音の性質と伝播メカニズム
Hello everyone, SCIENSPOT is a podcast that shines a spotlight on the latest scientific technology from Japan.
Your host is REN from SCIEN-TALK.
Today I'm gonna talk about sound of the music festival.
Did you know that the phenomenon was reported regarding Mrs. GREEN APPLE's concert held at Yokohama recently?
Mrs. GREEN APPLE is a very famous Japanese artist, and the deep bass sounds from the concert were heard all the way in Kawasaki, miles away.
This is quite surprising, and normally as sound travels away from its source, it rapidly diminishes and becomes almost inaudible at a distance.
This phenomenon, which defines the common sense, raises an interesting scientific question.
What mechanism allowed this bass sound to travel so far?
Let's start with the basic properties of sound.
Sound is defined as a longitudinal wave or the vibration of a pressure that travels through a medium like air.
When a speaker vibrates, it creates vibrations in the surrounding air, and these vibrations propagate through molecular collisions to adjacent molecules, moving through space as a sound wave.
You can imagine it like a chain reaction where energy is transformed from one molecule to the next.
There's a fundamental principle governing sound propagation called the inverse square law.
This law states that the intensity of sound decreases in proportion to the square of the distance from the source.
Specifically, if the distance doubles, the sound intensity and the sound pressure level decrease by 6 decibels.
This is because the acoustic energy emitted from a point source spreads out over an increasingly large spherical area as the distance increases, thinning the energy per unit area.
コンサートの音の減衰
However, the case of Mrs. Green Apple's concert shows complex atmospheric sound behavior that cannot be explained by this inverse square law alone, suggesting the need for more detailed scientific exploration.
So why was it only the deep bass that traveled so far?
The degree of sound attenuation or loss of acoustic energy largely depends on its frequency.
Sound attenuation is primarily caused by factors like absorption, where sound energy is converted into heat due to internal friction in the medium, and scattering, where sound waves are dispersed in all directions by tiny irregularities in the air.
These absorption and scattering effects are much greater for high-frequency sounds or high-pitched sounds.
Imagine high-pitched sounds as small balls trying to navigate a crowded path.
These short wavelengths mean frequent interactions with air molecules and tiny obstacles, causing them to lose energy quickly.
In contrast, low-frequency sounds like deep bass have very long wavelengths, resulting in less energy exchange with the medium, making them less prone to attenuation.
They are like large waves that can bypass minor obstacles relatively easily.
This is why even though high-frequency sounds were not audible, only the deep bass could propagate across such vast distances.
Even lower frequencies are called infrasound and they possess the ability to travel extremely long distances, even circling the Earth multiple times.
The key to understand why Mrs. Green Apple's concert bass reached Kawasaki lies in the reflection of sound waves, caused by atmospheric sound speed gradients.
音の伝播と風の影響
Specifically, vertical changes in wind and temperature are major factors that dramatically alter the path of sound propagations.
Imagine our atmosphere as having many layers, one on top of the other, and the weather, speed, and temperature are slightly different in each of these layers.
The sound speed is relative to the speed of the medium itself, so wind either adds to or subtracts from the atmospheric sound speed.
Crucially, wind speed is not uniform from the ground upwards due to friction with the ground. Wind speed typically increases with altitude.
This vertical wind speed gradient caused sound waves to bend, and in Mrs. Green Apple's case, there was a tailwind blowing from the concert venue towards Kawasaki.
In a downwind situation, the upper part of the sound wave encounters faster winds, increasing its relative speed.
This causes the sound wave to reflect downwards, tending to return to the ground.
It's as if an invisible highway for sound was created in the upper atmosphere, allowing sounds that would normally attenuate to travel much further.
Conversely, in a headwind, sound waves reflect upwards, creating an acoustic shadow zone, where sound is significantly attenuated or hardly heard on the ground.
Similar to wind, vertical changes in temperature also affect sound wave reflection.
Typically, on a clear afternoon, for example, the ground is warmed by the sun, leading to a negative temperature gradient, where the air temperature decreases with increasing altitude.
In this scenario, sound travels slower in the colder air afloat, causing sound waves to reflect upwards and away from the ground.
音の伝播メカニズムの解説
This makes it harder for sound to travel long distances.
However, under specific conditions, this usual temperature gradient can reverse a phenomenon known as a temperature inversion.
This occurs on clear nights, where the ground cools rapidly due to radiative cooling, or when the mass of warm air moves over a layer of colder air.
When a temperature inversion layer is present, the sound speed increases with altitude, causing sound waves to reflect downwards, tending to return to the ground.
This downward reflection allows sound to propagate much further than usual and be heard at a greater volume.
The reason why distant train crossing sounds are often heard clearly at night is due to this downward reflection of sound waves caused by a temperature inversion layer.
And what was particularly important in the Mrs. Green Apple cases was a phenomenon called duct propagation.
This is when sound waves are trapped within the horizontal layer called an atmospheric duct, which acts like a tunnel, allowing them to propagate with much less energy loss than normal.
In this phenomenon, sound waves reflect and arrive at distant locations as if falling from the sky.
This mechanism is not unique to sound waves. It also applies to electromagnetic waves, causing phenomena like tropophilic ducting, where distant radio broadcasts can be heard or relayed at sea.
To scientifically explain such complex sound propagation phenomena, researchers used a powerful numerous simulation tool called the parabolic equation method.
This method was originally developed for the electromagnetic wave problems and has since been applied to other scientific fields like ocean acoustics and outdoor acoustics.
音波の伝播と風の影響
In this case, the simulation took into account specific conditions such as the sound source location, the target areas, and most importantly, the atmospheric data from the time, especially wind direction and speed.
The simulation serves a crucial bridge between the theoretical sound wave propagation models and the complex real-world atmospheric conditions.
Finally, this surprising phenomenon of Mrs. Green Apple's concept base reaching Kawasaki can be primarily explained by the reflection of sound waves due to the wind gradient and the duct propagation phenomenon, where sound waves were trapped within the atmospheric duct.
Additionally, the characteristic of low-frequency sound being less prone to attenuation over long distances was a crucial factor enabling this long-range propagation.
So that's all for today's SciencePod.
I think today's topic is a little bit sensitive for the residents because the residents got angry with the sound of concerts.
So that's all for today's SciencePod.
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Thank you for listening and see you next time.
