Infrasound (Delayed):
These are low-frequency sound waves (below 20 Hz) that are inaudible
to humans. They travel long distances through the atmosphere and are
often used by scientists to calculate the energy of a bolide
explosion.
Electrophonic Meteor
Sound (Instantaneous): This is a "simultaneous" sound
(hissing or popping) heard at the exact moment the meteor is seen. It
isn't a true sound wave traveling through air; instead, it's caused
by Very Low Frequency (VLF) radio waves generated by the meteor’s
plasma trail that instantly vibrate local objects (like glasses or
hair) near the observer.
Delayed Sound (Delayed):
This is the conventional "sonic boom" or rumbling heard
several minutes after the visual sighting. Because sound travels much
slower than light (roughly 340 m/s), there is a significant lag
between seeing the flash and the physical shockwave reaching your
ears.
When a meteoroid enters
the atmosphere, it isn't just a rock falling; it is a kinetic energy
bomb.
The Physics of the
Airburst. An airburst occurs when the hydrodynamic pressure (the
force of the air pushing against the front of the meteor) exceeds the
structural integrity of the object.
Pancake Effect: As the
meteor fragments, its surface area increases exponentially. This
causes it to dump all its remaining kinetic energy into the
atmosphere almost instantly.
Altitude: Most
significant airbursts occur between 20 km and 50 km (Chelyabinsk was
at roughly 30 km). If the object is stronger (iron-rich) or larger,
it penetrates deeper (Tunguska was at 5–10 km), which drastically
increases ground damage.
Blast Force: The energy is measured in TNT equivalents.
Chelyabinsk (2013): ~500 kilotons (30x Hiroshima).
Tunguska (1908): 10–15 megatons (1,000x Hiroshima).
The Sound: Delayed vs.
Concurrent
This is where the physics
gets "spooky." Most people expect sound to follow the
"lightning and thunder" rule, but meteors offer two
distinct auditory experiences:
Delayed Sound (The Sonic
Boom). This is the standard shockwave. Since the meteor travels at
hypersonic speeds (up to 72km/s), it leaves a cone of pressurized air
behind it. Because sound travels at roughly 343m/s, witnesses often
see the flash and wait 2 to 3 minutes before the windows shatter from
the blast.
Concurrent Sound
(Electrophonic Meteors). For centuries, people reported hearing
"hissing," "sizzling," or "popping" at
the exact same moment they saw the flash. Since the meteor is 30 km
away, physical sound shouldn't reach them for 90 seconds.
There are two primary
scientific explanations for this:
1. Photoacoustic Coupling: The meteor’s light pulses so intensely
that it rapidly heats local objects near the listener (like hair,
leaves, or dark clothing). These objects then vibrate and create
"local" sound waves.
2. VLF Radio Waves: The plasma trail of the meteor generates Very Low
Frequency (VLF) electromagnetic radiation. This radiation travels at
the speed of light and can be "transduced" into sound by
nearby metallic objects (like a wire fence or even glasses) acting as
a natural antenna.
3. Comparison of Energy
Deposition
Feature; Chelyabinsk
(2013); Tunguska (1908)
Object Diameter: ~18–20
meters; ~50–80 meters
Burst Altitude: ~30
km; ~5–10 km
Energy Release: 500
Kilotons;10–15 Megatons
Primary Damage: Broken
glass/Infrasound; 2,000 km^2 of levelled forest.
The difference between
Infrasound and Electrophonic Sound.
Infrasound is a physical
"push" of air that arrives late, while Electrophonic sound
is an instant "radio signal" that your brain translates
into noise.
1. Infrasound: The Low-Frequency Hammer. Infrasound refers to sound
waves with a frequency below 20 Hz, which is the lower limit of human
hearing. The Mechanism: When a meteor fragments or creates a
shockwave, it displaces a massive amount of air. This creates a
low-frequency pressure wave. The "Long-Distance Traveler":
Because these waves have very long wavelengths, they aren't easily
absorbed by the atmosphere. They can travel thousands of kilometers.
Detection: While we generally can't "hear" them, we can
sometimes feel them as a strange pressure in the chest or ears.
Scientists use specialized "microbarometers"
(high-precision pressure sensors) to track them.Connection to Whales:
As we've discussed before, large cetaceans like Blue whales use
infrasound to communicate across entire ocean basins. A meteor
airburst essentially "screams" in the same frequency range
that whales use for long-distance calls.
2. Electrophonic Sound: The Instant Sizzle. As we touched on, these
are heard at the exact same moment the meteor is seen, defying the
speed of sound. The Mechanism: It is not a pressure wave traveling
through the air. Instead, the meteor’s plasma trail creates VLF
(Very Low Frequency) radio waves or intense light pulses. The
"Translation": These electromagnetic waves travel at the
speed of light. When they reach the ground, they interact with nearby
objects (like your hair, a fence, or even dry pine needles), causing
them to vibrate slightly or create "photoacoustic" effects.
The Experience: You hear a sharp pop, hiss, or crackle.
Key Differences at a
Glance
|
Feature
|
Infrasound
|
Electrophonic Sound
|
|
Speed
|
Speed of Sound (~343 m/s)
|
Speed of Light (~300,000 km/s)
|
|
Timing
|
Delayed (arrives minutes later)
|
Concurrent (heard instantly)
|
|
Audibility
|
Usually felt, not heard (below 20 Hz)
|
Clearly audible (hissing/popping)
|
|
Travel Distance
|
Global (can circle the Earth)
|
Local (only near the observer)
|
|
Medium
|
Air pressure waves
|
Electromagnetic/Light energy
|
Recent Study (2023): A
major analysis found that out of roughly 1,000 fireballs in the NASA
database, only about 65 distinct events produced a clear enough
infrasound signature to be pinpointed by the CTBTO arrays. This is
usually because the entry angle must be steep enough to "couple"
the energy into the lower atmosphere.
Measuring the "decibel"
level of a meteor at sea level is tricky because a meteor airburst
isn't just a loud noise—it is a supersonic shockwave.
At the point of the
airburst (high in the atmosphere), the sound is so intense that it
exceeds the physical limit of what "sound" can be.
1. The "Sound
Barrier" (194dB
In our atmosphere at sea level, the loudest possible "undistorted"
sound is approximately194dB
Why? At 194dB, the
"low pressure" part of the sound wave becomes a perfect
vacuum. If you try to go louder, the air can't physically move any
further back, and the sound wave turns into a shockwave (a wall of
moving air).
The Meteor: A major airburst like Tunguska or Chelyabinsk is
estimated to reach 300dB or more at the source. This is not "sound"
you would hear; it is energy that would vaporize or liquefy any
biological tissue instantly.
2. Estimated Levels at
Sea Level (Ground Level)
When the blast from an
airburst at 20–30 km altitude finally reaches the ground, the
decibel level depends on your distance from "Ground Zero."
|
Distance from Blast
|
Estimated Decibels (dB)
|
Physical Effect
|
|
Directly Underneath
|
170\180+dB
|
Immediate eardrum rupture, structural damage,
permanent hearing loss.
|
|
50km away
|
140\150dB
|
Pain threshold; similar to standing next to a
jet engine; windows shatter.
|
|
100km away
|
120\130dB
|
Deafening thunder; car alarms triggered;
potential minor ear damage.
|
3. The "Infrasound"
Component
While the audible "boom"
might be 130 dB, the infrasound (the part whales might sense) can
remain at high "perceived" energy levels for much longer.
Chelyabinsk (2013):
Even hundreds of kilometers away, the infrasound pressure was strong
enough to be detected by sensors as a "spike" that would
equate to roughly 90 dB if it were in the audible range.
To a human, this
feels like a sudden, phantom change in barometric pressure—your
ears "pop" or you feel a wave of nausea, even if you don't
"hear" a loud bang yet.
4. Comparison to Whale
Sonar
To give you a perspective
from previous conversations:
Sperm Whale Click: ~
230dB (underwater).
Meteor Airburst (at
source): ~ 300dB (in air).
Note: 300 dB in air
is vastly more powerful than 230 dB in water due to how the scales
are calculated and the density of the medium.