2014 sec 4 physics chapter 11 sound

2014-PhyNotes-11- Sound 1

2014 Sec 4 Physics Chapter 11

Sound Name: ____________________ ( ) Class: 4 / ___

Source: http://www.google.com/doodles/robert-moogs-78th-birthday

11 Sound 11.1 Sound waves 11.2 Speed of sound and echo 11.3 Ultrasound 11.4 Use of cathode ray oscilloscope

Specific Instructional Objectives

(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) relate loudness of a sound wave to its amplitude and pitch to its frequency

(d) explain that a medium is required in order to transmit sound waves and the speed of sound differs in air, liquids and solids

(e) describe a direct method for the determination of the speed of sound in air and make the necessary calculation

(f) describe how the reflection of sound may produce an echo, and how this may be used for measuring distances

(g) recall the approximate range of audible frequencies

(h) define ultrasound and describe one use of ultrasound, e.g. quality control and pre-natal scanning

(i) describe the use of a cathode-ray oscilloscope (c.r.o.) to display waveforms and to measure p.d.s and short intervals of time [detailed circuits, structure and operation of the c.r.o. are not required]

(j) interpret c.r.o. displays of waveforms, p.d.s and time intervals to solve related problems


Read up: Textbook: “All About Physics ‘O’ Level” Chapter 15, page 296-315. 11.1 Sound waves 11.11 Production and transmission of sound waves Note: * circle the correct option Sound waves are produced by the ……………………….. of objects, e.g. drum, tuning

fork or vocal cords.

Sound waves are propagated as *transverse / longitudinal waves which travel in a direction parallel to the direction of the vibrations.

Sound waves require a …………………………. to travel, so they are classified as *mechanical / electromagnetic waves.

The diagram below shows the positions of layers of air molecules before and after sound waves travel through them from the left towards the right.

No sound

Sound waves

travelling through air

When the vibrating object alternately pulls or pushes on the layers of air adjacent to it, it causes small but rapid changes in air ……………………………..

Where the air layers are pushed closer together (than normal), the air pressure becomes *higher / lower than normal, and a region of *compression / rarefaction is formed.

Where the air layers are pulled further apart (than normal), the air pressure becomes *higher / lower than normal, and a region of *compression / rarefaction is formed.

Example 11.1 On the diagram above showing sound waves travelling through air, mark & label (a) points of compression (with letter “C”) and points of rarefaction (with letter “R”); (b) a wavelength between 2 Cs and between 2 Rs.

positions of layers of air molecules

vibration

medium

pressure

The arrows below show the displacement of each layer of air molecules from their original (at rest) positions.

C CR


11.12 Properties of sound waves Sound waves can be represented by displacement-time, displacement-position and

pressure-position graphs.

Example 11.2 (a) Based on the positions of the layers of air when sound waves travel through it, as shown below, mark with arrows the displacements (if any) of each layer from its rest positions. (Convention: positive displacement towards the right.) (b) Hence, sketch the corresponding (i) displacement-time graph, (ii) displacement-position graph, and (iii) pressure-position graph.

No sound

Sound waves travelling through air

displacement

time 0

pressure

position 0

displacement

position 0

Negative displacement (to the left)

Positive displacement (to the right)

Zero displacement

of a layer of air molecules over time; not related to the different original positions (no sound)

of many layers of air molecules at different positions at an instant of time

C C

R

Air

Normal

Above

Below


Loudness or volume of sound waves increases as ………………………. of the vibrations increases. Since increased loudness is caused by more vigorous vibrations, it is a measure of the amount of ……………………… carried by the sound.

Pitch of the sound waves is determined by the number of waves formed in 1 s or its …………………………...

The characteristic of sound that allows us to differentiate between 2 different sources of sound with the same pitch and loudness is its ……………….. or timbre.

Sounds which differ only in their quality will have different harmonics but same frequency and amplitude. These can be displayed on a C.R.O.

11.2 Speed of sound & echo Sound waves need a …………….. to travel (or to be transmitted or propagated).

Speed of sound differs in gases, liquids or solids due to differences in the strength of the interatomic forces, closeness of the atoms/molecules, and temperature. Compression and rarefactions propagate faster in denser media.

When a sound wave is reflected by a large, hard and smooth surface, an echo is produced.

Speed of sound can be measured using direct or indirect methods. Compare these methods of determining the speed of sound below.

Direct method Indirect method Indirect method

Method Observer A fires a starting pistol

Observer B (at least 200 m away) starts a stopwatch on seeing the flash of the gun and stops the stopwatch on hearing the sound

Observer A faces a high smooth wall at least 50 m away and claps regularly to coincide with echoes.

Observer B times 50 claps.

Place a microphone at one end of a long hollow tube and a smooth flat surface at the other end.

Connect the microphone (sound sensor) to a laptop (installed with Addestation software with “Scope” simulator) .

Snap your finger next to microphone, and click to capture the image of sound and its echo.

Physical quantities to

measure

Formula to use

Possible sources of

error

amplitude

energy

frequency

quality

medium

Human reaction time in starting & stopping stopwatch

Speed v = d / t v = 2d / (t / 50) v = 2d / t

Human reaction time in starting & stopping stopwatch

d = Distance between A & B; t = Time taken to hear sound of pistol

d = distance between observer & wall; t = time interval of 50 claps

Error in locating exact position of (1) the microphone & (2) where the snap sound is produced at one end of tube

d = length of tube; t = time interval between the snap sound and its echo (2 peaks on C.R.O.).


11.3 Ultrasound

The human audible frequency range is between 20 Hz to 20 kHz.

Ultrasonic frequencies are those above ……………… and infrasound are those below ………………

Ultrasound allows imaging because of the difference in …………….. taken for the waves to return from different depths of the medium.

List and describe some common applications of ultrasound in the table below.

Use of ultrasound Description of use

11.4 Use of cathode ray oscilloscope

11.41 Displaying waveforms

Oscilloscopes are widely used for displaying waveforms (e.g. sound waveforms from a microphone). By selecting a suitable scale for the ……………………., we can display the voltage waveform by connecting to the Y-terminal input.

11.42 Measuring voltages

The figures below show the screen of an oscilloscope displaying different types of voltages when the time base is switched off. The gain control is set at 2 V/cm

In both cases, the magnitude of the input is …………….. V.

20 Hz20 kHz

time

time base

3.0

Source: a constant d.c. voltage (e.g. battery)

Source: an a.c. voltage (e.g. a generator)

V = height x Gain (or gain control or Y gain or Y setting) = 1.5 cm x 2 V /cm = 3.0 V

(C.R.O.)


14.33 Measuring Short Intervals of Time and Frequencies

The frequency of an a.c. voltage can be found by making horizontal measurements on the waveform displayed.

In the figure on the right, the time base control of the oscilloscope is switched on and set at ‘10 ms/cm’. This means that the spot takes 10 milliseconds to move 1 cm horizontally across the screen.

The distance occupied by one complete waveform on the screen is ……………..

Hence, the time taken for one complete waveform traced out is ……………….... and the frequency of the waveform is ………………....

Summary

In the space below, draw a mindmap or concept map to illustrate all the key ideas and

examples in this topic.

2.0 cm

20 msor 0.020 s

50 Hz

2.0 cm

Time = length x time base setting = 2.0 cm x 10 ms / cm = 20 ms


Exercises

1 To make a sufficiently detailed ultrasound image of a feotus in its mother’s uterus, a

physician has decided that a wavelength of 0.50 mm is needed. What frequency is required?

2 A fathometer measures the depth of water below a ship by sending out a pulse of ultrasound and measuring the time taken for its echo to return. A simple diagram of the arrangement is shown below. (a) Calculate the depth of the water if the time taken between transmission of a pulse of ultrasound and its echo arriving back from the sea bed is 0.80 s and the speed of ultrasound is 1500 m/s.

(b) The ship was using a CRO to measure the time taken for the echo to

return. (i) What does CRO stand for? ………………………………………………………

(ii) Identify and label the original pulse (O) and the returning echo (E) on the CRO display on the right.

(iii) State the scale of the time-base of the CRO …………………………………

3 The diagram below shows the waveform of an a.c. supply displayed on an

oscilloscope. If the gain control is set to 50 mV cm-1 and the time base control set to 10 ms cm-1, determine:

(a) the peak voltage of the a.c. supply.

(b) the peak-to-peak voltage of the a.c. supply.

(c) the frequency of the a.c. supply.

CRO display

sea bed

transmitter receiver

ship

Given speed of ultrasound in the body = 1540 m.


4 The screen of an oscilloscope is shown in the figure below.

The Y-plates are connected to an a.c. supply. The following settings are used.

Y-gain setting = 2 V/div

Time-base setting = 1 ms/div

(a) Calculate the frequency of the wave.

(b) If the time-base is set to 3 ms/div and the Y-gain is set to 1.5 V/div, draw the new waveform in the grid given below.

Answers: 1. 3.2 MHz 2(a) 600 m (b)(iii) 0.80 s/cm 3(a) 125 mV (b) 250 mV (c) 25 Hz 4(a) 83 Hz (b) 2 full waves

Discussion 1 How can you demonstrate that sound cannot travel without a medium? 2 (a) In the direct method to measure the speed of sound, explain why the flash can

be used as an indicator for the timer to start his stopwatch? (b) State the quantities to be measured in order to determine the speed of sound

by the direct method. (c) Explain how errors may be minimised in the above method. 3 Explain briefly how ultrasound foetal imaging is carried out.

3.1 MHz

This is a test done during pregnancy that uses reflected sound waves to produce an image of a foetus.

0.10 s/cm

2014 sec 4 physics chapter 11 sound

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