due to the peculiar nature of these speakers, we have opted not to do that, since the ‘reference axis,’ that is, the intended position of the listener with respect to the speaker, is quite different from that of conventional speakers. A ‘spin-o-rama’ curve set would not be flattering here, but that wouldn’t be completely fair to these speakers, since that is not how they are intended to be used. So, we explored the behavior of these speakers in a different order, one that better illustrates how and why the Polk T15s should be listened to above or below the speaker but not level with the speaker. Let’s look at a waterfall graph of the vertical dispersion of the T15:

Polk T15 Vertical Response +/- 100 degrees: 3D view 

The above graph depicts the Polk T15’s frequency response behavior on its vertical axis, where zero degrees is directly in front of the tweeter, negative degree values are below the tweeter, and positive degree values are above the tweeter. If you trace that zero-degree angle, you will see a significant dip at around 3 kHz to 5 kHz. That is a critical region for human vocal harmonics and instrument harmonics, and it is also serves as the basis for the fundamental for higher pitched notes. But, if you look at angles just a bit above and below the direct axis, the response in that region shores up. Typically what happens is that speakers will have a roughly flat response on their direct axis with that kind of dip at the regions where it is filled in, i.e., around 20 degrees.

The graph that might better illustrate this behavior is a polar map of the vertical dispersion:

 

Polar map of Polk T15 vertical response +/- 100 degrees off tweeter height axis

The above map looks at how the frequency response of the Polk T15 at vertical angles in 10-degree increments from 100 degrees above the tweeter to 100 degrees below the tweeter. The above graph shows the same information that the preceding vertical dispersion graphs do but depicts it in a way that offers new insight regarding this speaker’s behavior. Instead of using individual raised lines to illustrate amplitude, polar maps use color to portray amplitude, and this allows the use of a purely angle/frequency axis perspective. The advantage of these graphs is they can let us see broader trends of the speaker’s behavior more easily. What should be looked for on this graph are horizontal lines where the color does not change much; that is where the speaker will have the most even sound.

We again see that direct axis dip from 3 kHz to 5 kHz as a green blob near the center of the graph. Above and below it are solid red areas where the speaker’s output is much more level with the rest of its frequency range. This graph shows us where the speaker sounds ‘fuller’: at angles just above and below the direct axis. The T15 sounds best where the ear is around a 20-degree angle above or below the tweeter.

I contacted Polk Audio about this behavior and asked them about the intent of height placement regarding this response.

Their answer, from Stu Lumsden, VP of engineering:

We recognized that bookshelf speakers at this price are rarely placed on tall stands that position the speaker at the listener’s ear level. Usually the speaker is placed on furniture that results in positions slightly above or below ear level. And, as Audioholics astutely observes, this small speaker is a prime choice for surround channel and will likely be above ear level.

Let's emphasize here that this behavior was a deliberate design decision by Polk and not an accident or unintended side-effect. Let’s compare the horizontal dispersion at tweeter height versus horizontal dispersion at an angle 20 degrees below the tweeter to better see the consequences:

     

Polk T15 Horizontal Response +/- 100 degrees: tweeter height (left) and 20 degree low angle (right) 

What is occurring at tweeter height is that the tweeter is not in phase with the woofer, so their soundwaves are cancelling each other out thereby creating a null in the response. This will happen to every speaker where the drivers are separated by a specific distance; at some angle the distance difference places the phase in opposite polarity between the drivers for a frequency band where they overlap. Most speaker designs will make it so that the drivers are in phase on an axis level with the tweeter, and this is why it’s almost always suggested to use the speakers with tweeters at ear level, at least for the front stage. Since Polk believes that it will not often be the case where these speakers will be positioned in a way that their tweeters will be level in height with the listener’s ears, they have used the phase relationship between the drivers to shift phase-coherent sound to higher and low elevations. This is a clever design decision, but the only problem is that they don’t state this anywhere in the literature for the T15 speakers. The only line in their literature that hints at this behavior is this statement from the manual: “Ideal rear channel surround speaker placement is on a side wall, slightly behind and above the listening position.” This sort of behavior should be more explicitly stated in the product page and manual, hopefully with an illustration like so:

Once the proper elevation has been achieved, the frequency response is not bad for a speaker of this price point. There is a rise from just above 1 kHz to a bit below 4 kHz, and this does color the sound. I noted certain instruments like some brass stood out more so than usual when listening to orchestral music. This is a speaker resonance though and is the sort of thing that equalization can attenuate easily. As noted in the listening sessions, the treble region isn’t very pronounced with respect to the mids, so these speakers do not have a forward character. They were soft yet without omitting a lot of detail, and this response measurements explains why: a slightly lowered yet flattish treble response. The tweeter is doing its job just fine, and the woofer seems to be working okay, so the lump in the response looks like it’s due mainly to the crossover. This is not surprising given how simple the crossover is. A more sophisticated crossover may be able to deal with this problem better, but that would make for a more expensive speaker thus defeating the point of a speaker like this.

A more overall neutral response than a 20 degrees low angle can be had from using the speaker at an angle that positions the listener 20 degrees above tweeter height. The following graph compares the responses at 20 degrees high and 20 degrees low:

 

Polk T15 vertical response at 20 degrees above and below direct axis

Polar map of Polk T15 horizontal response at 20 degree angle below tweeter +/- 90 degrees

The above polar map shows the horizontal dispersion of the T15 at a 20-degree angle below the tweeter height. Dispersion at this angle is reasonably wide with a relatively uniform response stretching out to 40 degrees. There is some slight asymmetry exhibited here, and that is likely due to the asymmetrical placement of the port on the front baffle, but the audible consequences of that will be negligible. We do see some waist-banding around the crossover frequency between 4 and 5 kHz as the woofer starts to beam and the tweeter is still under the influence of the crossover filter. The story here is as long as you listen within 40 degrees off the center axis, you should get most of the sound. This speaker will cover a wide area with a reasonably full sound.

 

Polk T15 Low-Frequency Response 

The above graph shows the Polk T15’s amplitude response in bass frequencies. It was captured by using groundplane measurement techniques. The bass response looks to be slightly overdamped for a ported speaker, so it has a high roll-off frequency but gradual roll-off slope. A response like this makes sense to compensate for boundary-loading, in other words, placing these speakers near a surface should shore up the low-end of the response. Given that there is a keyhole mount in the back of the speaker, and also the fact that it is front-ported, this speaker really expects to be wall-mounted, and this sort of low-frequency response is more evidence for that case. Wall-mounting should produce a pretty strong response down to 80 Hz at the very least. Having the speakers on stands with a distance away from nearby surfaces may end up with a somewhat bass-shy sound, at least without the assistance of subwoofers.

  

Polk T15 Impedance and Phase Response 

The above graphs show the electrical behavior of the Polk T15 speaker. Polk specs this speaker at 8 ohms nominal, and that is a fair characterization. There are swathes spent at 6 ohm, but there are also regions spent way above 8 ohms, so it averages out. Phase behavior is also good; there are no sharp phase angles at impedance dips. Overall, this is a very benign electrical load that any amplifier could handle easily, as well it should be since these speakers are more than likely to be used with entry level AVRs that will not have the most robust amplification. Sensitivity was measured at 86.4 dB for 1 meter at 2.83v which does not quite match Polk’s 89 dB, but they specified their measurement at 1 meter for 1 watt, and that could produce differing results. Either way, it isn’t bad for budget bookshelf speakers, and the loudness will get a boost from boundary reinforcement for those who wall-mount them. The sensitivity is relatively good which will help, since, again, these speakers are not likely to be run on powerful amplifiers.

Conclusion

The Polk T15 is something of an odd duck to review as a stereo pair because of their direct-axis behavior. However, once they are positioned correctly for their design, they can produce a reasonably good sound for their cost. What the Polk T15 speakers set out to do, they accomplish. These are speakers designed for circumstances that are less than optimal for pristine high-fidelity listening; they are speakers for real-world conditions. Not many people actually have the correct room dimensions and floorspace for the ideal stand-off distances of speakers from walls, and those who would set aside that kind of room and floor space are probably shopping in higher price-ranges than $100 per pair. Polk has looked at how low-cost speakers are typically used in reality and has designed the T15 to accommodate those settings. In my experience there are speakers that have a more refined sound, even around the same price range, but they would not be able to cope with sub-optimal placement and listening conditions nearly as well as the T15s. 

The Polk T15 sound quality isn’t perfect, but it isn’t bad either, and at $100 per pair I am not going to nit-pick too much. In fact, their soundstage is remarkably wide, and their imaging abilities are respectably good. Their dialogue and vocal reproduction was satisfactorily clean and clear. The sound they produce is going to be a big step up from television set built-in speakers, and they should also be a significant upgrade from typical home-theater-in-a-box speaker sets. The caveat is, as has been repeated in this review, that they really should be used so that the listener is at roughly a 20-degree angle either above or below ear height. Most people who wall-mount front left and right bookshelf speakers for their televisions do so at a height that puts the speaker above the listener, so this angle occurs almost naturally. It’s also a nearly ideal mounting angle for surround speakers as well. The only problem is, as was mentioned before, Polk does not really explain this attribute. They mostly leave it to chance for the listener to find out this behavior. My advice to Polk is to better explain this recommended use in the manual and on the product page and, even better, with illustrations.

I want to emphasize that the Polk T15s are a very good choice for a surround speaker for a low-cost surround sound system. With their keyhole, they are easy to hang on a wall, so no need to spend extra for a bookshelf speaker mount. They have good sensitivity and an easy electrical load, so any AVR can power them just fine to loud levels. Their response at higher elevation with respect to the listener makes them perfect for conventional surround speaker roles where it is typically advised to place them at higher angles. Their front-firing port and overdamped bass response makes them a great fit for wall-mounting as well so the low-frequency response may not see as much of an over-boost as a regular bookshelf speaker. They have a wide horizontal dispersion, so they can cover a broad seating area with a full sound. All of these characteristics make them ideal for surround speakers for systems where the total system cost is less than $1,000 or so.

In the end, I leave the Polk T15s with a favorable impression. They have some unique design aspects that, while they ought to have been documented better by Polk, have some clever engineering and practical advantages. These are a fine choice for those one a slim budget who want wall-mountable speakers that need to be out of reach for children or no-fuss surround speakers that similarly have to be mounted high. The price is right, at $100 a pair ($69/pr currently on sale). I think that buyers who set the T15s up in a way that will bring out the best in them will end up enjoying these speakers a great deal. 

The Score Card

The scoring below is based on each piece of equipment doing the duty it is designed for. The numbers are weighed heavily with respect to the individual cost of each unit, thus giving a rating roughly equal to:

Performance × Price Factor/Value = Rating

Audioholics.com note: The ratings indicated below are based on subjective listening and objective testing of the product in question. The rating scale is based on performance/value ratio. If you notice better performing products in future reviews that have lower numbers in certain areas, be aware that the value factor is most likely the culprit. Other Audioholics reviewers may rate products solely based on performance, and each reviewer has his/her own system for ratings.

Audioholics Rating Scale

• — Excellent
• — Very Good
• — Good
• — Fair
• — Poor

Metric	Rating
Build Quality
Appearance
Treble Extension
Treble Smoothness
Midrange Accuracy
Bass Extension
Bass Accuracy
Imaging
Dynamic Range
Fit and Finish
Performance
Value