Close your eyes for a moment and pretend you are picnicking, during the mid-afternoon, with family and friends in Central Park. About a two football fields away you can see a softball game. You can see the men and women cheering as ballplayers smack the ball and race around the bases. You see a player slide into home plate and you watch the umpire signal him out. You can see all this action, but you cannot hear it.
Later that night as the temperature cools and other softball teams are playing under the lights you can see and hear everything clearly. Why is this so? Could it be, during the day, you were situated in an acoustic shadow? Sound travels faster in warmer air. So the sound travels faster in the air close to the ground. The reverse happens at night. At night the ground cools quickly. The higher air is warmer than the air close to the ground.
During the day the sound travels faster near the ground. This causes the sound wave to refract upwards. At night the opposite happens. The sound further from the ground travels faster at night causing the sound wave to refract back towards the earth. In the s Osborne Reynolds performed the first sound wave refraction recorded test.
He placed a ringing bell, one foot above the ground, and crawled twenty yards. He had to lift his head in order to hear the ringing sound. He then crawled seventy yards and had to stand before he could hear the ringing sound. In Charles D. Ross published a book called Civil War Acoustic Shadows.
In this article we are going to explore the phenomenon of sound wave refraction and how it relates to the sounds of war the formation of acoustic shadows. In the early 20 th century, with the use of hot air balloons, scientists started to learn more about our atmosphere and its different layers. In atmospheric temperature changes were documented while studying meteors. The layers that house our ozone tend to be warmer due to the captured radiation from the sun.
Wind also plays an important role in the refraction of sound waves and ultimately on the distance they travel.
Wind traveling directly into an oncoming sound wave will make it refract upward more sharply. Wind traveling in the same direction as a sound wave will make the sound wave refraction more gradual.
In the upper atmosphere a strong wind traveling in the direction of the wave will push the wave further and faster. You can only hear it at night. The reason for this upward diversion of sound in the daytime versus the downward diversion at night is the strong dependency of the speed of sound in the atmosphere on temperature.
The atmosphere acts like a lens that focuses sound energy upwards during the day, but keeps it at ground level during the night. I would tend to agree that background noise is a factor, but rather than reducing, adding to the sound you are trying to make sense of.
So part of that may be how your brain is able to filter the information from the background noise. So by reduction in the absorption of energy by air molecules in the path of the sound, more energy will reach your ear in the colder temperature.
If we suppose that the phenomenon you describe is related with wave interference. A wave is a kind of mechanical disturbance in the medium through which it is travelling. A sound wave consists of areas of relatively high and low energy, in the form of relatively high and low pressure.
To understand how sound is produced, consider a speaker. The cone or diaphragm of a speaker vibrates inwardly and outwardly in response to an electrical signal. These vibrations are typically very small, only visible with larger speakers. However, they all impart energy to the air in the same way.
When the cone moves outward, it pushes the air forward that originally occupied that space. This air becomes locally compressed, forming a region of relatively high pressure. When the cone moves inward again, it recoils from the space that it occupied and leaves behind a partial vacuum, a region of relatively low pressure. The frequency and amplitude of the vibrations change the characteristics of the wave that is formed, and hence the sound that we perceive.
When we hear the sound, our ears are being bombarded with air molecules of rapidly varying pressures. The signal is sent to the brain where it is interpreted. As the sound wave progresses through the air, its energy slowly dissipates. This is why sound is louder closer to the source and quieter further from the source.
Wave interference occurs when two or more waves disturb the same air molecules. If a relatively high energy part of one wave combines with a relatively low energy part of another, the result is a region of air with an average of the two.
In the most extreme case, the resulting pressure is indistinguishable from that of the undisturbed air, and is therefore undetectable by the ear. This situation is known as total destructive interference. In practice, however, interference is almost always partial. Similarly, if two high- or two low-energy parts of a wave combine, they can be summative.
This opposite process is known as constructive interference. To visualise this process, you may want to look at a video or two on-line. You may wonder, it is because it is quieter at night than in the daytime. Therefore it is easy to hear the sound far away. However, it is only one of the reasons.
Actually, sound transmits farther at night may be related to refraction of sound waves! First, sound is the vibration of air, and it is a kind of wave motion.
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