Summary
We represent a sound wave by drawing lines like the coils of a slinky spring. But we should remember that, although layers of air oscillate, the individual atoms do not.
Get the basics
Pressure wave
A sound wave is the propagation of changes in pressure, however it is often represented as displacement of air. This is a fair assumption on a macroscopic scale.
However, it is not the air molecules themselves that are transmitting the longitudinal wave, as molecules move with random motion.
Pressure or displacement
Here a sound wave is represented by layers of air showing how displacement is related to changing pressure. The wavelength is the distance between two compressions.
Reflection
When a sound wave meets a wall, it pushes against the wall and the wall pushes back.
Refraction
The velocity of sound in air is related to temperature. If a sound passes from cold air to warm air, its direction will change.
Diffraction
The wavelength of sound is in the order of metres, so sound waves are diffracted by openings such as doors.
Interference
Two sound waves will interfere to produce loud and quiet areas.
This is the principle involved in noise-cancelling headphones, but can also cause quiet patches at live music venues.
Stretch for 7
Standing waves - closed pipes
When a sound wave meets the end of a closed pipe, it is reflected. The reflected and incident waves interfere to produce a standing wave.
Standing waves - open pipes
Sound waves are also reflected off an open end.
Test yourself
Use quizzes to practise application of theory.
START QUIZ!
Complete the explanation of a sound wave using the words below (not all are used):
pressure, vibrating, expand, compressed, temperature
A sound wave is formed when a object causes a change in air . Layers of air molecules become and then causing the next layer of air to be the propagation of change in is a sound wave.
Complete the following explanation of how the velocity of sound is related to temperature using the words below (not all are used):
speed, compressed, pressure, temperature, mass
Sound is a wave. If a piston is moved inwards the gas next to the piston is , this compression will propagate through the gas causing the on the opposite end of the cylinder to increase. The time taken for the to change is dependent on the of the gas atoms and the of the gas atoms is dependent on .
The average speed of the molecules of a certain gas is 1000 ms-1
The velocity of sound in that gas is:
The speed of propagation of pressure must be less than the speed of the molecules.
340 ms-1 is the speed at room temp and the approximate speed of the molecules is 600 ms-1 . If the molecules have speed of 1000 ms-1 then the velocity of sound will be greater than the velocity in air.
The diagram represents layers of air. The dots are the undisturbed positions and the lines the positions as a sound wave propagates.
The layer represented by the blue line has:
The line is in its original position but in the middle of a compression
Calculate the wavelength of a 680 Hz sound
velocity of sound = 340 ms-1
λ = v/f
When a sound wave travelling in air hits a wall the air next to the wall
The air next to the wall is trapped by the wall but other molecules can crowd in to increase the pressure.
The animation shows a standing wave in sound.
The nodes marked are
Its not obvious but these are points where the air layers do not move, however the pressure is changing.
The speed of sound in helium is greater than in air. If you measure the sound from a 600 Hz tuning fork in a room full of helium the sound would be
The frequency of sound is still the same as the tuning fork but the wavelegth is longer f = v/λ
I have noticed that if I go fishing in a boat on a still night I can hear people talking quietly in another boat over 1 km away.
This effect is due to:
Refraction through the layers of different temperature air.
It might be possible to hear the effect of diffraction when sound passes through:
a door is the same order of magnitude as sound waves
Two speakers are arranged 10 m apart.
Identical sounds with wavelength 40 cm are played from both speakers
At the mid point the waves add to give maximum loudness. How far to the left would you have to move to find the first quiet spot?
10 cm to the left the right sound will travel 5.1 m and the left will travel 4.9 m
path difference is 0.2m which is half a wavelength.
Identical sounds of wavelength 50 cm are produced from two speakers as shown
Where will the waves cancel out
The largest possible path difference is 20 cm which is less than λ/2
Air is blown across the open end of a 20 cm long closed pipe.
What is the wavelength of the lowest note possible
Air is blown across the open end of a 20 cm long closed pipe.
If air is blown gently a low note is produced if blown hard a higher one is heard.
How much higher is the second note?
The next note is the 3rd harmonic
A 600 Hz note is played by excting the first harmonic on an open ended pipe.
What is the frequency of the pipe if played with one end closed?
wavelength of 1st harmonic is x2 in a closed pipe so frequency is half
The end of a closed pipe is
pressure can change but displacement can´t
How many nodes are there in the 3rd harmonic of a closed pipe?
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