This invention relates generally to a diaphragm for a loudspeaker and, more particularly, to a loudspeaker diaphragm provided with a core member which is made from a foamed resin material molded into a frustum configuration. 
Diaphragms for use in loudspeakers are known in which a foamed resin material, such as foaming polystyrene, is molded generally into a frusto-conical configuration to be used for the core member of the diaphragm.  Such diaphragms typically have the same mass as conventional diaphragms of paper and the like which are also molded into a frusto-conical configuration. 
Although these foamed core members provided certain advantages over conventional paper core members, for example, hindering the formation of a split vibration and improving the acoustic characteristics thereof, they disadvantageously have a low vibration or acoustic propagation speed and a relatively low natural resonance frequency, while retaining a high resonance sharpness Q. Thus, the acoustic output-frequency characteristic of the foamed diaphragms have a remarkable peak dip, particularly in the high-pitched tone region, and do not establish a satisfactory flat response. 
The foamed diaphragm usually has its back or small-diameter portion connected to a voice coil bobbin and is driven thereby.  However, the distance between the back or small-diameter end portion and the flat front surface of the frustum at its center differs from the distance between the back portion and the front surface at the periphery of the latter, resulting in a degradation of the phase characteristic of the acoustic output.  Further, since the voice coil bobbin is adhesively affixed directly to the back end of the foamed core member, it is difficult to maintain the bobbin in a fixed condition with any satisfactory degree of accuracy, and the resulting movement with respect to the adhesively fixed condition affects the acoustic output-frequency characteristic in the high-pitched tone region. 
Furthermore, since the foamed resin material has a generally poor heat resistance, it is difficult to use such material in a diaphragm of a large output speaker because of the large amount of heat transferred from the bobbin to the core member.