Abstract:
A mechanical two way loudspeaker includes a low frequency diaphragm arranged coaxially with one another which are driven by the same voice coil. In order to flatten the sound pressure versus frequency characteristic of the loud speaker in the high frequency range, the low frequency diaphragm is provided with a flat sound wave radiation surface. The flat surface may be of a honey-comb configuration, or may be made of a low density synthetic foam.

Description:
BACKGROUND OF THE INVENTION 
     One example of a conventional mechanical two-way loudspeaker is shown in FIG. 1. A conical diaphragm 4&#34; for the low frequency range (hereinafter referred to as the low frequency diaphragm) is fixedly secured through an edge 3 to the mounting edge 2A of a frame 2 fixedly mounted on a magnetic circuit 1. A diaphragm 5 for the high frequency range (hereinafter referred to as the high frequency diaphragm) is fixedly secured through an edge 6 to the low frequency diaphragm 4&#34; in such a manner that the diaphragms 4&#34; and 5 are coaxial with one other. A voice coil bobbin 7 fixedly secured to the inner peripheral edges of the low frequency diaphragm 4&#34; and the high frequency diaphragm 5, and a voice coil 8 wound on the voice coil bobbin 7 are inserted into the magnetic gap 1A of the magnetic circuit 1. In FIG. 1, reference numeral 10 designates a damper which supports the voice coil bobbin 7. 
     When a current bearing sound signals flows in the voice coil 8 in the magnetic gap 1A of the magnetic circuit 1, it is vibrated in response to the sound current according to Fleming&#39;s left-hand rule, so that the high frequency diaphragm 5 and the low frequency diaphragm 4&#34; are operated sharing the vibration; more specifically, in the high frequency range, the low frequency diaphragm 4&#34; is not sufficiently vibrated, while the high frequency diaphragm 5 is primarily vibrated, and in the low frequency range both the low frequency diaphragm 4&#34; and the high frequency diaphragm 5 are vibrated. This is a specific feature of the conventional loudspeaker. 
     However, in the above-described conventional mechanical two-way loudspeaker, the sound pressure from the low frequency diaphragm 4&#34; which is conical and provided behind the high frequency diaphragm 5, interferes with the sound pressure vs. frequency characteristic of the high frequency diaphragm 5 during vibration, as a result of which this characteristic is not flat, as is desired. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of this invention is to provide a mechanical two-way loudspeaker in which the sound pressure vs. frequency characteristic of the high frequency diaphragm in the high frequency range is made flat, by making the sound wave radiation surface of the low frequency diaphragm flat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view illustrating one example of a conventional mechanical two-way loudspeaker; 
     FIG. 2 is a sectional view of one example of a mechanical two-way loudspeaker according to the invention; 
     FIG. 3 is a sectional view showing another example of the mechanical two-way loudspeaker according to the invention; and 
     FIG. 4 is a plan view of the loudspeaker shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the invention will be described with reference to FIG. 2. 
     In this embodiment, the low frequency diaphragm 4 has an acoustic wave radiation surface which is in the form of a flat plate. The internal structure of the low frequency diaphragm 4 is provided in a honey-comb configuration in order to increase its rigidity because the low frequency diaphragm 4 undergoes rigid motion, as a whole, in the low frequency range. In FIG. 2, reference numeral 11 designates a sub-cone for driving the low frequency diaphragm 4. The sub-cone 11 has its inner periphery bonded to a voice coil bobbin 7 with adhesive or the like, and its outer periphery bonded to the lower surface of the low frequency diaphragm 4, to support the latter. More specifically, the outer periphery of the sub-cone 11 is bonded to the low frequency diaphragm 4 so that the sub-cone 11 supports the latter at the node of the free vibration of the low frequency diaphragm, or near the node, whereby divisional vibration, which otherwise may be caused during the vibration of the low frequency diaphragm 4, is prevented, i.e., the low frequency diaphragm 4 is efficiently vibrated. The outer periphery of the low frequency diaphragm 4 is supported through an edge 3 by the mounting edge 2A of the frame 2, and the inner periphery is supported through an edge 6 by the outer periphery of the high frequency diaphragm 5. In FIG. 2, those components which have been described with reference to FIG. 1 are designated by the same reference numerals or characters, and therefore the detailed description thereof will be omitted. 
     The loudspeaker, according to this first embodiment, is constructed as described above. When a sound current flows in the voice coil 8, the voice coil 8 in the magnetic gap 1A of the magnetic circuit 1 is vibrated according to Fleming&#39;s lefthand rule in response to the magnetic flux induced therein, and simultaneously the voice coil bobbin 7 is vibrated. In the high frequency range, in association with the vibration of the voice coil bobbin 7, only the high frequency diaphragm 5 is vibrated to reproduce high frequency sound. However, in this case, the vibration is not sufficiently transmitted to the low frequency diaphragm 4 because of the compliance of the vicinity of the neck of the sub-cone 11, the outer periphery of which is bonded to the lower surface of the low frequency diaphragm 4, and accordingly the low frequency diaphragm 4 does not comply with the vibration of the voice coil bobbin 7. The low frequency diaphragm 4, which is coaxial with the conical high frequency diaphragm 5 and surrounds the latter has a sound wave radiation surface which is in the form of a flat plate, as described above. Therefore, in this loudspeaker, unlike the mechanical two-way loudspeaker in which the high frequency diaphragm 5 and the low frequency diaphragm 4&#34; are coaxial with each other and overlap each other, the sound pressure vs. frequency characteristic of the high frequency diaphragm 5 in the high frequency range is not affected by the low frequency diaphragm 4, and accordingly the sound pressure vs. frequency characteristic in the high frequency range of the loudspeaker is flat. 
     Sound reproduction in the low frequency range proceeds as follows: When a sound current flows in the voice coil 8, the low frequency diaphragm 4 having the flat-plate-shaped sound wave radiation surface is vibrated by the voice coil bobbin 7 and the sub-cone 11, while the high frequency diaphragm 5 is vibrated by the voice coil bobbin 7; that is, the diaphragms make rigid motion as a whole, to reproduce low frequency sounds. 
     The flat-plate-shaped low frequency diaphragm 4 is substantially in the form of a ring, and the inner periphery is fixedly connected through the edge 6 to the high frequency diaphragm 5 which is driven simultaneously. Therefore, when the loudspeaker is driven, a large vibration is liable to occur near the inner periphery; however, it is prevented in this embodiment because one sub-cone 11 is bonded to the substantial middle of the low frequency diaphragm 4. Thus, large vibration prevention can be achieved without increasing the number of sub-cones or the mass of the sub-cone. 
     In the above-described embodiment, the low frequency diaphragm 4 having the flat-plate-shaped sound wave radiation surface is not integral with the edge 3; however, if the components 3 and 4 are formed as one unit, installing the low frequency diaphragm 4 on the frame 2 can readily be achieved in assembling the loudspeaker. 
     FIGS. 3 and 4 show a second embodiment of the invention. In this embodiment, the low frequency diaphragm 4&#39; is made of a foamed synthetic resin which is light-weight and large in internal loss, in such a manner that it has an upper surface, which is the sound wave radiation surface, in the form of a flat plate. The upper surface is square or rectangular, as shown in FIG. 4. With this low frequency diaphragm 4&#39;, unlike one which is flat and circular, the vibrating area can be increased for a given size enclosure. 
     Similarly as in the first embodiment, the sound wave radiation surface of the low frequency diaphragm 4&#39; is flat. Therefore, the sound pressure vs. frequency characterisic of the high frequency diaphragm 5 in the high frequency range is not affected by the low frequency diaphragm 4&#39;, and accordingly the sound pressure vs. frequency characteristic of the loudspeaker in the high frequency range is flat.