The cone of a loudspeaker is not perfectly
rigid. During the reproduction of
high frequencies, the outer edges of the cone are unable to follow the extremely
rapid motions at the center where the voice coil is attached.
The cone flexes, and standing waves appear in the cone that generate
modulation distortion. Unfortunately,
the human ear is particularly sensitive to this form of sound distortion and
listening to the loudspeaker becomes intolerable. To address this problem, a coil of heavy gauge
wire called an inductor
(Fig. 8) is connected between the amplifier and one of the voice coil
terminals (usually the positive terminal).
The inductor offers high resistance only to the passage of high frequency
signals from the amplifier. An inductor
is usually used with a woofer, a
loudspeaker that efficiently produces low frequency tones with a heavy cone,
a high-power voice coil and a large magnet.
Inductor value or inductance is expressed in units called
microhenries (mH) with the Greek letter m and uppercase
letter H. Higher inductance values
block a wider range of frequencies from amplifier signals. Woofer cones have
poor high and midrange frequency performance, and usually require large inductors
with values in millihenries (mH). Typical values range from 300mH for a 3-inch speaker to 2.2mH for an 8-inch woofer. (Note: 1000mH = 1mH).
The thin voice coil wire and bobbin of a tweeter are easily damaged by heat. To protect the voice coil from the high power levels in low frequency signals, an electronic filter called a capacitor (Fig. 9) is connected between the amplifier and one of the voice coil terminals (usually the negative terminal). A capacitor operates in a reverse fashion of an inductor, and offers resistance only to the passage of low frequency signals from the amplifier. Capacitor value or capacitance is expressed in units called microfarads with the Greek letter m and uppercase letter F. Typical values range between 2.2mF and 10mF. Fig. 9 illustrates a commonly used 4.7mF 16V (4.7 microfarad, 16 volt) capacitor.
Fig. 10 is a schematic diagram of an electrical
circuit or crossover network for a loudspeaker. The crossover network
divides the amplifier signal into high and low frequencies for the tweeter
and woofer. The values of the components set the dividing or crossover
frequency. Many circuits have
crossover frequencies between 3,000 to 4,000 Hz (3 - 4 kHz).
The crossover network in Fig. 10 is suitable for a loudspeaker
with a 4 to 6.5 inch (100mm to 165mm) woofer.
Circuit components include:
