09/22/20
Tan TY
Sound
a) Describe the production of sound by vibrating sources
b) Describe the longitudinal nature of sound waves in terms of the processes of compression and rarefaction.
c) Explain that a medium is required in order to transmit sound waves and the speed of sound differs in air, liquids and solids.
d) Describe a direct method for the determination of the speed of sound in air and make the necessary calculation
e) Relate loudness of a sound wave to its amplitude and pitch to its frequency
f) Describe how the reflection of sound may produce an echo and how this may be used for measuring distances
Sound
Sound is produced by vibration
– Feel your throat as you talk
– Observe a tuning fork
Sound is a longitudinal wave
– Vibrating object alternatively pushes and pulls the adjacent particles
– Particles move in a direction parallel to wave motion
– Energy is passed from particle to particle
Have you tried listening to a tape recording of your own voice? Does it
sound like your own voice? Why?
Sound
Sound waves are longitudinal waves.
– A compression zone forms when layers of air
are pushed close together, forming areas of high pressure.
– A rarefaction zone forms when layers of air are
pulled apart, forming areas of low pressure.
– Distance between two consecutive
compressions or rarefactions is the wavelength.
– Displacement-distance & Displacement-time
graph
– Eg.15.1 (pg. 296)
Bell Jar Experiment
Suspend an electric bell in a glass bell jar.
Switch on the bell and sound can be heard from the jar.
When air is slowly removed using a
vacuum pump, the sound becomes fainter Finally no sound can be heard, even
though the hammer can be seen hitting the bell.
When air is reintroduced into the jar, sound from the bell can be heard again.
Thus without a medium (air), sound cannot propagate. This shows that sound requires a medium to propagate.
Think about why the bell should be
suspended in the air, and why the wires used should be thin?
Description of experiment not
Properties of Sound
Speed of sound
Varies in different mediums, propagate faster in denser medium.
ratio of speed: gas (1x), liquid (5x), solid (15x)
Dependant on temperature, travels faster through hot air
Not dependent on pressure
Detection of sound
hearing range/audible frequency: 20Hz to 20kHz
changing pressure of air makes the eardrum vibrate. The vibration is passed on until it is picked up by special sensory cells. Information on frequency and amplitude is transmitted along the auditory nerve to the brain as electrical signals.
An oscilloscope (CRO) can give a graphical display of sound.
– waveform shows the relationship of air pressure and time
Determine Speed of Sound in Air
Direct method
Two people (A and B) are positioned 1 kilometre apart in an open field.
A fires a pistol. Observer at point B starts the stopwatch when he sees
the flash, stops when he hears the sound.
Speed = distance between A and B / time taken
The two people switch roles, with B firing while A taking time. This
reduces error due to wind.
Repeat experiment a number of times and take the average.
Example
A person stands on top of a tall building. He fires a gun. Another person
stands on top of another tall building starts a stopwatch when he sees the flash from the gun, and stops the watch when he hears the sound from the gun. Given that the two people are 2.5 km apart, the time taken is 7.49 s. Calculate the speed of sound in air.
Total distance =
Total time taken =
Speed of sound =
Echo
Formation of echo Sound waves also experience reflection and refraction
An echo is a reflected sound that you hear a short time interval after the
original sound
The following factors will affect an echo – Size of the surface
– Smoothness of surface – Hardness of surface
– Distance/ position of surface
Reverberation: when you are too close to the reflecting surface, the echo
follows the closely behind the original sound. It is difficult to differentiate the two, it sounds like a prolonged sound rather than two distinct sounds.
Echo time: time required for the sound wave to travel to the reflecting
surface and back. Can you state the equation to calculate the speed of sound, given the echo time?
Determine Speed of Sound in Air
Two methods: using direct method/ echo method Note the following:
– Total distance travelled by the wave – Total time taken by the wave
– Assumptions and precautions
Echo method
2 people standing about 50m in front of a high flat wall.
One will clap two wooden blocks to make a sound. As soon as he hears
the echo, he will clap the blocks again. He will clap 50 times.
The second person will start a stopwatch when the first clap is made. He
will stop the watch when he hears the echo from the 50th clap.
Total distance travelled by the sound waves = 2 x 50m x 50 = 5000m Total time taken = time on stopwatch
Speed = total distance/ total time
Example
A girl standing 50m in front of a wall claps, and continues to clap when
she hears the echo. Another girls takes the time between the first and fifty-first clap. Calculate the speed of sound. Time on the stopwatch shows 15.00 s.
– Total dist = 50x2 = 100m (for 1 complete journey)
– Time for 1 clap = (15.00 / 50)s
– Speed = total dist/ time
= (2 x 50) / (15.00 / 50)= 333.3 m/s
Look at Eg. 15.2 and 15.3 (pg. 301)
Applications
Echo sounder (sonar)
– Use to measure the depth of seabed. Sends pulses of sound wave
(usually ultrasound) towards the seabed and measures the echo time.
Electronic tape measure (FYI)
– Measures distance between two objects, or distance between the
instrument and the object.
Radar (FYI)
– Detect position of aircraft/ ship using microwaves.
– The echo time is taken, given the speed of sound, the distance of
the objects can be calculated.
Ultrasound
Ultrasound scanning to detect abnormal growth or checking
the development of the foetus.
– A transmitter sends pulses of ultrasound into the body.
– A detector picks up the pulses reflected from the body.
– Different layers and different composition of the body will result in
different reflected pulses.
– Signals are processed by a computer and shown using a image.
Explore ultrasound
Ultrasound Applications
Cleaning (FYI)
– Machinery is immerse in a liquid
– High power ultrasound send through the liquid
– Vibrations used to dislodge dirt and grease
Metal testing
– Pulse of ultrasound sent through a metal plate
– Pulse is reflected from the other end of metal plate
– Sensor detects the the reflected pulse
– If there is a flaw, two pulses will be detected
One pulse from the other end of the plate
The 3 factors distinguishes the various sounds
Note how the factors can be varied
Note the changes in waveforms
Loudness
The source of vibration puts more energy
Into the system
Results in a larger amplitude of vibration
Hence a louder sound
Unit of sound: bel (B), and decibel (db). Refer to table on pg. 275.
A tuning fork on a table produces a louder sound. – Vibration of fork makes the table surface vibrate – More air near table vibrate
– Hence louder sound heard
– Why does the sound dies away faster?
Three Factors
Pitch
Higher the frequency, higher the pitch
Frequency can be increase by:
– Increasing the tension
– Decreasing the length
– Using thinner string of smaller
mass per unit length
Quality
Different instruments produces sounds different quality/ timbre
Sound of the same frequency, but produced by different instruments
will have different waveforms
Try Q. 5 pg. 312
LTA to mitigate noise along elevated MRT tracks
Residents in Singapore who live close to MRT viaducts and are disturbed by the noise of the comings and goings
of trains could be in for some reprieve.
The Land Transport Authority (LTA) announced on Tuesday plans to mitigate railway noise on elevated tracks.
It will embark on a noise measurement study spanning the entire length of elevated MRT tracks island-wide and
covering more than 500 blocks of flats located close to the elevated viaducts or turnouts.
"With an ageing MRT system that has been in operation since 1987 and increased train frequencies to better
serve commuters, LTA recognises that some residents living particularly close to the MRT viaducts may experience increased noise levels," said LTA in a statement.
The study, to be completed by the second quarter of 2012, will help LTA identify locations requiring special
attention.
Currently, LTA has installed Low Noise Barriers at the new viaduct and platform at Jurong East MRT station to
reduce noise to the same levels before the implementation of the Jurong Modification Project.
Responding to feedback, LTA will also pilot Low Noise Barriers along the open tracks at Bishan MRT station. It will
call for a tender for the installation of the barriers this month.
This study is conducted concurrently with the on-going multi-agency study jointly led by the National Environment
Agency and LTA on road and rail noise, and which is expected to be completed by the first quarter of 2012.
What is SMRT doing to minimise track noise?
We do a couple of things, mostly in preventive
maintenance. In wheel profiling, we grind the wheels daily to maintain an even roundness and diameter for each wheel to allow the trains to travel smoothly and with less noise. Train wheels are also mounted with sound
absorbers to reduce screeching sounds.
One of the biggest contributors of track noise is uneven track surface. To rectify this, we use a Rail Grinding
Vehicle to grind track surface for smoother and less noisy train operations. Typically, a section of a track is ground every six to 12 months.
A Rail Grinding Vehicle grinds track surface and helps reduce noise from train operations
Our maintenance staff inspects the tracks daily to check for track defects.
Is the MRT jingle appealing or annoying? (2010)
"Train is coming, train is coming...", I found myself singing to this jingle ever since I heard it last Sunday
at Dhoby Ghaut MRT Station. The jingle stood out with the Dim Sum Dollies’ bright, chirpy vocals and infectious melody, and ends with “Start queuing!”, reminding commuters to be a gracious commuter.
The jingle is part of a programme helmed by the Public Transport Council, supported by LTA, SMRT,
SBS Transit and the Singapore Kindness Movement, to promote gracious behaviour on buses and trains. This year's slogan is "Love Your Ride", and you might have seen colourful posters and a music video (see below) starring the programme ambassadors, Dim Sum Dollies, at MRT stations, bus
interchanges, bus-stops, in trains and in buses...
Back to the jingle: how are we reacting to this new jingle (it replaces the previous spoken “The train is
approaching the platform” announcement that most of us are familiar with)?
Alicia, 19, initially liked the creative jingle. However, the more she heard it, the more she found herself
irritated by it. She also thought that Singaporeans might need some time to get used to it.
Both Yiin Shin, 18 and Felicia, 19 dismiss the jingle as “annoying”, “distracting” and “totally redundant”.
They prefer the spoken announcements used previously, which was simple and effective. Yiin Shin also observes that electronic signboards at train stations and bus stops allow commuters to know about approaching trains and buses.
Challenge
To investigate rock layers underground, an explosion is made on the ground
surface. The figure below shows the arrangement.
Sound from the explosion may travel to the detector through the air (path 1),
through the earth (path 2), or by reflection from a layer of rock (path 3). Some sound travel into the layer of rock by path 4. Time taken by the sound is shown the table.
a. Explain why sound arrives first at the detector along path 2. [1]
b. Given that the speed of sound in air is 340m/s, calculate the distance between the explosion and the detector. [2]
c. From (ii), calculate the speed of sound in earth. [2]
d. When sound wave travel along path 4, would the speed be faster or slower compared to path 3. Explain. [3]
e. Explain how the diagram shows that sound travel faster in the layer of rock compared to the earth layer. [2]
Path 1 Path 2 Path 3
Time taken for sound to travel
