Sound / Waves + Wave motion Flashcards
(70 cards)
1
Q
Doppler effect
A
Apparent change in freq. of waves due to motion of the source or observer
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
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
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
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
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
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)
8
Q
Speed
A
distance travelled in relation to time
9
Q
Speed of sound
A
speed of sound in air: 340 m/s
will be diff. in diff. media
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
11
Q
Sound properties
A
longitudinal wave
compression, rarefaction
molecules vibrate parallel to direction of wave
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
13
Q
Nodes
A
Points of zero disturbance,
destructive interference
14
Q
Antinodes
A
Points of maximum disturbance,
constructive interference
15
Q
What does speed of sound depend on?
A
depends on material that it passes thru + temperature of that material
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
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
18
Q
Characteristics of notes
A
- Amplitude and loudness
- Frequency and pitch
- Quality and overtones
- Frequency limits of audibility
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.
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
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
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.
23
Q
Dog whistle
A
- ultrasonic: above 20,000Hz
- Dogs + bats can hear up to 35,000
- Dog whistles operate at ultrasonic freq.
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
25
Resonance
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
26
Fundamental frequency of a stretched string formula
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
```
27
Fundamental frequency f
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
28
Closed pipe
a pipe closed at one end and open at the other
29
Pipe closed at one end
- 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
Stationery waves in a closed pipe
- 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
Speed of sound in air
- 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
Pipes open at both ends
All harmonics may be present
eg. tin whistle, flute, recorder are pipes open at both ends
33
Sound intensity level
- 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
Ear protection
- 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
what happens when you double sound intensity
Doubling sound intensity increases sound intensity levels by 3dB
36
Pipe open at both ends equation
λₙ = 2L/n
| for fundamental freq, n = 1
37
Pipe closed at one end equation
λₙ = 4L/n where n = odd numbers
| for fundamental freq, n = 1
38
types of electromagnetic radiation
learn off table, which is longest wavelength and highest freq and vice versa
39
demonstrate the doppler effect
-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
what causes the red shift in spectrum of a distant star
- stars move relative to earth
| - longer wavelength if star receding from earth
41
application of doppler effect
- calculate speeds of stars or galaxies
- speed traps
- radar
- medical imaging
- blood flow measurement (echo-cardiogram)
- temperature measurement
- underwater acoustics
42
how is infra-red radiation detected?
- thermometer
- temperature sensor/probe
- photographic film/plate
- by its heating effect
43
describe how an emission line spectrum is produced
- monatomic gas receives energy + electrons move/jump to higher level/state (excited)
- electromagnetic radiation/energy/photon emitted on return
44
red lije emitted has wavelength of 656 nm. Spectrum of moving star has wavelength of 720 nm. Is the star approaching the earth? why?
no, wavelength has increased
45
a sound wave is diffracted as it passes through a doorway but a light wave is not. Why?
-wavelength of light is much less than wavelength of sound
46
what two phenomena occur when light passes through pair of narrow slits?
- diffraction
| - interference
47
a pattern is formed on the screen when ligh passes through narrow slits. Explain how pattern is formed
- waves overlap
- constructive interference gives bright fringes
- destructive interference gives dark fringes
48
diffraction experiment: effect of increasing wavelength of light
- distance between fringes increases
| - pattern more spread out
49
diffraction experiment: effect of increasing distance between slits
- distance between fringes decreases
| - pattern less spread out
50
experiment to demonstrate resonance
- two tuning forks of same freq on a wooden board
- set one tuning fork vibrating
- second tuning fork starts vibrating
51
waves in a pip open at both ends vs closed at one end diagrams
in notes
52
relationship between sound intensity and distance from source to an observer
I ∝ 1/d²
53
Sound intensity
power per unit area
I = P/A
notation
54
experiment to demonstrate light waves are transverse waves + experiment to demonstrate polarisation
- light source + two pieces of polaroid
- rotate one polaroid relative to other + light intensity decreases (to zero)
- polarisation indicates transverse waves
55
factors that affect freq of a stretched string
- tension
| - mass per unit length
56
diagram of string when it vibrates at its second harmonic (2005 q12c)
3 nodes, 2 antinodes
57
the human ear is more sensitive to certain frequencies of sound. How is this taken into account when measuring sound intensity levels?
- dBa scale is used
| - sound level meter modified so that it responds more to sounds between 2kHz and 4kHz
58
Explain why a musical tune does not sound the same when played on diff instruments
diff instruments emit diff combinations of overtones/harmonicsCoh
59
conditions necessary for total destructive interference to occur
- same amplitude
| - out of phase when crest meets trough
60
what type of harmonics is produced by a clarinet (closed at one end)
odd
61
The frequencies were identified from a sound: 550 Hz, 1100 Hz, and 1651 Hz. What name is given to this set of frequencies?
overtones / harmonics
62
what are stationary waves?
- amplitude of wave at any point is constant
| - there is no net transfer of energy
63
how are stationary waves produced?
waves with same freq + amplitude travelling in opposite directions meetq
64
freq of a stretched string depends on...
tension
length
mass per unit length
65
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
diagrams of first and other harmonic for open and closed pipes
-all harmonics for open pipe, only odd harmonics for closed pipe
66
application of stress polarisation
checking for defects
67
standing wave in a pipe calculations
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
sound intensity formula (not in log tables)
SI = power / area
69
apparent freq max and min
use both c-u and c+u to find max and min
70
finding wavelength of stretched string given the length of it
treat it as like pipe open at both ends, wavelength = 2l
