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LC Physics > Sound / Waves + Wave motion > Flashcards

Sound / Waves + Wave motion Flashcards

1
Q

Doppler effect

A

Apparent change in freq. of waves due to motion of the source or observer

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2
Q

doppler effect formula

A

f’ = fc / c±u

f = actual freq.
f' = apparent freq.
u = speed of source
c = speed of waves

c - u if moving towards observer
c + u if moving away from observer

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3
Q

Dopppler effect applications

A
  • —Red shift:
  • If a star is moving away from us, wavelength of the light emitted will seem longer than normal light, will be redder
  • Can be used to find speed of stas
  • —Speed traps:
  • Microwaves are emitted and reflect off vehicles,
  • Have diff wavelengths than the original waves
  • Allows the speed to be found
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4
Q

Standing at A

A
  • Crests are closer together than crests from stationary source
  • Thus, wavelength is less than wavelength of waves from stationary source
  • Since speed of waves is same, follows that to a stationary observer, freq. of waves from approaching source must be greater than freq. from stationary source
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5
Q

Standing at B

A
  • Behind moving source, opposite happens
  • Crests are further apart than crests from stationary source
  • Wavelength is greater than wavelength of waves from stationary source
  • Thus freq. is less than freq. from stationary source
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6
Q

Standing waves

A
  • Freq of a standing wave is same as freq of wave that made it
  • distance between one node/antinode and the next is λ/2
  • Distance between a node + next antinode is λ/4
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7
Q

Wave nature

A

-sound travels through waves, so therefore it acts like a wave
-Does all things other waves do:
reflection (echo)
refraction (hear better on cold night)
diffraction (hear around corners)
interference (can block sounds)

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8
Q

Speed

A

distance travelled in relation to time

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9
Q

Speed of sound

A

speed of sound in air: 340 m/s

will be diff. in diff. media

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10
Q

Frequency of sound

A

same as that of the vibrating source producing it

eg. sound wave emitted from a vibrating tuning fork is same as freq. printed on fork

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11
Q

Sound properties

A

longitudinal wave

compression, rarefaction

molecules vibrate parallel to direction of wave

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12
Q

Stationary waves

A

-An example of interference
-Produced when two waves interfere which:
have same amplitude + wavelength
travel in opposite directions
-stationary wave pattern is produced
-tend to store energy

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13
Q

Nodes

A

Points of zero disturbance,

destructive interference

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14
Q

Antinodes

A

Points of maximum disturbance,

constructive interference

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15
Q

What does speed of sound depend on?

A

depends on material that it passes thru + temperature of that material

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16
Q

Acoustics

A
  • Science of designing theatres + concert halls to have the right amount of reflection and absorption of sound
  • sound must be 20-20,000 Hz for us to hear
  • Reflection can enhance or ruin sound of music or play
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17
Q

Use of destructive interference (noise pollution) + method

A

Use: Large bg noise can be reduced using interference

Method:

  • Mic picks up sample of the noise. sound wave of same freq. + amplitude is made
  • new noise is played on a speaker so its crests are in sync w. original waves troughs
  • results in either total or near silence
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18
Q

Characteristics of notes

A
  1. Amplitude and loudness
  2. Frequency and pitch
  3. Quality and overtones
  4. Frequency limits of audibility
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19
Q

Amplitude and loudness

A
  • Sound is a longitudinal wave
  • Molecules vibrate parallel to direction owave
  • Amplitude of wave is maximum displacement of a molecule from its resting point
  • Loudness of a sound depends on amplitude
  • Greater the amp, greater the loudness
  • Loudness also depends on the freq.
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20
Q

Frequency and pitch

A
  • Freq of a sound wave is same as that of vibrating source producing it
  • If a note is high, it is said to have high pitch, if low, low pitch
  • The higher the freq, higher the pitch
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21
Q

Quality and overtones

A
  • Sometimes source give off more than one sound
  • Can emit original freq + their multiples
  • Frequencies which are multiples of a certain freq are called overtones of that freq
  • If f is a freq, 2f is the first overtone, 3f is the second
  • Quality of a musical note depends on number of overtones emitted, less overtones, purer the sound
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22
Q

Frequency limits of audibility

A
  • Highest + lowest freqs. that can be heard by a normal human ear: 20 Hz - 20,000 Hz
  • Upper value decrease with age
  • Above 20,000 Hz is called ultrasonic, cant hear above that freq.
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23
Q

Dog whistle

A
  • ultrasonic: above 20,000Hz
  • Dogs + bats can hear up to 35,000
  • Dog whistles operate at ultrasonic freq.
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24
Q

Natural frequency

A

Natural freq. of a vibration is the freq. that an object will tend to vibrate at if it is free of outside influences

eg. if a swing is free to move back + forth, it does so at natural freq

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25
Q

Resonance

A

Transfer of energy between two bodies of the same/similar natural frequency

If freq. is applied to an object that is the same or close to the natural freq. of that object, it will start to vibrate

eg. put a vibrating tuning fork near another

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26
Q

Fundamental frequency of a stretched string formula

A

f = 1/2l x √T/μ

f = fundamental frequency
l = length of wire between bridges
T = tension read off the Newton spring balances
μ = mass per unit length of the wire
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27
Q

Fundamental frequency f

A

called the first harmonic, multiples of this are overtones or harmonics

eg. 2f = second harmonics + first overtone
3f = third harmonic + second overtone

diagrams in hardback

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28
Q

Closed pipe

A

a pipe closed at one end and open at the other

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29
Q

Pipe closed at one end

A
  • if vibrating tuning fork is placed at the open end, a longitudinal wave travels down + is reflectedback up
  • Incident + reflected ray interfere
  • If length of pipe is adjusted, resonance occurs –> stationary longitudinal wave
30
Q

Stationery waves in a closed pipe

A
  • Since distance between node + next antinode in stationary wave is λ/4, length of pipe in first diagram = λ/4 so l = λ/4
  • Longer the pipe, lower the freq. (lower the note emitted)
  • Only odd numbered harmonics exist in a closed pipe eg. clarinet, trombone, saxophone
31
Q

Speed of sound in air

A
  • We use stationary waves in a closed pipe to find the speed of sound in air, but we must take into consideration that antinode will not reside at exactly the top of the pipe.
  • will be a little distance outside pipe, called the end correction
  • found to be equal to 0.3 d, where d is the internal diameter of the pipe
  • So, λ/f = l + 0.3 d instead of λ/4 = l
  • Thus λ = 4(l + 0.3d) is used in c = fλ to find the speed of the wave
32
Q

Pipes open at both ends

A

All harmonics may be present

eg. tin whistle, flute, recorder are pipes open at both ends

33
Q

Sound intensity level

A
  • Threshold of hearing - lowest intensity to which human ear can respond when f = 1kHz
  • Freq. response of the ear.
  • Sound intensity level decibels
  • dB(A) scale is used bc it is adapted to the ears’ freq. response
34
Q

Ear protection

A
  • Example of sound intensities dangerous to our ears: lawnmowers, concerts, drills, planes (anything above 74dB)
  • Hearing impairment can happen when a person is exposed to loud sounds
  • Ear protection is req. by law in industry -> ear plugs, larger head gear
35
Q

what happens when you double sound intensity

A

Doubling sound intensity increases sound intensity levels by 3dB

36
Q

Pipe open at both ends equation

A

λₙ = 2L/n

for fundamental freq, n = 1

37
Q

Pipe closed at one end equation

A

λₙ = 4L/n where n = odd numbers

for fundamental freq, n = 1

38
Q

types of electromagnetic radiation

A

learn off table, which is longest wavelength and highest freq and vice versa

39
Q

demonstrate the doppler effect

A

-source of sound eg buzzer
-swing source attached to string
-note freq change instant source passes observer
(source may also be propelled longitudinally along a string, etc)

40
Q

what causes the red shift in spectrum of a distant star

A
  • stars move relative to earth

- longer wavelength if star receding from earth

41
Q

application of doppler effect

A
  • calculate speeds of stars or galaxies
  • speed traps
  • radar
  • medical imaging
  • blood flow measurement (echo-cardiogram)
  • temperature measurement
  • underwater acoustics
42
Q

how is infra-red radiation detected?

A
  • thermometer
  • temperature sensor/probe
  • photographic film/plate
  • by its heating effect
43
Q

describe how an emission line spectrum is produced

A
  • monatomic gas receives energy + electrons move/jump to higher level/state (excited)
  • electromagnetic radiation/energy/photon emitted on return
44
Q

red lije emitted has wavelength of 656 nm. Spectrum of moving star has wavelength of 720 nm. Is the star approaching the earth? why?

A

no, wavelength has increased

45
Q

a sound wave is diffracted as it passes through a doorway but a light wave is not. Why?

A

-wavelength of light is much less than wavelength of sound

46
Q

what two phenomena occur when light passes through pair of narrow slits?

A
  • diffraction

- interference

47
Q

a pattern is formed on the screen when ligh passes through narrow slits. Explain how pattern is formed

A
  • waves overlap
  • constructive interference gives bright fringes
  • destructive interference gives dark fringes
48
Q

diffraction experiment: effect of increasing wavelength of light

A
  • distance between fringes increases

- pattern more spread out

49
Q

diffraction experiment: effect of increasing distance between slits

A
  • distance between fringes decreases

- pattern less spread out

50
Q

experiment to demonstrate resonance

A
  • two tuning forks of same freq on a wooden board
  • set one tuning fork vibrating
  • second tuning fork starts vibrating
51
Q

waves in a pip open at both ends vs closed at one end diagrams

A

in notes

52
Q

relationship between sound intensity and distance from source to an observer

A

I ∝ 1/d²

53
Q

Sound intensity

A

power per unit area

I = P/A
notation

54
Q

experiment to demonstrate light waves are transverse waves + experiment to demonstrate polarisation

A
  • light source + two pieces of polaroid
  • rotate one polaroid relative to other + light intensity decreases (to zero)
  • polarisation indicates transverse waves
55
Q

factors that affect freq of a stretched string

A
  • tension

- mass per unit length

56
Q

diagram of string when it vibrates at its second harmonic (2005 q12c)

A

3 nodes, 2 antinodes

57
Q

the human ear is more sensitive to certain frequencies of sound. How is this taken into account when measuring sound intensity levels?

A
  • dBa scale is used

- sound level meter modified so that it responds more to sounds between 2kHz and 4kHz

58
Q

Explain why a musical tune does not sound the same when played on diff instruments

A

diff instruments emit diff combinations of overtones/harmonicsCoh

59
Q

conditions necessary for total destructive interference to occur

A
  • same amplitude

- out of phase when crest meets trough

60
Q

what type of harmonics is produced by a clarinet (closed at one end)

A

odd

61
Q

The frequencies were identified from a sound: 550 Hz, 1100 Hz, and 1651 Hz. What name is given to this set of frequencies?

A

overtones / harmonics

62
Q

what are stationary waves?

A
  • amplitude of wave at any point is constant

- there is no net transfer of energy

63
Q

how are stationary waves produced?

A

waves with same freq + amplitude travelling in opposite directions meetq

64
Q

freq of a stretched string depends on…

A

tension
length
mass per unit length

65
Q

explain, with aid of labelled diagrams, why pipe open at only one end produces half the no. of harmonics as pip open at both ends

A

diagrams of first and other harmonic for open and closed pipes

-all harmonics for open pipe, only odd harmonics for closed pipe

66
Q

application of stress polarisation

A

checking for defects

67
Q

standing wave in a pipe calculations

A

count how much of a wavelength is in the pipe and use that in calculations if you are not finding the fundamental freq

ie learn off those pipe and wavelengths diagrams

68
Q

sound intensity formula (not in log tables)

A

SI = power / area

69
Q

apparent freq max and min

A

use both c-u and c+u to find max and min

70
Q

finding wavelength of stretched string given the length of it

A

treat it as like pipe open at both ends, wavelength = 2l

LC Physics (25 decks)

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