5.14 Non-biological Sound Wave Production
141
The ‘starred’ sound levels were not measured, but rather calculated from power.
The maximum sine wave sound level at 191 dB occurs when a pressure variation
matches the atmospheric pressure, so that the lowest pressure in such a wave is
zero. Higher top pressures have finite zero-pressure regions with little air, caused by
layers of air slamming together and then scattering apart. The maximum amplitude
condition for a longitudinal wave gave an upper limit of 204 dB. (See Sect. 5.6.)
Both of these limits are indicative only, since the waves become non-linear. (If the
sound produces a ten per cent variation in the atmospheric pressure, it will be at a
sound level of 171 dB.)
Much higher pressure variations can be sustained in water than in air. Whales can
produce sound levels up to 188 dB.
Life on Earth suffers the vicissitudes of astronomical and geological turmoil.
Earthquakes are an example which produce high-energy sound waves within the
Earth that can destroy lives and environments.
Earthquakes are generated by the movement and slippage of large masses
of material under our feet. Tectonic plate movement can build energy through
increasing stress of solids in the Earth. When the stressed material finally breaks, the
slippage is usually rather jerky, not smooth. There results large and energetic sound
waves propagating outward from the slipping region. The sound reflects, refracts and
gets partially absorbed through various solids and liquids in the Earth. The sound
wave has both longitudinal and transverse components, each moving at different
speeds, and when they reach the Earth’s surface, surface waves propagate at still
another speed.
The energy released by an earthquake can be rather well determined by soundwave-detecting devices called seismometers. Because this energy measured in
joules extends over many decades in powers of ten, a logarithmic scale is used
to characterize the magnitude of the quake. Such a scale was invented in 1935
by Charles Richter. 22 Barely-detected quakes have a Richter magnitude of about
1.0, and quakes which produce total destruction of even sturdy buildings having a
magnitude above 9.0.
To calculate the energy released when the Richter magnitude has the value M ,
use
E = 10
1.448M +4.7993 J .
(5.47)
A magnitude 9 earthquake releases 6.78 × 10 17 J. If the quake lasts 10 s, the power
generated would be about 6.78 × 10 16 W, which is comparable to the sound power
that was released to the air by the Krakatoa 1883 volcano eruption.
22 C.F. Richter, An instrumental earthquake magnitude scale, Bulletin of the Seismological Society
of America 25:1–2, 1–32 (1935).
141
The ‘starred’ sound levels were not measured, but rather calculated from power.
The maximum sine wave sound level at 191 dB occurs when a pressure variation
matches the atmospheric pressure, so that the lowest pressure in such a wave is
zero. Higher top pressures have finite zero-pressure regions with little air, caused by
layers of air slamming together and then scattering apart. The maximum amplitude
condition for a longitudinal wave gave an upper limit of 204 dB. (See Sect. 5.6.)
Both of these limits are indicative only, since the waves become non-linear. (If the
sound produces a ten per cent variation in the atmospheric pressure, it will be at a
sound level of 171 dB.)
Much higher pressure variations can be sustained in water than in air. Whales can
produce sound levels up to 188 dB.
Life on Earth suffers the vicissitudes of astronomical and geological turmoil.
Earthquakes are an example which produce high-energy sound waves within the
Earth that can destroy lives and environments.
Earthquakes are generated by the movement and slippage of large masses
of material under our feet. Tectonic plate movement can build energy through
increasing stress of solids in the Earth. When the stressed material finally breaks, the
slippage is usually rather jerky, not smooth. There results large and energetic sound
waves propagating outward from the slipping region. The sound reflects, refracts and
gets partially absorbed through various solids and liquids in the Earth. The sound
wave has both longitudinal and transverse components, each moving at different
speeds, and when they reach the Earth’s surface, surface waves propagate at still
another speed.
The energy released by an earthquake can be rather well determined by soundwave-detecting devices called seismometers. Because this energy measured in
joules extends over many decades in powers of ten, a logarithmic scale is used
to characterize the magnitude of the quake. Such a scale was invented in 1935
by Charles Richter. 22 Barely-detected quakes have a Richter magnitude of about
1.0, and quakes which produce total destruction of even sturdy buildings having a
magnitude above 9.0.
To calculate the energy released when the Richter magnitude has the value M ,
use
E = 10
1.448M +4.7993 J .
(5.47)
A magnitude 9 earthquake releases 6.78 × 10 17 J. If the quake lasts 10 s, the power
generated would be about 6.78 × 10 16 W, which is comparable to the sound power
that was released to the air by the Krakatoa 1883 volcano eruption.
22 C.F. Richter, An instrumental earthquake magnitude scale, Bulletin of the Seismological Society
of America 25:1–2, 1–32 (1935).