Activity: Sound waves
Sound will be introduced as an example of a longitudinal wave, and you will investigate its wave nature. You will also derive the equations for the harmonic frequencies of standing waves in pipes - and measure the speed of sound.
Need to know
When air is disturbed it causes a change in pressure which in turn disturbs the surrounding air resulting in propagation of changing pressure throughout the medium. This can be simulated using Algodoo.
In the simulation you can see the disturbance spreading down the tube.
- How would you describe the motion of the gas molecules?
- How can you tell that energy is transferred?
- Does the pulse reflect off the left end?
- Explain why the speed of sound is dependent on the temperature.
If the end is made to move with SHM then a continuous wave will be sent along the tube, the individual molecules wouldn't oscillate but layers of molecules would. This can be demonstrating by observing a candle flame licker when placed next to a loud speaker, here the flicker is shown in slow motion.
https://youtu.be/TuOpSD-cXMc
Notice how the flames move in the same direction as the wave, this means sound is a longitudinal wave.
It is often more convenient to represent the displacement of the layers of gas rather than the changing pressure. Use the simulation below to investigate the relationship between the wave and the displacement position graph.
Notice how there is a zero displacement at the centre of a compression and rarefaction.
- Where is the maximum pressure?
Sound is not just something to be measured it can also be experienced with our senses, the loudness and pitch of a sound are related to the physical quantities of amplitude and frequency:
High pitch > High frequency
Loud > Large amplitude
Speed = 340 m s-1
Use the simulation below to investigate frequency and amplitude. The graphs are displacement time graphs.
- Calculate the wavelength of a 600 Hz sound.
Reflection
Reflection of sound is called an echo, a room with no reflection is called an anechoic chamber.
Some interesting TOK aspects in that video, how our physical environment affects the way we think (or not).
Refraction
Refraction of sound isn't an everyday experience but if you are ever sitting in a boat on a still evening you might notice that sounds seem amplified and this is due to refraction. On a still evening the cold air settles near the water with the warmer air on top. Sound waves travelling up through the air are refracted so much by the different layers that they are directed back down again. What you hear is the sound directly from the source plus the sound refracted down.
This can be demonstrated in the ripple tank simulation.
Here you can see the different layers. The wave speed increases with increasing height.
From Paul Falstad's ripple tank
You have seen that a sound wave reflects off the end of a pipe, when a reflected wave adds with the incident wave a standing wave can be formed.
- A node is a position of zero displacement. Why is a node formed at the closed end?
The simulation below shows the possible waves (harmonics) in a closed pipe, use the slider to change between harmonics. Note the position of the nodes.
1st Harmonic
- Given that the pipe length is L what is the wavelength of the first harmonic?
- If the velocity of sound is v what is the frequency of the first harmonic?
3rd Harmonic
- What is the wavelength of the 3rd harmonic?
- Show that the frequency of the 3rd harmonic is 3 x the frequency of the 1st.
- Why are only odd harmonics possible?