5.16 Physical Sound Wave Detection
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be attenuated, an independent set of microphones would be needed near the signal
sources, spatially separated from the sources of noise. Headphones can pass a signal
source to the ears from interior speakers, while an exterior set of microphones on
the headphones detects the ambient noise to be used to generate the noise canceling
waves generated inside the headphones. Buildings in earthquake zones which have
active movable counterweights can be used to mollify passing earthquake waves.
Multiple pickup microphones (with at least four pickups on the vertices of a
tetrahedral) can be used to electronically suppress the signals of one or more sources
which are not at the same location. Such an arrangement can also be employed
to determine the sound source locations from just the audio signals of the four
microphones.
5.16.2 Radiation Pressure Devices
It is possible to directly detect sound pressure by recording that pressure on an
elastic surface. If the surface responds to the pressure, then the displacement of the
surface will be proportional to the sound pressure of an impinging wave.
The levitation of a small ball of fluid in a vertical standing wave of sound
demonstrates that the sound pressure may be sufficient to overcome gravity.
Such levitation devices are useful for the manufacture of small semiconductors
levitated while cooling. The levitated objects must be significantly smaller than
the wavelength of the sound. Ultrasonics is typically used, with sound levels above
150 dB.
The force on a small particle, such as a biological cell, smaller than the
wavelength of the sound, is proportional to the negative gradient of the difference
between sound energy in particle and the sound energy which would be in the same
fluid volume without the particle. In turn, the sound intensity is proportional to the
square of the sound pressure.
Acoustical radiation pressure has been used to move individual biological cells
to desired positions, serving as acoustical tweezers.
5.16.3 Heat Detectors
Most commonly, sound energy is dissipated into heat (quasi-random motion in
atoms and molecules). A ‘bolometer’ (or calorimeter) is a device which measures
the heat produced by particle beams or radiation. To measure the heat production,
a very sensitive thermometer can be employed, such as those constructed from
a semiconductor. The detector material is weakly coupled to a heat reservoir at
ambient temperature or colder. The absorbed radiation raises the temperature of
the semiconductor, which changes the semiconductor conduction characteristics.
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be attenuated, an independent set of microphones would be needed near the signal
sources, spatially separated from the sources of noise. Headphones can pass a signal
source to the ears from interior speakers, while an exterior set of microphones on
the headphones detects the ambient noise to be used to generate the noise canceling
waves generated inside the headphones. Buildings in earthquake zones which have
active movable counterweights can be used to mollify passing earthquake waves.
Multiple pickup microphones (with at least four pickups on the vertices of a
tetrahedral) can be used to electronically suppress the signals of one or more sources
which are not at the same location. Such an arrangement can also be employed
to determine the sound source locations from just the audio signals of the four
microphones.
5.16.2 Radiation Pressure Devices
It is possible to directly detect sound pressure by recording that pressure on an
elastic surface. If the surface responds to the pressure, then the displacement of the
surface will be proportional to the sound pressure of an impinging wave.
The levitation of a small ball of fluid in a vertical standing wave of sound
demonstrates that the sound pressure may be sufficient to overcome gravity.
Such levitation devices are useful for the manufacture of small semiconductors
levitated while cooling. The levitated objects must be significantly smaller than
the wavelength of the sound. Ultrasonics is typically used, with sound levels above
150 dB.
The force on a small particle, such as a biological cell, smaller than the
wavelength of the sound, is proportional to the negative gradient of the difference
between sound energy in particle and the sound energy which would be in the same
fluid volume without the particle. In turn, the sound intensity is proportional to the
square of the sound pressure.
Acoustical radiation pressure has been used to move individual biological cells
to desired positions, serving as acoustical tweezers.
5.16.3 Heat Detectors
Most commonly, sound energy is dissipated into heat (quasi-random motion in
atoms and molecules). A ‘bolometer’ (or calorimeter) is a device which measures
the heat produced by particle beams or radiation. To measure the heat production,
a very sensitive thermometer can be employed, such as those constructed from
a semiconductor. The detector material is weakly coupled to a heat reservoir at
ambient temperature or colder. The absorbed radiation raises the temperature of
the semiconductor, which changes the semiconductor conduction characteristics.