Basic design, Target specs, Materials & FAQ:

Target specs (meet or exceed)

87dB/2.83V 1m
300-20kHz
105dB SPL max
4 ohms

Design

The driver will be a push-pull mid/tweeter planar. Overall, the magnet system is the same as is used by Patten (2), Gold Ribbon Concepts (4) and Carver in their ALS. Unlike the Patten or GRS (but like numerous others) the driver will be slightly corrugated horizontally to increase longitudinal stiffness and impede the generation of standing waves. Also unlike both of these, multiple conductors will be in each gap. Where Patten uses thin wire and packing tape, here ~0.3 mil Al foil will be used and bonded to the diaphram film (Kapton). The basic system will look from the top something like:

The diaphram is be held together by an adjustable frame so that the horizontal tension can be adjusted. The frame can be made of MDF (or particleboard) or maybe hardwood.

Materials

Diaphram approximately 3" x 44" (or 7.6cm x 112cm). Kapton is clearly the material of choice. It's avail in thicknesses down to 0.3 mil, resists heat incredibly (something like 700 deg. F for short term) and will not stretch.
~0.3mil Al foil. Get from film caps. Maybe $2US
Ceramic 5 block magnets. I've ordered 300 (a few extra) #CB65's from the Magnet Source. They're 0.39 x 0.43 x 1.875", oriented along the 0.39" dimension. 0.43 will be pretty flush to the 0.5" steel middle strip. The cost for both speakers will be ~$120US.
1/8" steel in 2" and 1/2" widths. Total of 32' of the 2" and 20' of the 1/2". I got it for $40US.
3M Adhesive transfer tape #9460. I purchased this from the John Shelly Co. in Boston (800 525 0202). They've got a $25 minimum order and a roll of the tape went for about $7. There's another 3M tape, #976 that is an "office adhesive transfer tape" that may be just as good and more readily available.
Thin foam for dampening. ~$2 US
MDF / particle board for frame ~$7 US
Hardware for assembly ~$5 US

Total cost: under $200US/pair

FAQ?

Q: What's the difference between a ribbon and a planar?

A: In a ribbon design the conductor is the diaphram. In a planar, the conductor can be bonded to a diaphram to increase the surface area and/or mass and add dampening

Q: Why pick a planar and not a ribbon?

A1: You can get a much higher impedance using multiple parallel runs
A2: You can increase your surface area and increase your efficiency
A3: The diaphram material can help dampen the system and hold the conductor nicely within the gap. Since you can use longer runs of conductor, for a given impedance your power handling is much higher since you can use a wider or thicker conductor. Also, very tall drivers are possible.

Q: Why pick a planar and not a dynamic?

A1: Moving mass is orders of magnitude smaller. "Control" over the diaphram is much tighter.
A2: Vertical dispersion is very poor, knocking out ceiling and floor effects. Reflections within 10-20ms smear your image (Hass effect) and ceilings and hard floors are notorious for doing this.
A3: The impedance is almost purely resistive.
A4: Need something to use up all that power on your amp...

Q: Why not just build Patten's design (Ref 2)?

A1: Where's the fun in that?
A2: I'm dubious about the use of wire held between Mylar and packing tape. In time the tape will fail. Also, the wire will dissipate less heat than a strip of Al bonded to a sheet, lowering your power handling.
A3: The metalwork can be done less expensively. I'm a poor grad student.
A4: There is no provision for controlling or adjusting the horizontal tension of the diaphram.
A5: I'll learn a lot more this way.

Target specs (meet or exceed)
Design
Materials
FAQ?

Planar mid/tweeter - 15 OCT 1996

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