O level physics Wave and Sound

• an imaginary line on a live that joints all points that are in the same phase.
• It is usually drawn by joining the wave crests.

Reflection of waves

• When water waves get reflected, the only thing that changes is the direction.
• The wavelength, frequency and speed remains the same throughout.
  
• Sponges are used to absorb the reflections of the water waves.

Refraction of waves

• When water waves get refracted (move from deep to shallow water), the speed and the wavelength changes.
  
• The frequency of the wave does not change

Electromagnetic Spectrum

• Electromagnetic waves are transverse waves. They are electric and magnetic fields that oscillate at 90° to each other.
• They transfer energy from one place to another.
• They can travel through vacuum (do not require any medium to travel)
• They travel at 3.0 x 10⁸ per second in vacuum. They will slow down when travelling through water or glass.
• The wave equation is applicable here too.
• They obey the laws of reflection and refraction.
• They carry no electric charge (they are neither positively or negatively charged)
• Their frequencies do not change when travelling from one medium to another. Only their speeds and wavelength will change.

Most important equation

Sound

• Sound is a form of energy.
  
• The energy is passed from 1 point to another as a wave.
• Sound is an example of longitudinal wave.
  
• Sound is produced by vibrating sources placed in a medium (air).
  
• It travels in air through a series of compressions or rarefactions.
• Compressions: Air molecules are close together, forms high pressure.
  
• Rarefactions: Air molecules are far apart, forms low pressure.
  

Speed of sound differs in different medium

Air: 330 – 340m/s
Water: 1500m/s
Glass: 5000m/s

Speed of sound differs because:

• Differences in strength of interatomic forces
  
• Closeness of atoms in the 3 states
  
• Temperature

– The Wave Equation can also be used to find the speed of sound
– The speed of sound is solids like metals are so fast that we can assume/ignore the time it takes to travel a distance.

Echoes

• Echoes refer to the repetition of a sound resulting from reflection of the sound waves.
• Echoes are formed when a sound is reflected off a hard and flat surface.
• Reverberation occurs when the surface is too close, causing any reflected sound to follow closely behind the direct sound and prolonging the original sound.

Ultrasound

• The range of frequencies which a person can hear is known as the range of audibility. 
  • Human: Between 20 Hz and 20 kHz
    
  • Dog: <20 kHz
    
  • Bats: Between 10 kHz and 120 kHz.

• Ultrasound is the sounds with frequencies above the upper limit of the human range of audibility.
  
• Its small wavelength means less diffraction and the echo formed is more precise in direction.
• Applications for ultrasound include:
  • Determining depth of seabed
    
  • Locating sunken ships / shoals of fish
    
  • Cleaning small dirt from jewellery
  • Quality control (checking for cracks) in concrete
    
  • Medical applications (development of foetus)

Loudness

• a factor distinguishing between various sounds.
  
• The larger the amplitude of vibration, the louder the sound
  
• Sound is measured by decibels (dB).

Pitch

• a factor distinguishing various sounds
  
• The higher the frequency of a note, the higher the pitch
  
• Pitch is measured in hertz (Hz).