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5 Acoustics in Biology and Medicine
5.14.2 Electromechanical Devices
• Electromagnetic Speakers: An electromagnetic speaker produces sound by the
vibration of a stiff cone of thin fibrous material pushing and pulling the air. A
thin axial cylinder is glued to the cone near its vertex. The cone and cylinder
are suspended at the large circumference of the cone by a very flexible material
which allows the cone to move freely (to some limit) in the direction of the axis of
the cone. Around the cylinder is a thin coil of copper wire, attached to the frame
of the speaker by very flexible copper wires. Within the hollow cylinder is a fixed
permanent magnet, with a north-south axis along the cone axis. The magnet is
not centered in the coil, but displaced outward a small distance. When a current
is passed through the coil, the magnetic field that current produces either attracts
or opposes the permanent field, causing the cylinder to push the cone in or out.
The cone acts as an impedance matching device, i.e. the cylinder and cone match
the impedance of the surrounding air far more than the cylinder by itself.
• Electrostatic devices: If two metal plates are put in parallel with a gap between,
they can act as a capacitor. If one plate is made thin and held elastically, it can
be made to vibrate by an alternating current fed into the capacitor. The surface of
the vibrating plate pushes and pulls air, producing sound.
• Magnetostriction devices: Magnetostriction occurs when there is a change in
the length of a ferromagnetic material due to an external magnetic field. An
unmagnetized rod will expansion if put into magnetic field. Thus, an alternating
field will cause such a rod to vibrate at twice the frequency of the field. If the
rod is already magnetized, the alternating field can generate vibrations in the
rod with the same frequency as the field. If the field’s frequency matches the
natural frequency of vibration of the rod, resonance will occur, making the rod’s
vibration a maximum. Longitudinal sound waves are generated in the rod, which
are emitted from the ends of the rod as sound waves in the adjacent material.
Magnetostrictive devices have outputs up to 50 W/cm 2 .
A magnetostrictive generator is usually constructed around a inductorcapacitor circuit stimulated by an electronic oscillator. The coil of the inductor
contains a rod of ferromagnetic material such as pure nickel, or nickel-copper
alloys (invar and monel). The oscillator frequency f R = 1/(2π
√
LC) can
be adjusted by changing the inductance L of the coil, or the capacitance
C. The natural frequencies of a rod vibrating longitudinally are given by
f n = (n/2l)
√
Y/ρ, where Y is the Young’s modulus of the rod, ρ is its density,
l is its length, and n is an integer giving the harmonic index for that frequency.
• Piezoelectric devices: Piezoelectric generation of ultrasound is possible by
placing an alternating electric field in the same direction as the piezoelectric axis
of the crystal. Resonance, and thus the maximum amplitude of vibration, occurs
when the electric field frequency matches the mechanical vibrational frequency
of the crystal along the axis of the field. A low frequency limit of about 10 kHz
comes from the difficulty in exciting thick slabs of crystal. An upper frequency
limit of about 10 MHz comes from having to make crystals too thin to sustain
5 Acoustics in Biology and Medicine
5.14.2 Electromechanical Devices
• Electromagnetic Speakers: An electromagnetic speaker produces sound by the
vibration of a stiff cone of thin fibrous material pushing and pulling the air. A
thin axial cylinder is glued to the cone near its vertex. The cone and cylinder
are suspended at the large circumference of the cone by a very flexible material
which allows the cone to move freely (to some limit) in the direction of the axis of
the cone. Around the cylinder is a thin coil of copper wire, attached to the frame
of the speaker by very flexible copper wires. Within the hollow cylinder is a fixed
permanent magnet, with a north-south axis along the cone axis. The magnet is
not centered in the coil, but displaced outward a small distance. When a current
is passed through the coil, the magnetic field that current produces either attracts
or opposes the permanent field, causing the cylinder to push the cone in or out.
The cone acts as an impedance matching device, i.e. the cylinder and cone match
the impedance of the surrounding air far more than the cylinder by itself.
• Electrostatic devices: If two metal plates are put in parallel with a gap between,
they can act as a capacitor. If one plate is made thin and held elastically, it can
be made to vibrate by an alternating current fed into the capacitor. The surface of
the vibrating plate pushes and pulls air, producing sound.
• Magnetostriction devices: Magnetostriction occurs when there is a change in
the length of a ferromagnetic material due to an external magnetic field. An
unmagnetized rod will expansion if put into magnetic field. Thus, an alternating
field will cause such a rod to vibrate at twice the frequency of the field. If the
rod is already magnetized, the alternating field can generate vibrations in the
rod with the same frequency as the field. If the field’s frequency matches the
natural frequency of vibration of the rod, resonance will occur, making the rod’s
vibration a maximum. Longitudinal sound waves are generated in the rod, which
are emitted from the ends of the rod as sound waves in the adjacent material.
Magnetostrictive devices have outputs up to 50 W/cm 2 .
A magnetostrictive generator is usually constructed around a inductorcapacitor circuit stimulated by an electronic oscillator. The coil of the inductor
contains a rod of ferromagnetic material such as pure nickel, or nickel-copper
alloys (invar and monel). The oscillator frequency f R = 1/(2π
√
LC) can
be adjusted by changing the inductance L of the coil, or the capacitance
C. The natural frequencies of a rod vibrating longitudinally are given by
f n = (n/2l)
√
Y/ρ, where Y is the Young’s modulus of the rod, ρ is its density,
l is its length, and n is an integer giving the harmonic index for that frequency.
• Piezoelectric devices: Piezoelectric generation of ultrasound is possible by
placing an alternating electric field in the same direction as the piezoelectric axis
of the crystal. Resonance, and thus the maximum amplitude of vibration, occurs
when the electric field frequency matches the mechanical vibrational frequency
of the crystal along the axis of the field. A low frequency limit of about 10 kHz
comes from the difficulty in exciting thick slabs of crystal. An upper frequency
limit of about 10 MHz comes from having to make crystals too thin to sustain