Image Processing Reference
In-Depth Information
Figure5.9 ROMANIS acoustic imaging sensor. (CourtesyofMandarChitreand
it does not require that the target be illuminated with artificially produced sound
waves, and it can produce images at much higher frame rates, making it capable of
imaging moving objects.
We have seen examples of active acoustic imaging (high resolution sonar), and
acoustic imaging that is neither active nor passive (acoustic noise imaging). Now
we will examine an example of passive acoustic imaging. Figure 5.11 is a series of
acoustic “pictures” of a TGV (Train à Grande Vitesse, or high-speed train) trainset
imaged with SYNTACAN, an acoustic microphone array that can produce visual
images of moving objects that emit sound. The physical scale of the images is
6 m by 300 m, which is slightly larger than the trainset. The microphone array
data build up a picture line by line, like an inkjet printer, using the motion of the
object (in this case, the train) as the scanner. The TGV is especially loud because
of its high speed, and these data help engineers to understand the various sources
of noise in the TGV trains and to study methods of noise reduction. The sound
waves that produced these images range in wavelength from 0.75 m for the top
image to 10 m in the bottom. Note that the resolution of the image decreases as the
wavelength increases. The pseudocolor indicates the sound intensity, where dark
red indicates the highest intensity and blue the lowest. At the higher frequencies,
we see the finest spatial resolution in the images. This is a recurring characteristic in
imaging that is a manifestation ofdiffraction: for a fixed detector array geometry,
the shorter the wavelength, the finer the angular resolution of the resulting images
tends to be. Note that the most intense noise sources in the image are the power
units at each end of the train. The pantographs (electrical wipers that engage the
overhead power lines) at each end are sources of aerodynamically induced noise,
and the “bogies” (wheelsets) with 17 m spacing are sources of high-pitched rolling
wheel noise.
Figure 5.12 shows the SYNTACAN microphone array suspended from a
manlifter. Instrumentation closer to the tracks measures the speed of the train so
that the noise data from the microphones can be interpreted into an acoustic image.
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