Monthly Archives: March 2014
87.WHAT IS THE SOUND BARRIER?
Have you ever heard a loud sound like an explosion when a fast airplane was flying overhead? This sound is produced by a plane as it passes the sound barrier. Now what is this sound barrier and why does the plane produce this Boom?
Sound, as you know, travels at a certain speed. The speed depends on the altitude. At sea level, the speed of sound is about 760 miles (1,220 kilometers) per hour, and at a height of 11,000 meters, it’s 660 miles (1,060 kilometers per hour. When a plane flies at speeds close to the speed of sound, it undergoes certain kinds of stress. When it is flying at ordinary (slower) speeds, the forward part of the plane sends out a pressure wave. This wave travels at the speed of sound.
86.WHAT IS PITCH?
If all sound is caused by vibration, why is one sound simply a noise and another sound pleasing or musical? The answer is that it depends on how the sound-making object vibrates. When the vibration is very regular (that is, when the sounding body sends out waves at absolutely regular intervals), the result is a musical sound. If the vibration is not regular, or very sudden, the effect on our ears makes us call it a noise.
Sounds differ in many ways. We know, for example, when a sound is near and when it’s far away. We can also tell that certain sounds are high and shrill and others are deep and low. And we can tell the sound of one musical instrument from another. Well, these three differences between one sound and another are loudness, pitch, and tone quality.
85.WHAT IS MISTLETOE?
In millions of homes in Europe and the United States, the mistletoe is hung up at Christmastime. According to a happy custom, when a girl is standing under the mistletoe, a man is allowed to kiss her.
Curiously enough, the use of the mistletoe on holidays and ceremonial occasions goes back to quite ancient times. When the Romans invaded Britain and Gaul (now called France), the people who lived there were called Celts. These Celts were organized under a strong order of priests called Druids.
