Loudspeakers 693woven materials, these must be sealed and the sealant is usually used to provide some
mechanical termination of the cone.
The front suspension represents a discontinuity in the diaphragm system and because
it has its own mass and compliance, it is capable of a separate resonance. When this
takes place it presents a very high impedance to the edge of the cone, reducing its output
and causing a dip in the response. Because of its nonlinearity, it radiates considerable
distortion, especially at low frequencies where the amplitude is greatest. The requirements
are high fl exibility and high interval losses. Probably the most successful material is
plasticized PVC, using a very stable nonmigrant plasticizer such as dibutyl sebacate.
The rear suspension is the major restoring force, the radial width is usually at least
twice that of the front suspension, and is a multiroll concentrically corrugated fabric
disc, impregnated with a phenolic resin. The weave of the material, number of
corrugations, diameter, and amount of impregnant determine the stiffness. It should
provide a substantially linear restoring force over the designed maximum amplitude of
displacement. The whole structure behaves mechanically as a series resonant circuit. The
mass is determined by the weight of the cone, voice coil, and former, and the stiffness
by the combined effects of the rear suspension and the annulus, the Q of the circuit being
determined almost wholly by the losses of the restoring force.
23.15 Voice Coil
The dimensions of a voice coil are determined primarily by the rated power handling of
the loudspeaker. It must be emphasized that with direct radiators, 95–99% of the input
electrical power is dissipated in the form of heat; even with the most effi cient horn loaded
units a minimum of 50% is used for heating purposes only. Figure 23.12 shows voice coil
temperature versus input power. The limiting temperature is set by:
- Maximum temperature rating of the former: 100°C for paper-based materials;
150°C for “ Nomex, ” which is an aromatic polyamide; 250°C for polyimide; - Temperature rating of the wire enamel: maximum 220°C for ML insulation,
down to 110°C for self-bonding and self-fl uxing wires; - Adhesive; from 110°C for cyanoacrylic to 250°C for those with polyimide
base; and - Mechanical expansion of the voice coil diameter at elevated temperatures.