Sound Principle

• Sound  Principle
  l              Sound is a kind of wave. 
  13.1 Introduction to Wave Theory
  1.       Types of waves
  l              There are two types of waves, transverse and longitudinal. 
  l              When the direction of travel of the wave is perpendicular to the motion of the source, the wave is transverse.
  l              When the travel of the wave is parallel to the motion of the source, the wave is longitudinal. Sound is typical example of this type of wave.
  l              Both types of waves require the action of an oscillating or vibrating source, the motion of the source is called simple harmonic motion.
  
  
  2. Aspects of Periodic Waves
  l             Both types of waves are recurring cycles of the same motion, thus we called it periodic waves. The motion can be represented by the sine wave in mathematics.
  Terms used to descript waves:  
  l              Cycle: a cycle is a complete sequence of motion that repeats itself
  l              Wave-length(λ): wave-length is the length in metres in one cycle
  l              Period(T): period is the time to complete one cycle and is measured in seconds
  l              Frequency: frequency is the number of cycles in a given time, which is measured in cycles per second. The unit is named hertz (Hz).
  l              Amplitude: the amplitude of wave is the maximum disturbance of a wave from it zero point(negative or positive), measured in metres(m)
  
  
  3. Wave Equation
  l             v =λ/T   =>  v =λf  (f=1/T)
  
  
  13.2 The Transmission and Speed of Sound
  1.  Medium Dependence
  l             Sound is a longitudinal wave that requires a medium to travel in.
  l             In air, only possible type of vibration is longitudinal. Air cannot sustain a transverse motion, otherwise air particles will drift off.
  l             The dense areas of air transmitting a sound wave are called compressions and the less dense areas are called rarefactions.
  l             Factors affecting the speed of sound are: the type of medium(gas, liquid or solid), temperature and density.
  l             Generally speaking, the stiffer the material, the faster the speed of sound.
  
  
  2.       Dependence of Speed on Temperature
  l             : v = 332m/s + (0.6 m/s / °C )(T °C)
  
  
  13.3 Mach Number and the Sound Barrier
  
  
  l             When objects (such as aircrafts and cars) get close to or go faster than the speed of sound, a different unit is often used to describe their speed. It has called Mach Number.
  l             1 mach = one time of speed of sound
  l             Subsonic: speed < mach 1
  l             Supersonic: speed > mach 1
  l             Hypersonic: speed > mach 5
  
  
  1.           Sound Barrier
  l             The term “sound barrier” is used in aviation to describe the buildup of sound waves in front of a plane as its speed close to the speed of sound. (The first plane to break the sound barrier was the Bell XS-1 experimental rocket on October 14,1947; the fastest plane in the world is X-43A, which has a speed greater than 7 mach with the best record of 9.7 mach(11000kph))
  
  
  2.   Sonic Boom
  l             When a plane breaks the sound barrier, it will leave a pressure wake behind it. The waves will spread out and move down towards the ground. We will hear it as a loud "boom".