Remaining compliance is provided by a pair of opposition-mounted seven-inch-diameter spiders made from Dupont cotton material. These are progressive-type spiders that decrease their compliance (have more resistance) as they reach the end of their travel. This is especially important if you plan to use the MA1200XL in a vented box. Coupling the cone to the motor system is a 311/44-inch (82mm)-diameter dual voice coil wound with copper-clad aluminum wire on a high temperature Kapton former. The dual two-layer voice coils are terminated with tinsel wire woven into the top spider and terminated to two sets of platinum-plated terminal blocks. Each voice coil has two terminal blocks, which have two screw-down connectors that accept up to 12-gauge wire.
In The Lab This section is divided into two parts, Klippel analysis and LEAP 5 analysis. Using the Klippel analyzer (on loan from Klippel GmbH), Pat Turmire, CA&E reviewer and CEO of Red Rock Acoustics, performed the Klippel analysis on the MA Audio MA1200XL DVC subwoofer. Ideally, the Bl curve should be centered on the 0mm point (where the cone is positioned when there is no signal) and symmetrically decrease with the same slopes in both directions of voice coil travel. When a woofer shows an offset it may mean that the magnetic and mechanical systems have not been completely optimized. Motor strength could therefore decrease faster in one direction (usually the outward direction) than in the other, and this means more distortion at high operating levels than if it were operating perfectly symmetrically and providing equal motor strength in both directions. However, sometimes an offset is deliberately used to keep the motor more linear as it hits its primary operating range of 90-110dB. Also, it's good to keep in mind that the human ear is not very sensitive to distortion at low frequencies and 10-15% THD (Total Harmonic Distortion) really is not audible with program material, which helps to keep the concept of linearity in a better perspective.
When looking at the MA1200XL Bl(x) curve, we can see the symmetry is almost perfect. The BL shows equal energy in each direction. Note that the XBl or measured Xmax from Bl limits can also be calculated from these curves. For the MA1200XL, this is at 17.5mm, which is higher than the rating in MA Audio's literature of 14mm and a bit higher than the +15% value that is used in the tests below. Another curve that the Klippel generates is the Kms(x) or compliance symmetry. This is similar to the Bl(x) curve; however, it shows how linear and symmetrical the suspension parts (foam surround and spider) are. Ideally the Bl(x) and Kms(x) curves are similar, which means the speaker is well controlled and stable at high power levels. For many woofers this is not the case. However, for this one, the curves are nearly perfect complements of each other. During the Klippel test this really proved to be true. In free air with more than 400 watts RMS driving the speaker to 22+mm of excursion, the MA1200XL stayed well behaved, with no bottoming or mechanical noise.
Next I measured the T/S (Thiele/Small for Dr. Neville Thiele and Dr. Richard Small, two very well known names in loudspeaker engineering and both from Down Under) parameters for the MA1200XL. This was accomplished using the LinearX LMS (Loudspeaker Measurement System) analyzer with Windows 9x software, the new LEAP 5.0 Enclosure Shop CAD software, and the LinearX VIBox for measuring dynamic impedance (impedance at different voltages). Testing consisted of performing impedance sweeps with LMS, with the driver clamped in a rigid stand for the free-air measurement, and then with the addition of 75 grams of modeling clay attached to the cone. These impedance curves were made using the current source (admittance) measurement method. Instead of getting the impedance curve by a single measurement as a voltage change across a large 600-1000-ohm resistor, this method is done by measuring voltage and current separately at a series of successively higher voltage levels and each two-curve pair (voltage and current) is divided to produce an impedance curve. Measurements were taken at 1V, 1V with added mass, 3V, 5V, 10V, 15V, 20V, 25V and 30V (for a total of 18 separate sweeps with the LMS analyzer: nine voltage sweeps and nine current sweeps). This is done not only because it's better to have the woofer driven through an amplifier like it will be in "real life," but also because measuring impedance at different voltages gives you a lot more information about the dynamic behavior of the woofer. The MA1200XL was getting fairly non-linear at 30V in free air (no box), but since LEAP 5 was able to closely curve-fit the data, it was included. This body of data was copy/pasted into the LEAP 5 software and the parameter derivation utility used to create the T/S parameters and computer box simulation data.
The MA1200XL parameters found were used to create computer box simulations using LinearX's LEAP 5 Enclosure Shop (learn more about this at www.linearx.com). According to the manual, this MA woofer was intended for high SPL performance in both small sealed and small vented boxes. However, with a Qts of 0.52, I decided to go with a pair of sealed boxes. I programmed LEAP 5 to simulate the woofer's operation in a 0.75ft3 and a 1.25ft3 sealed enclosure, neither with any fill material such as fiberglass. This also was the minimum and maximum sealed box volume recommended by MA Audio in the woofer manual.
Loaded into the 0.75ft3 sealed box, the MA1200XL yielded a low-frequency roll-off of 56.4Hz with a box Qtc of 1.03. The graph curves showed the SPL at 2.83V (blue curves) in half-space, 2.83V in a small 154-cubic-foot car compartment, about the size of a Mitsubishi Eclipse GT, and at the SPL at a power level required to get maximum linear excursion. Increasing the voltage input to the 0.75ft3 computer simulation to 108V pushed the woofer excursion to Xmax +15% and yielded a loud 118dB.
The larger 1.25ft3 box simulation produced a lower F3 of 49.2Hz with a lower box Qtc of 0.88. Since larger boxes take less voltage for them to over excur, the 1.25ft3 computer simulation required only 87V to reach a maximum linear SPL of 116dB, loud enough to tweak your ears a bit but still 24dB below the threshold of pain. Since these numbers are based on steady state (sine wave) analysis, the performance with program material will be at least 2-3dB greater before noticeable distortion.
MA Audio has a good reputation and the XL series has the right kind of features in this price range at $200. The MA1200XL produces some good numbers and looks to me to be very well built. Performance on paper is one thing, and how it sounds playing music is another. So, how does the MA1200XL sound, Eric? -VD
Subjective Well Vance, this is an interesting and cool looking subwoofer. From the packaging barrel that the MA1200XL comes in, to its massive frame and magnet structure, the MA1200XL has a commanding presence.
Installation To get MA Audio's official recommendation for the enclosure size and type to achieve maximum sound quality, I spoke to marketing coordinator and one-time star of The Discovery Channel's hit TV show, Monster Garage, Kevin Kuenzie. Kevin, a man of very few words (he kind of reminds me of my brother Patrick), recommended either a ported enclosure of 1.0ft3 with dual 4-inch-round ports, 16 inches in length, or a sealed enclosure with a net volume of 1.1ft3.