Audio Engineering

Loudspeakers 695

either inward or outward depending on the direction of the current. The magnitude of this
force is

FBli Newtons),( (23.24)

where B is fl ux density in Webers/m^2 ,/is conductor length in meters, and/is current in amperes.

It should be noted that l refers only to that portion of the coil situated in the magnetic
fi eld. As shown later, to reduce distortion the coil is often longer than the working
magnetic fi eld defi ned by the gap dimensions.

If the coil is free to move, its velocity will be determined by the applied force F and the
mechanical impedanceZm. The mechanical impedance Zm will be a function of total mass
(Lm ) of the system, that is, voice coil and former, diaphragm, air loading, etc., resistance
Rm due to losses in the suspension and radiation, and total stiffness (1/ Cm ) due to the
restoring force.

Using normal circuit theory, the impedance will be

ZRjL

mm mCm

ω .

ω

⎡ 1

⎣

⎢

⎢

⎤

⎦

⎥

⎥

⎡

⎣

⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

(23.25)

Front suspension

Cone

Rear suspension

Cradle

Voice coil

Figure 23.13 : Moving coil loudspeaker cone, suspensions, and voice coil assembly.

S

Coil

S

N

N

Current

i F

Magnet

Figure 23.14 : Moving coil motor element.