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Fig. 1. Field Mount Dub'l Cone In-a-Car Speaker.
plastic, spun aluminum and rubber fabricated paper. But all of these proved ineffective because they affected the sound quality. It was then decided to superimpose a second cone in front of the first and it is upon this theory that this paper is presented. The Dub'l Cone In-a-Car Speaker unit is practical for the following reasons:
1 . It protects the inner cone from both the sun and the rain.
2. The addition of the second cone plus a larger permanent magnet than is required for a single cone gives a very pleasing response.
3. When the outer cone becomes sunbaked and deteriorated it can be replaced in the field with a new cone and gasket and the life of the inner cone and the speaker unit itself prolonged indefinitely.
The Dub'l Cone Speaker unit (Fig. 1) has an aluminum voice coil, a resinimpregnated diaphragm, and both cones are treated with water repellent. Waterproof cork gaskets provide a resilient seal between both speaker cones and between the inner speaker cone and the housing. The housing itself is an all-weld construction which gives rigidity to the entire speaker under all kinds of temperature variations. Ordinarily single
TREATED
DIAPHRAGM
1.30 QZ.
MAGNET
ALUMINUM VOICE COIL
Figure 2.
cone speakers employ a 0.68-oz magnet. Because of the added load of the second cone the magnet in the Dub'l Cone Speaker is 1.3 oz and the magnet material used is selected Alnico V which is two and one-half times more powerful than previous magnetic materials. Thus every point in an in-a-car speaker unit that can be attacked by hot sun or rain has been protected (Fig. 2).
A coincidental result of the second or outer cone was the action it takes at the extreme high end of the response curve. It brings about a much sharper cutoff. In addition the mass-loading through the second cone causes a shifting of the resonance point with a slight additional response in the 500 to 1000-cycle range.
A comparison of the frequency response of a single-cone speaker with that of a Dub'l Cone Speaker is shown in Fig. 3. The solid line is the response of the Dub'l Cone Speaker and the dotted line, response of the same speaker with the outer cone removed. This graphically demonstrates that the Dub'l Cone has greater response between 100 and 200 cycles with a definite cutoff at 4000 cycles. The single-cone speaker while reaching 5000 cycles definitely does not have as sharp a cutoff. The Dub'l Cone Speaker thus eliminates the undesirable high-frequency noises which may be present on the sound track, giving a cleaner reproduction than would otherwise be possible.
The Dub'l Cone Speaker unit is mounted in a die-cast aluminum case. The front half of the case is provided
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June 1953 Journal of the SMPTE Vol. 60