Passage 7: Sound
Sound is critically important in marine ecosystems as a method of communication
for animals, submarines, and other technological instruments. Researchers
conducted an experiment to study sound waves in various aquatic contexts, with a
special focus on the speed of sound across varying media.
A speaker emitting high-frequency sound was utilized to generate sound waves at
a fixed frequency. Hydrophones, strategically placed at varying distances from the
radio, measured sound intensity and speed in freshwater, salt water, and air.
The first phase of experimental data collection involved measuring the speed of
sound in these three media at the same temperature. Results showed that there
was a significant difference between all three, with the most significant difference
between either water medium and air. Next, researchers varied the temperature
and salinity of the water in the tanks, measuring the speed of sound at regular
increments.
They also used ultrasound waves to map the seafloor, detecting obstacles. Findings
are tabulated in Table 1:
Attenuation in signal intensity was attributed to absorption and scattering of waves
in the water (as signal intensity decreased with increasing depth). Background noise
in the water column affected the precision of the measurements; regions with high
biological activity reduced measurement accuracy.
In an attempt to map the ocean floor with ultrasound waves, the researchers
generated sound waves with a 5 MHz frequency. They noticed that the
presence of thermoclines (layers of water with higher temperatures) influenced
how the waves propagated. How did the presence of thermoclines affect the
speed and wavelength of the ultrasound waves, and how did this affect the
accuracy of seafloor accuracy?
A) The speed and wavelength decreased, resulting in overestimated distances
in mapping the seafloor
B) The speed remained constant, but the wavelength increased, causing
distortion in imaging
C) The speed and wavelength both increased, but imaging was unaffected
D) The speed of ultrasound waves increased, and the wavelength increases,
resulting in underestimated distances in mapping the seafloor
Correct answer: D. The speed of sound generally increases in a
medium due to increased kinetic energy. If ultrasound waves go from a medium
of low temperature to high temperature (into the thermocline) the speed of the
wave increases. According to the wave equation v = fλ, increase in speed while
maintaining constant frequency (an important assumption) necessarily results in an
increase in wavelength.
Ultrasound imaging uses the difference in time between when an ultrasound wave
is sent versus when it is returned to the receiver. This difference in time allows
imaging of the contours of a surface (e.g. the seafloor). Assuming that the speed
increases, the waves would return to the receiver faster, so less distance in the
surface’s contour would be detected.
.svg)
