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5 Acoustics in Biology and Medicine
with very small sizes, high sensitivity, omnidirectionality, and very little external
power, they have come to be used in hearing aids and body-sound detectors.
Just as for radiators of sound, if the detection of a wide range of sound
frequencies is desired, there should be no mechanical resonances in the frequency
range of the detector. Also, the detector material, such as the diaphragm, must be
in reasonable impedance-matching to the air (or water if microphone is embedded
in tissue). These considerations limit the frequency range of the sound a given
microphone is designed to detect, as well as the sensitivity of the microphone.
Piezoelectric crystals are good radiators and good detectors of single ultrasonic
frequencies. A crystal, such as quartz, is first ground into a rectangular shape and
to have a natural mechanical vibrational frequency at the desired frequency to be
detected. Electric plates are then put on opposite sides of the crystal. A passing
sound wave will squeeze and expand the crystal, causing an electric field to be
generated in proportion. This field, in turn, causes charges to be pushed from or
pulled onto the plates, making a small current. These devices are used in ultrasonic
probes, both for generation and for detection.
Magnetostrictive devices use the change in shape of a magnetic material, such as
cobalt, in response to a change in the external magnetic field to produce a sound.
Such devices are capable of producing intense ultrasonics waves. They are used in
underwater sonar and in surgical tools which employ high-intensity ultrasound for
cutting tissue and stopping bleeding.
Sometimes the presence of sounds, including the human voice, can change
the operation of electronic instruments. This occurs when variations in the sound
pressure changes the distance between conductors in the circuit, and varying the
capacitance between some of those conductors changes the operation of the circuit.
For example, the frequency of an internal oscillator may be fixed by an ‘LC’ circuit,
for which f ≈ 1/(2π
√
LC), with L an inductance and C a capacitance. This
undesirable phenomenon is call ‘microphonics’.
Microphones with true noise cancellation require an auxiliary microphone
immersed in the noisy region. The signal from the noisy pickup is compared to
that of the primary microphone to isolate the sound which does not contain the
noise. If one knows the frequency distribution of the noise, then frequency filtering
(either analog or digital) can be employed to reduce the noise without using a second
microphone. Of course, the source signal of the noise might also be reduced or
eliminated. Wind shields on microphones keep moving air from whistling through
microphone guards. (‘Hums’ are usually produced by poor grounding of amplifiers
which use 60cps AC power sources.) Screeching comes from positive feedback
between sound-producing speakers and nearby microphones.
Since sound is a material wave, destructive interference of that wave is possible,
not just at one nodal point, but over a region. A matrix of active devices can be
constructed which detects a wave over a surface and then generates a wave at each
detected frequency and amplitude that is 180 ◦ out of phase with that incoming wave,
effectively canceling most Fourier components. The matrix of detectors would have
to be denser on the active surface than the shortest wavelength squared. This is
the idea behind noise cancellation devices. To allow a signal to pass and noise to
5 Acoustics in Biology and Medicine
with very small sizes, high sensitivity, omnidirectionality, and very little external
power, they have come to be used in hearing aids and body-sound detectors.
Just as for radiators of sound, if the detection of a wide range of sound
frequencies is desired, there should be no mechanical resonances in the frequency
range of the detector. Also, the detector material, such as the diaphragm, must be
in reasonable impedance-matching to the air (or water if microphone is embedded
in tissue). These considerations limit the frequency range of the sound a given
microphone is designed to detect, as well as the sensitivity of the microphone.
Piezoelectric crystals are good radiators and good detectors of single ultrasonic
frequencies. A crystal, such as quartz, is first ground into a rectangular shape and
to have a natural mechanical vibrational frequency at the desired frequency to be
detected. Electric plates are then put on opposite sides of the crystal. A passing
sound wave will squeeze and expand the crystal, causing an electric field to be
generated in proportion. This field, in turn, causes charges to be pushed from or
pulled onto the plates, making a small current. These devices are used in ultrasonic
probes, both for generation and for detection.
Magnetostrictive devices use the change in shape of a magnetic material, such as
cobalt, in response to a change in the external magnetic field to produce a sound.
Such devices are capable of producing intense ultrasonics waves. They are used in
underwater sonar and in surgical tools which employ high-intensity ultrasound for
cutting tissue and stopping bleeding.
Sometimes the presence of sounds, including the human voice, can change
the operation of electronic instruments. This occurs when variations in the sound
pressure changes the distance between conductors in the circuit, and varying the
capacitance between some of those conductors changes the operation of the circuit.
For example, the frequency of an internal oscillator may be fixed by an ‘LC’ circuit,
for which f ≈ 1/(2π
√
LC), with L an inductance and C a capacitance. This
undesirable phenomenon is call ‘microphonics’.
Microphones with true noise cancellation require an auxiliary microphone
immersed in the noisy region. The signal from the noisy pickup is compared to
that of the primary microphone to isolate the sound which does not contain the
noise. If one knows the frequency distribution of the noise, then frequency filtering
(either analog or digital) can be employed to reduce the noise without using a second
microphone. Of course, the source signal of the noise might also be reduced or
eliminated. Wind shields on microphones keep moving air from whistling through
microphone guards. (‘Hums’ are usually produced by poor grounding of amplifiers
which use 60cps AC power sources.) Screeching comes from positive feedback
between sound-producing speakers and nearby microphones.
Since sound is a material wave, destructive interference of that wave is possible,
not just at one nodal point, but over a region. A matrix of active devices can be
constructed which detects a wave over a surface and then generates a wave at each
detected frequency and amplitude that is 180 ◦ out of phase with that incoming wave,
effectively canceling most Fourier components. The matrix of detectors would have
to be denser on the active surface than the shortest wavelength squared. This is
the idea behind noise cancellation devices. To allow a signal to pass and noise to