Sound - Leaving Cert Physics
Sound
Sound is a form of energy produced by vibrating objects.
Sound travels as a longitudinal wave motion (can not be polarised).
Sound is a wave motion because
1. Reflection echos
2. Refraction sound is heard more clearly on a cold night
3. Diffraction we hear around corners
4. Interference rotate a vibrating tuning fork near your ear
sound rises constructive interference
sound falls destructive interference
To show the interference of sound and hence that sound is a wave motion
Walk slowly from A to B
Result : Sound rises constructive interference
Sound falls destructive interference
Conclusion : Speakers act as coherent sources producing an interference pattern
Note : Noise may be reduced using destructive interference
Sound of the same frequency and amplitude
is created electronically
crest of noise meets trough of sound producing destructive interference
To show that sound needs a medium to travel through
The air is removed by the vacuum pump
Result : We can still see the bell is ringing
But we can no longer hear the bell
Conclusion : Light is an electromagnetic wave it can travel through a vacuum
Sound is a mechanical wave it can not travel through a vacuum
Speed of sound in air = 340 m s-1
i) In general the more dense the medium the faster sound travels
ii) The speed of sound in a gas increases with temperature
Refraction of sound :
Resonance : is the response of a body to vibrations of its own natural frequency
e.g. singer breaks glass , soldiers on bridge
To demonstrate resonance :
1. Get two tuning forks of the same frequency
2. Set one vibrating
3. Hold it close to but not touching the second fork
Result : The second tuning fork starts vibrating
Characteristics of notes
Loudness : depends on the amplitude of the wave
Pitch : depends on the frequency of the wave
High pitch == high (big) frequency
Low pitch == low (small ) frequency
Overtones : are frequencies that are multiples of a certain frequency
( any resonant frequency above the fundamental frequency )
f = the given frequency
2f = the first overtone
3f = the second overtone
Quality : The quality of a note depends on the number of overtones that are present
Fundamental Frequency : Is the simplest mode of vibration of a string or the air in a pipe
Harmonics : Are frequencies that are multiples of the fundamental frequency
f = the first harmonic
2f = the second harmonic
3f = the third harmonic
Stationary waves in a string :
[pic]
Fundamental frequency Second harmonic Third harmonic
First harmonic
Stationary waves in a closed pipe : ( trombone , saxophone )
Only odd numbered harmonics present
L = (/4 L = 3(/4 L = 5(/4
[pic]
Stationary waves in an open pipe : ( tin whistle , flute )
All harmonics may be present
L = (/2 L = ( L = 3(/2
[pic]
Frequency limits of audibility : are the highest and lowest frequencies
that can be heard by a normal human ear
Threshold of hearing : is the smallest sound intensity detectable
by the average human ear at a frequency of 1 kHz
Sound intensity = Power ( Watt )
Area ( square meter)
Note : The sound is emitted in a sphere so the area = 4(r2
Power = Work ( energy ) (Joule)
Time (second)
Problem on Sound intensity [pic]
Answer : Sound intensity = Power Watt (meter )2
Area
Loudness depends on the amplitude of the wave
Pitch depends on the frequency of the wave
Power = 2.6 x 10-3 W Sound is emitted in a sphere around the source => Area = 4 ( r2
r = 8.6m
Sound intensity = Power = 2.6 x 10 -3 = 2.8 x 10 -6 W m-2
Area 4 ( (8.6)2
Doubling the sound intensity ( sound intensity level increases by 3dB
See also 2007 Q 12 b
Note : Sound intensites measured in Wm2 give a very large range
a more convient scale is the Bel
Sound intensity level : Is measured in Bels or decibels
This scale allows a large range of intensities to be represented
by a small range of numbers
Note : (sound intensity) x 2 = sound intensity level increases by 3 dB
Decibel adapted scale : The dBA scale is used in sound level meters.
The meter responds more to sounds with frequencies
between 2,000 Hz and 4,000Hz just like the ear.
Factors that determine the fundamental frequency of a string
1. f ( 1 Frequency is inversely proportional to the length
L
2. f ( ( T Frequency is directly proportional to the square root of the tension
3. f ( 1 frequency is inversely proportional to the
( ( square root of the mass per unit length
Combine the 3 factors f = 1 (T L = length (metre) T = tension (newtons)
2L (( ( = mass per unit length (Kg per m )
½ = constant of proportionality
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