Ch 20 SOUND

• Sound is a compression wave in an elastic medium. These can include solids, liquids and gases or a plasma.

Human hearing can detect sound waves with frequencies between 20 Hz and 20,000Hz (20kHz).

Frequencies below 20Hz are called infrasonic. They are inaudible to human hearing. (We can’t hear them!)

• Frequencies above 20,000Hz are called ultrasonic. They are also inaudible to human hearing.

(Again, we can’t hear them, but some animal can!)

Media That Transmit Sound

•Any elastic material can transmit sound.

•Steel is a very good conductor of sound.

•Water is not as good a conductor as steel, but is better than air.

•Air is a poor conductor of sound

Speed of Sound

• Sound waves will have different speeds in different media. The speed also varies with temperature.

•Sound travels 15 times faster in steel than in air, and it travels 4 times faster in water than in air.

•Sound travels at about 330 m/s in air at 0 oC and 340 m/s in air at 20 oC.

Reflection of Sound•Echo - The reflection of sound.

•E.G. Singing in the shower.•Sound reflects off of a smooth surface at the same angle at which it is incident.

•(i.e. The angle of reflection equals the angle of incidence.)

Refraction• Refraction is the bending of the sound

wave. Sound waves bend when parts of the wave front travel at different speeds.

•This occurs when there are:

–Uneven Winds.

–Uneven Temperatures.

•The speed of sound is slightly faster in warm air than in cool air.

Energy in Sound Waves

• Sound generally transmits only a small amount of energy.

•(10,000,000 people talking at the same time = energy needed to light a common flashlight).

•Sound energy dissipates to thermal energy (heat) while sound travels through air.

• High frequency sound dissipates to thermal energy more rapidly than low frequency sound.

•Hence, low frequency sound has a greater range in air than high frequency sound. (Fog horns typically produce low frequency sound, but have a long range)

Forced Vibrations• The setting up of vibrations in an

object by a vibrating force.

Examples:

1) Sounding board of a musical instrument.

2) The box of a music box.

Natural Frequency A frequency at which an elastic

object naturally tends to vibrate if it is disturbed and the disturbing force is removed.

The natural frequency of an object depends on such factors as the elasticity and shape of the object.

• When the frequency of a forced vibration (driving frequency) matches the object’s natural frequency, a dramatic increase in amplitude occurs.

This is known as resonance.

Example: Pushing someone on a swing.

Resonance• The response of a body when a

forcing frequency matches its natural frequency.

Sound Interference

• Sound waves can interfere with each other just like transverse waves interfere.

+

+

Transverse Wave Interference

Longitudinal (Sound) wave interference

Beats

• When two tones of slightly different frequencies are sounded together, a fluctuation of the loudness of the combined sounds is heard.

•Alternating loud and faint sound can be heard. This periodic variation in loudness is called beats.

Beats

f1

f2

fbeat= |f1 - f2|

Standing Waves on a String

STANDING WAVES ON A STRING

SUPERPOSITION OF WAVES

Fundamental (1st harmonic)

2nd harmonic (1st overtone)

3rd harmonic (2nd overtone)

NODES

Antinodes

Parts of a Standing Wave on a String

L =

Fundamental (1st harmonic)

2nd harmonic (1st overtone)

L = /2

L1= /4

Tuning fork

Standing Waves in an air column – open tube at one end

Fundamental (first harmonic)

L2= 3/4

Third harmonic- open at one end

End of Chapter 19

Simulations

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