Zaph|Audio - Bargain Aluminum MTM

Modeled on-axis frequency response is relatively smooth with the exception of a mild peak-dip combo centered on 4.5kHz. This is partially a box diffraction issue but it is primarily caused by cavity effect. The center frequency and amplitude of cavity effect can vary based on surround design, cone profile and driver position. Cavity effect works the same as baffle diffraction - the sound wave from the tweeter follows the baffle and releases when it hits the woofer cone edge and/or surround.

No software can model cavity effect (currently), and it's the main inaccuracy issue with current baffle diffraction simulators. (and midbass inaccuracy because none model cabinet depth) The good news is that it often smooths out off axis in much the same way that baffle diffraction changes off axis. You will see this in the measurements below.

The reverse null shows the crossover point, and the polar response shows the vertical off axis nulls. Vertical response stays relatively smooth with the severe nulls at 45 degrees based on woofer c/c spacing. Should this design be used horizontally as a center channel, about +/- 20 degrees is acceptable before the midrange starts drooping and dialog becomes hard to hear. This is generally pretty good for a large MTM.

Given the low distortion of the drivers, a tonal balance close to flat at the listening spot works pretty well. Lesser drivers would have to vary from flat to remove the harsh edge from their tonal balance.

Measured response

Measured on-axis frequency response - 5ms gated window

Measured response is pretty close to modeled response aside from a few minor variations due to component tolerances. Showing the several horizontal off axis measurements was important for us to see what happens to the response in the 2-5kHz range. Specifically, I knew it would smooth out off axis, but it was a question of how much and where the optimum response curve is. This will help us position the speakers in our listening room. The last image linked above shows only the 15 degrees off axis curve with the tweeters to the inside, which can be considered the optimum. In that case, the response curve is nearly perfect. Power response is relatively smooth for these speakers, without much midrange droop happening until around 60 degrees off axis.

Bass tuning options

Modeled low end response in sealed box with excursion @ 1,2,4,8,16,32,64,128 watts

While extension is pretty good for both sealed and vented options, the trade off is low power handling. In both cases, not much more than 20 watts is going to cause overload in the low bass region at certain frequencies. If you're carefull a 50-100 watt amp would probably be a good match if you're running full range. If you're running a subwoofer with an active crossover, those limitions disappear and you can dump a lot more power into them.

The DA175 woofers are higher Qts and in general that's going to mean sealed enclosures are more suitable. In fact, unreasonably large sealed enclosures are going to be required if you intend to keep the box Qtc low. Every enclosure for these is going to represent a compromise. 1 cu ft total (1/2 cu ft per woofer) is a trade off between reasonable sealed enclosure size and box Qtc. Trying to model a vented enclosure for these reveals huge volume requirements. Using enclosures that big will also result in even lower power handling and probably sloppy sounding bass. Therefore the enclosure size of 2 cu ft that I selected for vented is a tradeoff between size and peaking response. With the tuning at 32Hz, we get only mild peaking in the 50-80Hz range, good extension and power handling close to the sealed version. This setup, while not optimum for these woofers, will sound pretty darn good to someone running them full range without subwoofers.

Enclosure

Sealed system on stands

The above two enclosure types are presented here. See the bass tuning options above for more information.

In the vented enclosure, Whispermat or Sonic Barrier is the optimal damping material on top. In the bottom of the vented enclosure where I specify 8" thickness of damping, packed dacron fiber will work fine. This will absorb the lengthwise pressure node. In the sealed enclosure, dacron fiber will work fine but "Acousti-Stuf" will work better due to it's better absorbtive qualities. Stuff liberally but keep a good 5" away from the bass drivers.

Options

Room Placement
Nothing too out of the ordinary here, and my recommendations are similar to most of my other systems. Keep the rear of the enclosure at least a foot out from the back wall and maybe 3 feet from the side wall. The woofer's three X, Y and Z room placement dimensions are from the floor, back wall and side wall. These should be as different as possible and not even multiples of each other for the smoothest in-room midbass response.

The speakers can be facing straight forward or toed in slightly but should not be aimed directly at the listener. As mentioned above, the offset tweeters should be placed to the inside for the smoothest response, and optimal listening should be somewhere between 15 and 30 degrees off axis. As with most 2-way MTM designs, the tweeters should be level with the ears, or even a little above if you want a balance between sitting and standing listening positions. Adjust your stand height or floor standing enclosure accordingly.

Summary
This is a high value system. Not just by the low cost of the woofers, but by the simplicity of the crossovers. As always I aim for low distortion and this system delivers.

Comparisons are inevitable with the popular L18 system shown elsewhere on this web site. That system uses the same tweeter and the $70 Seas aluminum woofer. It essentially comes down to the DA175 vs the L18. Both linear and nonlinear distortion are similar in these drivers within the passband. There are a few areas where the L18 does a little better, and that's primarly in the area of dynamics. This is something my measurements don't show well, as I don't do distortion plots at different SPLs, nor do I have Klippel equipment available to me. But, I know the L18 maintains a better BL curve due to it's T-shaped pole piece and longer Xmax, and thermal compression is less of an issue due to it's ventilated spider and phase plug. It's not hard to see the difference in build quality that leads to better dynamics and higher power handling in the L18.

There is an area where the DA175 seriously outperforms the L18, and that's obviously price. The DA175 was named the value leader in the 6.5"-7" driver comparison, and with good reason. Using the DA175's in pairs helps hide the deficiencies of low sensitivity and low power handling and still costs half as much as a L18. If the size and sexy looks of an all aluminum MTM system fit your listening area, and your budget is limited to about $175 for drivers and crossover parts, this design could suit you just fine.

Enjoy!

Page done by John "Zaph" Krutke © 2006
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