IB DP Physics: SL复习笔记4.2.4 Sound Waves

Sound Waves

  • Sound waves are longitudinal waves and, as such, require a medium in which to propagate
  • Sound waves are generated by oscillating sources, which produce a change in density of the surrounding medium
  • The sound wave then travels with a series of compressions and rarefactions


A sound wave travelling through air

  • Sound waves form a continuous spectrum based on their frequency


The spectrum of sound waves

  • Humans can only hear sounds with frequencies in the range 20 Hz - 20 kHz, known as the audible range
  • Sounds with frequencies below and above this range cannot be detected by the human ear

Pitch and Volume

  • The frequency of a sound wave is related to its pitch
    • Sounds with a high pitch have a high frequency (or short wavelength)
    • Sounds with a low pitch have a low frequency (or long wavelength)
  • The amplitude of a sound wave is related to its volume
    • Sounds with a large amplitude have a high volume
    • Sounds with a small amplitude have a low volume


Pitch and amplitude of sound

Speed of Sound

  • Sound waves travel at a speed of about 340 m s–1 in air at room temperature
    • The higher the air temperature, the greater the speed of sound
    • The is because the average kinetic energy of the particles is higher
  • Sound travels the fastest through solids, since solid particles are closely packed and can pass the oscillations onto their neighbours much faster
  • Sound travels the slowest in gases, since gas particles are spread out and less efficient in transferring the oscillations to their neighbours


  • Sound waves reflect off hard surfaces
  • This phenomenon is known as echo
  • Echo can be used to obtain an experimental value of the speed of sound. This is calculated using the equation


  • Where:
    • v = speed of sound in metres per second (m s–1)
    • d = distance between the sound source and the hard surface (m)
    • t = time taken to travel from the source to the hard surface and back (s)

Measuring the Speed of Sound Experimentally - Fast Timer

  • The speed of sound can be measured using a fast timer (one which can measure to the nearest millisecond or even microsecond)
  • Two microphones separated 1 m apart are connected to a fast timer
    • The first microphone triggers the timer to start
    • The second microphone triggers the timer to stop


  • A hammer is made to strike a plate
  • The sound waves from the plate travel to the two microphones triggering the first and then the second
    • The time delay will be around 3.2 ms
  • The speed of the waves can be calculated by rearranging the equation: distance = speed × time

Measuring the Speed of Sound Experimentally - Double Beam Oscilloscope

  • Two microphones are connected to the input of a double beam oscilloscope
  • A signal generator is connected to a loudspeaker and set to a frequency between 500 Hz and 2.0 kHz
    • One of the microphones is close to the loud speaker
    • The other microphone is 1 m away


  • There will be two traces that appear on the screen
  • The traces are compared as the second microphone is moved back and forth in line with the first microphone and the speaker
  • Use a ruler to measure the distance that the second microphone needs to move for the traces to be in phase then out and phase and back in phase again
    • This distance is equal to the wavelength of the wave
  • The speed of the waves are therefore calculated using c = fλ

Worked Example

A person stands 50 m from a wall. The person claps their hands repeatedly, and changes the clapping frequency until the echoes are synchronised with the claps. A mobile phone application measures the time between the claps, which is t = 0.30 s. Determine the speed of sound.

Step 1: Write down the known quantities

    • Distance between the person and the wall, d = 50 m
    • Time between the claps, t = 0.30 s

Step 2: Write down the "echo equation"


Step 3: Substitute the numbers into the above equation and calculate the speed of sound v


v = 330 m s–1