Polk Audio SR6500 Car Speakers

So how do these stack up?

These are, in my opinion, perfect. The Qts is ideally damped-not too low and dry and not too high and peaky, The Fs is low enough to give good midbass and the efficiency is high enough to keep up with good-sized woofers with lots of power.

Klippel testing has given us a new way to look at a speaker's linearity under power. The non-linear Motor Force (BL [X] and Compliance (Cms [X] are graphical representations of what the speaker is doing while it is playing. They tell us if everything is balanced and working together. Both of these curves should be centered on the X(mm) 0 line and have essentially the same shape on either side of this point. The dashed lines on the graphs are the mirrors of the actual test and make it easy to see if the curves match the ideal.

The Bl (X) curve (Graph 1) for the woofer is almost perfect, with a shift of about 0.4mm. This is the best that I have tested with a linear range over 4mm. The Cms (X) as seen in Graph 2 is perfect. There is no offset and the shape is perfectly symmetrical. More importantly, the two curves are perfectly matched. They track each other over the entire range of the linear travel.

The Bl (X) and Cms (X) curves can also be used to predict the maximum excursion for the speaker (Xmax). It is defined by Klippel as the point at which the compliance or Bl nonlinearities create 10 percent harmonic or intermodulation distortion. This happens when the Bl has dropped to 82 percent of its x=0 value (XBl) or Cms has dropped to 75 percent (Xc). For the SR6500 woofer the XBl point is 4mm and the Xc point is 4.6mm. These numbers mean that you can use this component set full range with a medium-sized amp and get decent bass, or you can use them with a subwoofer and active crossover and they will simply rock.

Frequency Response

I start by measuring the speakers without crossover to see what the exotic materials were bringing to the table. Graph 3 shows the woofer and tweeter response on-axis. This is pretty much textbook stuff-unbelievable flat tweeter response, great woofer response with a slightly rising shape and no breakup problems on the top end. The wide overlapping range from 1kHz to 6 kHz means that the crossover can be effective and give a smooth transition on- and off-axis.

Graph 4 shows the individual responses together after that crossover, also textbook stuff. The 3.2kHz crossover point offers perfect symmetry. The tweeter is a little hotter so there is good versatility.

Graph 5 shows the system response on-axis. This is as good as it gets. A (very) small dip at 1kHz probably caused by an edge hole on the woofer is the only deviation from a flat line. Even this dip is way smaller than most 6 1/2" woofers.

Graph 6 shows the three tweeter attenuation settings available by changing the + lead of the tweeter at the crossover. The settings include a +1, 0, -1.5 and -3 dB.

I test component sets and coaxes on-axis and both vertical and horizontal off-axis to see if the crossover design is flexible enough to work in the wide variety of car audio installation positions. This is where most component sets fall apart. If the crossover isn't designed well, the on-axis response may look great, but the off-axis response will be awful. This is where the SR6500 really shines.

Graph 7 shows the response 30 degrees off-axis with the tweeter mounted above the woofer. If you leave the tweeter facing the same direction as the woofer, you get a slight roll-off above 10kHz, as shown in the graph. However if you use the included angled plates to aim the tweeter, you get the same perfect response that you get on-axis.

Graph 8 shows the tweeter mounted next to the woofer (horizontal). This by far the harder of the two off-axis positions because of the additional time delay difference between components. In this test I used the angle tweeter mount and reversed the phase of the tweeter as suggested in the manual. Again this response is pretty much perfect.

I also test harmonic distortion at 1/2 rated RMS power to get some idea of where the limits are in the system. The SR6500 is rated higher than most component sets I have tested. My tests at 60 watts showed very low distortion levels-below 3% in the mid and upper band.

Because the SR6500 is convertible to a coax, I decided to mount the tweeter and see just how bad it got. You see coaxes have the disadvantage of playing havoc with the woofer's response by putting a big chunk of plastic in front of it and having the tweeter mounted at the end of a post that creates huge amounts of diffraction. The end result is huge peaks and valleys in the response curve. This of course is the reason that component sets sound better than coaxes.

Graph 10 of the coax response, off-axis 30 degrees just like it would be in a car, completely blew me away. The response is flat with the exception of a very small dip at 3kHz. The tweeter is still smooth and there are virtually no diffraction dips. This is the best coax response I have seen. I have spent some time figuring out what they did and my guess is that it is a combination of the smooth tweeter post and combination of large woofer voice coil diameter and relatively small diameter tweeter fascia diameter. If you want a truly high-end speaker but need to have a coax, this is truly a no-compromise solution.

Graph 11 shows a system impedance that is smooth and easy on any amplifier (of course).

So what is my final opinion based on the tests?

On-Axis Response
10
Off-Axis Vertical
10
Off-Axis Horizontal Mount
10
Flexibility (above 3 averaged)
10
XBL
9.9
XC
10
Distortion
9.9
Parameters
10

Here it is: The Polk Audio SR6500 component sets are the best component sets and coaxes I have tested: the best performance, the best cosmetics, the best engineered, and so far the only thing I have seriously considered putting in my own car. If these seem expensive to you at $899, save your money and get the best. If these seem inexpensive to you and you are considering spending more-even a lot more-save your money. It doesn't get any better.

Polk guys, if I get to keep these, they are going in my truck. If I have to send them back, I am buying two pairs-one to put in my truck, and another to take apart and copy for my next customer. -PT

For the subjective portion of this test report, read the December 2005 issue.