Acoustics

ACOUSTICSThe Science of Sound

Imagine our world without sound.

Sound brings things to life.

WHAT THEN IS SOUND?

Sound Defined

All sounds are produced by the

vibrations of material objects.

It is a longitudinal wave that carries

energy produced by vibrations of matter

and is propagated through a

medium such as solid, liquid or gas.

Not all sounds are audible

Audible range for humans is between 20

Hz to 20000 Hz

Lower than this range is classified as

INFRASONIC

Higher than this range is classified as

ULTRASONIC

The reason for hearing

Some animals can detect ULTRASONIC frequencies

50,000 Hz 100,000 Hz

How about INFRASONICS?

Sources include earthquakes, thunder,

volcanoes and waves produced by

vibrating heavy machinery

May pose damage to human body including

irritation of internal organs

Sound is a pressure wave

Compression

Rarefaction

THE SPEED OF SOUND

Prelude

Sound is a mechanical wave.

Its propagation depends on the properties

of a medium.

Can you hear me?

How about here?

Factors Affecting Sound Speed

Phase of the medium

Density and elasticity of the medium

Temperature

Phase of the Medium

Density and Elasticity of the Medium (Solid)

The denser and more elastic the medium,

the faster the transmission of sound.

v – velocity of sound

Y – elastic modulus of

the solid medium

ρ – density of the solid

medium

Sample Problem 1

Find the speed of sound in an aluminum rod

at a temperature of 20°C. The Young’s

modulus for aluminum is 70 x 109 N/m2 and

its density is 2.7 x 103 kg/m3.

Density and Elasticity of the Medium (Liquid)

The denser and more elastic the medium,

the faster the transmission of sound.

v – velocity of sound

B – bulk modulus of the

solid medium

ρ – density of the liquid

medium

Sample Problem 2

Find the speed of sound in water at a

temperature of 20°C. The bulk modulus of

water is 2.2 x 109 N/m2 and its density is 1.0 x

103 kg/m3.

The Speed of Sound in Various Solid Media

Medium Speed (m/s)

Aluminum 5100

Copper 3500

Iron 4500

Glass 5200

Polystyren

e

1850

The Speed of Sound in Various Liquid Media

Medium Speed (m/s)

Alcohol 1125

Mercury 1400

Water 1500

Sound Speed in Air and Temperature

As temperature increases, sound speed in

air increases at a definite rate.

Vsound = (Vsound at 0°C + 0.6T°C)m/s

Vsound at 0°C = 331 m/s

Sample Problem 3

In the cold months of December and

January, the temperature in Baguio can go

down to 10°C. What is the speed of sound in

air?

Sample Problem 4

An automatic focus camera is able to focus on objects by use of an ultrasonic sound wave. The camera sends out sound waves that reflect off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. If the temperature of the air is 20°C and the infrasonic frequency returns to the camera 0.150 seconds after leaving the camera, how far away is the object?