# Engineering Acoustics/Moving Coil Loudspeaker

# Moving Coil Transducer

The purpose of the acoustic transducer is to convert electrical energy
into acoustic energy. Many variations of acoustic transducers exist,
although the most common is the moving coil-permanent magnet transducer.
The classic loudspeaker is of the moving coil-permanent magnet type.

The classic electrodynamic loudspeaker driver can be divided into three
key components:

1\) The Magnet Motor Drive System

2\) The Loudspeaker Cone System

3\) The Loudspeaker Suspension

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![](loud_speaker.gif "loud_speaker.gif")

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Figure 1 Cut-away of a moving coil-permanent magnet loudspeaker

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## The Magnet Motor Drive System

The main purpose of the Magnet Motor Drive System is to establish a
symmetrical magnetic field in which the voice coil will operate. The
Magnet Motor Drive System is comprised of a front focusing plate,
permanent magnet, back plate, and a pole piece. In figure 2, the
assembled drive system is illustrated. In most cases, the back plate and
the pole piece are built into one piece called the yoke. The yoke and
the front focusing plate are normally made of a very soft cast iron.
Iron is a material that is used in conjunction with magnetic structures
because the iron is easily saturated when exposed to a magnetic field.
Notice in figure 2, that an air gap was intentionally left between the
front focusing plate and the yoke. The magnetic field is coupled through
the air gap. The magnetic field strength (B) of the air gap is typically
optimized for uniformity across the gap. \[1\]

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Figure 2 Permanent Magnet Structure

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When a coil of wire with a current flowing is place inside the permanent
magnetic field, a force is produced. B is the magnetic field strength, l
is the length of the coil, and I is the current flowing through the
coil.

$F = Bli$

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![](Magnet2.gif "Magnet2.gif")

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Figure 3 Voice Coil Mounted in Permanent Magnetic Structure

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The coil is excited with the AC signal that is intended for sound
reproduction, when the changing magnetic field of the coil interacts
with the permanent magnetic field then the coil moves back and forth in
order to reproduce the input signal. The coil of a loudspeaker is known
as the voice coil.

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Figure 4 Photograph - Voice Coil

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## The Loudspeaker Cone System

On a typical loudspeaker, the cone serves the purpose of creating a
larger radiating area allowing more air to be moved when excited by the
voice coil. The cone serves a piston that is excited by the voice coil.
The cone then displaces air creating a sound wave. In an ideal
environment, the cone should be infinitely rigid and have zero mass, but
in reality neither is true. Cone materials vary from carbon fiber,
paper, bamboo, and just about any other material that can be shaped into
a stiff conical shape. The loudspeaker cone is a very critical part of
the loudspeaker. Since the cone is not infinitely rigid, it tends to
have different types of resonance modes form at different frequencies,
which in turn alters and colors the reproduction of the sound waves. The
shape of the cone directly influences the directivity and frequency
response of the loudspeaker. When the cone is attached to the voice
coil, a large gap above the voice coil is left exposed. This could be a
problem if foreign particles make their way into the air gap of the
voice coil and the permanent magnet structure. The solution to this
problem is to place what is known as a dust cap on the cone to cover the
air gap. Below a figure of the cone and dust cap are shown.

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![](loud_cone.gif "loud_cone.gif")

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Figure 6 Cone and Dust Cap attached to Voice Coil

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## The Loudspeaker Suspension

Most moving coil loudspeakers have a two piece suspension system, also
known as a flexure system. The combination of the two flexures allows
the voice coil to maintain linear travel as the voice coil is energized
and provide a restoring force for the voice coil system. The two piece
system consists of large flexible membrane surrounding the outside edge
of the cone, called the surround, and an additional flexure connected
directly to the voice coil, called the spider. The surround has another
purpose and that is to seal the loudspeaker when mounted in an
enclosure. Commonly, the surround is made of a variety of different
materials, such as, folded paper, cloth, rubber, and foam. Construction
of the spider consists of different woven cloth or synthetic materials
that are compressed to form a flexible membrane. The following two
figures illustrate where the suspension components are physically at on
the loudspeaker and how they function as the loudspeaker operates.

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![](loud_suspension.gif "loud_suspension.gif")

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Figure 7 Loudspeaker Suspension System

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![](loudspk.gif "loudspk.gif")

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Figure 8 Moving Loudspeaker

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## Modeling the Loudspeaker as a Lumped System

Before implementing a loudspeaker into a specific application, a series
of parameters characterizing the loudspeaker must be extracted. The
equivalent circuit of the loudspeaker is key when developing enclosures.
The circuit models all aspects of the loudspeaker through an equivalent
electrical, mechanical, and acoustical circuit. Figure 9 shows how the
three equivalent circuits are connected. The electrical circuit is
comprised of the DC resistance of the voice coil, Re, the imaginary part
of the voice coil inductance, Le, and the real part of the voice coil
inductance, Revc. The mechanical system has electrical components that
model different physical parameters of the loudspeaker. In the
mechanical circuit, Mm, is the electrical capacitance due to the moving
mass, Cm, is the electrical inductance due to the compliance of the
moving mass, and Rm, is the electrical resistance due to the suspension
system. In the acoustical equivalent circuit, Ma models the air mass and
Ra models the radiation impedance\[2\]. This equivalent circuit allows
insight into what parameters change the characteristics of the
loudspeaker. Figure 10 shows the electrical input impedance as a
function of frequency developed using the equivalent circuit of the
loudspeaker.

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![](Eq_circuit.gif "Eq_circuit.gif")

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Figure 9 Loudspeaker Analogous Circuit

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![](Freq_resp.gif "Freq_resp.gif")

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Figure 10 Electrical Input Impedance

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## References

\[1\] The Loudspeaker Design Cookbook 5th Edition; Dickason, Vance.,
Audio Amateur Press, 1997. \[2\] Beranek, L. L. Acoustics. 2nd ed.
Acoustical Society of America, Woodbridge, NY. 1993.
