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.