694 Chapter 23
Fortunately, there is in-built semiprotection for the assembly, namely temperature
coeffi cient of resistance of the wire, which is 0.4% for a 1°C rise in temperature! Thus
at a temperature of 250°C above ambient, the voice coil resistance has doubled, and
for a constant voltage input (which is the norm for modern amplifi ers), the indicated
power E^2 / Rnom is twice the actual power. Note that Rnom is the manufacturer’s specifi ed
resistance and is, or should be, the value at the series resonant frequency Rco where the
input impedance is minimum and resistive.
23.16 Moving Coil Loudspeaker.....................................................................................
Figure 23.13 shows the structural features of a moving coil direct radiator loudspeaker,
and for purposes of analysis it will be convenient to divide it into two parts: the “ motor ”
or “ drive ” unit and the acoustic radiator, or diaphragm, described earlier.
The driving system consists of a solenoid situated in a radial magnetic fi eld. It is free
(within certain restraints) to move axially (see Figure 23.14 ). When a current is
passed through the coil, a magnetic fi eld will be generated, with the magnitude being
directly proportional to //, and this will react with the steady fi eld of the permanent
magnet and a mechanical force will be developed that will tend to move the coil axially,
30025020015010050(^0) 0 20406080100120
25 37 50 Nomex
former
Aluminium
former
50
50 (black)
75
100 mm dia.
Watts (true)
Δ T
°C
Figure 23.12 : Voice coil temperature versus input power (300 Hz).