DIY planar magnetic speakers

The Monsoon MM-700 is a subwoofer/satellite system with a pair of unenclosed planar magnetic drivers. These planar-magnetic elements are capable of covering nearly the full audible frequency range with low distortion, controlled diaphragm resonances, and without dealing with multiple drivers and crossovers. The system is no longer in production but may be found on ebay for $50-$100.

This project involves disassembling the satellites and installing the planar-magnetic elements in compact speaker enclosures to fully realize their potential and to eliminate need for the subwoofer and its built in stereo amplifier.

Dipole cancellation

Why not use the stock system? The output from the rear of planar magnetic elements is 180 degrees out of phase with the front output. At frequencies whose wavelength is longer than the dimensions of the panel (about 2.5 kHz in this case), rear output begins to diffract around the panel and partially cancels the front output. This produces a falling response as seen in a measurement of the stock satellites made at 1/2 meter distance:

Unless Monsoon applies drastic electronic equalization to the satellites or crosses over to the subwoofer at 1 kHz, there will be a suckout in the system response. Either of these is a major compromise.

Now let's look at a measurement of the planar magnetic element with the mic a few millimeters from the diaphragm. Close-miking avoids the effects of dipole cancellation, and shows the amazing potential of these drivers:

We can approach this ideal behavior by enclosing the driver's rear output, as is normally done with conventional dynamic drivers.

Enclosure

I built a somewhat crude 5 liter enclosure 136mm wide x 300mm high x 180mm deep, made from 12mm plywood. I lined the walls with a couple of layers of 80 mil auto sound deadening mat to dampen wall resonances and stuffed the box with fiberfill to help reduce internal standing waves. A palm router was used to make room for the irregular driver chassis. Even then a bead of form-a-gasket was required to get a good seal.

Damping

As with the smaller MH-500 elements I used to build planar magnetic headphones, the MM-700 drivers require additional damping to avoid a large peak at the bottom of their frequency range. Monsoon accomplishes this by sandwiching a layer of felt with a screw-on grill behind the driver. Here is the close-mic measurement overlaid with a measurement after removing the rear felt layer and grill (green):

I ended up gluing the felt to the rear of the metal driver chassis, since the screw-on rear grill wouldn't fit in the enclosure cut-out I made.

Diffraction loss

When mounted in a bookshelf speaker enclosure most drivers will exhibit a gentle 6 dB "response step" around 1 kHz to 4 kHz due to a phenomenon termed diffraction loss. This is seen in the white curve below, showing the driver in its enclosure at a 16" distance. Diffraction loss can be compensated by using a paralleled inductor/resistor combination set in series with the driver. The magenta curve below shows the same measurement after adding a network consisting of a 0.8 mH air core inductor and 8 ohm 5w wirewound resistor.

As you can see this reduces the response above 2 kHz by about 8 dB, which is a bit excessive. I ended up using a 6 ohm resistor instead of an 8 ohm resistor, so the final reduction was 6 dB as desired. I glued these to the rear-mounted binding post cup using construction adhesive:

Vent tuning

The driver only reaches down to about 200 Hz on its own. By using a vent we can extend the response deeper. I experimented with a range of vent tuning frequencies and determined that the best compromise for this enclosure/driver combination was a vent tuned to a high 120 Hz. Since the enclosure uses 12mm plywood, a simple 35mm diameter hole in the rear produces such a tuning without need for a longer port tube. This graph shows the complex sum of the vent output and close-miked diaphragm output for various port tunings, weighted for their relative surface area.

Directivity

The MM-700 planar-magnetic element is much larger than a conventional tweeter and is therefore more directional. This proves to be a benefit, since the 1 meter on-axis response (white) has a large treble peak while a flatter overall response is seen 15 degrees above axis (yellow) or 30 degrees horizontally off-axis (red) thanks to this directivity:

Note that the microphone I used for this measurement is a little "hot" and is partially responsible for the treble peak in this graph.

Conclusion

Here are a few strengths and weaknesses of these speakers.

Weaknesses:

• Bass - even with a vented enclosure the bass is lightweight and cuts off quickly below 100 Hz. For real bass a subwoofer will be required.
• Unevent treble - large treble peak on axis, mitigated by...
• Directivity - fortunately if mounted on a desktop aiming straight ahead the listener will be above and to the side of the main axis, and the driver's limited directivity helps smooth the treble
• Efficiency and Power Handling - while dynamic, these tiny speakers will not go very loud

Strengths:

• Compact size - suitable for desktop/computer use
• Low driver resonances - planar magnetic drivers act as near-ideal pistons, resulting in great clarity
• Low distortion - the large surface area enables small excursion and generally low distortion
• Channel matching - very well matched L and R speaker response, resulting in sharp imaging
• Uniform impedance - making the speaker unaffected by the high output impedance of some tube amps
• Industrial appearance - in the handmade wood cabinets I think they have an interesting steam-punk look

So how do they sound? I'm using these as desktop speakers to listen to while working on a computer. For this purpose, I don't need high output levels, deep bass, or wide dispersion. I have them aimed straight ahead so am listening above and to the side of the main axis, which tames the uneven treble. Here the speakers are well balanced without calling attention to any individual frequency range. While limited in bass, they produce enough to follow complex prog rock bass guitar parts. The excellent channel matching, lack of driver resonances, and control of enclosure resonances result in a sharp and convincing stereo image with plenty of depth. The speaker's major strength is the midband: female vocals are realistic and focused, while solo piano is positively magical.

PS: Line arrays

An approach taken by some dedicated hobbyists is stacking several planar magnetic drivers in 6 to 8-foot tall vertical arrays to approximate an ideal line source. Line sources are known for providing a stable stereo image over a large listening area. As a bonus this approach helps iron out response irregularities, mitigates directivity issues, and increases efficiency and power handling. Monsoon MM-700 elements are a popular choice for these designs.

As with the single driver case though, dipole cancellation sets in early and requires either a very wide baffle or a large enclosure with challenging wall resonances and internal standing waves. Another consideration is the added cost of supplying 6 or 8 drivers per channel.