Page 235 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 11 Acoustic Devices 197

Schematic Symbol
Open Horn Schematic Symbol
Reverse Horn Outer Pole
Magnet Terminals
Protective Housing
Coil Magnet
Bridge Piece
Diaphragm Inner Pole
Suspension
Pole Piece
Primary Horn Corrugated Foil
Figure 11-13 Folded Horn Loudspeaker

In some applications, the length and size of the horn can be
prohibitive. In these cases, the horn is folded, as shown in
Figure 11-13. The primary horn feeds a reverse horn which,
in turn, feeds the open horn. This arrangement effectively
Figure 11-15 Ribbon Element Loudspeaker
shortens the horn length to one-third of its unfolded length.
These types of speakers are very common in outdoor and
industrial settings.
Another common use for the folded horn is in handheld signal. As the ribbon moves, it pumps the air and creates
public address systems, or megaphones. Figure 11-14 shows sound pulses which mirror the electrical signal.
a typical commercial megaphone. The loudspeaker is Planar loudspeakers are made by stretching a large plastic
mounted on the front of a housing that encloses an amplifier, diaphragm across a frame. Pancake coils are bonded to the
battery, and microphone. A volume control and handle are diaphragm and a series of strip magnets are mounted in close
mounted to the housing. Most units have a trigger switch to proximity to the coils. As a signal is applied to the coils, the
turn the amplifier on and off. diaphragm deflects and produces sound. These types of speak-
ers are reasonably efficient because of their large diaphragm.
Figure 11-16 shows how a planar speaker is constructed.
Amplifier, Battery, and
Folded Horn Driver Housing
Volume and
Power Switch

Microphone
Schematic Symbol

Pancake Coil
Magnet
Microphone Trigger
Handle

External Microphone Connector
External Power Supply Connector Diaphragm
Figure 11-14 Handheld Public Address System or
“Megaphone”

Ribbon element loudspeakers are generally used in high Terminals
performance, high frequency, sound reproduction, such as
home and studio applications. Figure 11-15 shows a stylized
view of a typical ribbon tweeter. A corrugated, metalized rib- Pancake Coil
bon is positioned between the poles of a strong permanent
magnet. A signal is applied across the length of the ribbon and
the foil deflects in reference to the polarity and current of the Figure 11-16 Planar Loudspeaker