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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