Can sound hurt?


 



 



             The loudness of sounds is measured in units called decibels (dB).Ordinary speech has a sound intensity of about 20 decibels, while a noisy room full of people produces about 70 decibels. The quietest sound you can hear is about 20 decibels. Loud music at a rock concert reaches a level of about 125 decibels, while sounds of 140 decibels or more actually cause pain in the ear. This level of sound can produce permanent hearing damage if it continues, which is why people operating noisy machinery have to wear ear protectors.




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How can sound detect submarines?


         



                Sound waves travel very well through water, so they are used to detect submarines or even shoals of fish. A sonar device under a ship sends out sound waves that travel down through the water. The sound waves are reflected back from any solid object they reach, such as a submarine, shoals of fish or the seabed. The echoes are received by the ship and can be used to ‘draw’ an image of the object, and its location, on a screen.



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What causes thunder?


 



               Thunder is produced when lightning heats up the surrounding air very rapidly, causing it to expand faster than the speed of sound. This produces a ‘sonic boom’ like the one caused by a supersonic aircraft. If you count the seconds between seeing a flash of lightning and hearing thunder and divide this number by 3, it will tell you how many kilometres away from the lightning you are. Sound travels at about 1220 km/h, while light from the lighting strike travels so fast that you see it almost instantaneously.




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What is the sound barrier?


               At sea level, sound travels though the air at around 340 m pet second, and slightly slower at high altitudes. The denser the substance, the faster sound will travel through it. Sound travels at 1,500 m per second through water, for example. When travelling at such very high speeds, an aircraft begins to build up a huge wave of compressed air in front of it. This was known as the sound barrier because it was an obstacle to high-speed flight. When the aircraft exceeds the speed of sound, it leaves the built-up waves of pressure behind, and these break away, forming a ‘sonic boom’. The speed of sound in air is called Mach 1, and the speed of supersonic aircraft is measured in Mach numbers. Concorde is the only commercial aircraft to have broken the sound barrier.



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What is the Doppler Effect?


               You will have noticed that as a car travels quickly towards you, the sound of its engine gets higher, and then becomes deeper after the car has passed by. This is called the Doppler Effect. What happens is that as the car approaches, the frequency of the sound of its engine increases as the wavelength of the sound decreases. Each successive sound wave is a little shorter as the car comes towards you. Then as the car moves away, the process is reversed; the frequency decreases while the wavelength of the sound becomes longer.



               You will not notice the Doppler Effect if a supersonic aircraft flies past, because you cannot hear the sound of the plane until it is actually flying away from you.



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What is an echo?


             



                   Like light, sound can be reflected from certain surfaces. Hard surfaces such as rock or the side of a building reflect sound well; as the sound bounces back you hear an echo. The delay in hearing the echo is due to the comparatively slow speed of sound. Soft materials absorb sound and will not produce an echo. This is why recording studios are lined with felt material, which prevents any unwanted noise from interfering with the recording.



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How many sounds can we hear?


               Compared to other animals, the human ear is not very sensitive. We can hear sounds with a frequency of up to 20 kilohertz (kHz) — a normal speaking voice is about 1 kHz. Bats, dogs and some insects can hear sounds has are pitched much higher, which we cannot hear at all. Children can usually hear a bat squeaking, although this sound can sometimes be as high as 120 kHz, but in adults the ability to hear these high-pitched sounds is usually lost. ‘Supersonic’ dog whistles are used to call dogs, and their sound is so high-pitched that humans cannot hear it at all.




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What is sound?


               Sound is a form of vibration passing through the air or some other material. Sound travels in the form of waves but, unlike electromagnetic waves, sound waves cannot pass through a vacuum. The frequency of a sound wave controls its pitch. Long wavelengths produce deep sounds, while short wavelengths produce higher sounds. The loudness of a sound depends on the height of the sound waves, or their amplitude. The higher the amplitude, the louder the sound.



               Sound vibrations are received in your ears. They are conveyed to a mechanism inside your ears that first amplifies them, and then converts the vibrations into signals that your brain interprets as sound.



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