Warm up

1. Differentiate between infrasonic and ultrasonic.

2. Compare and contrast a rarefaction and a compression.

3. What is the source of all sounds?

Homework

Read and notes pg. 26.4 – 26.9

Supplement pg.

SOUNDChapter 26

THE ORIGIN OF SOUND

All sounds are produced by the vibrations of objects

A.k.a a disturbance Waves are created by some form

of a disturbance or vibration

THE ORIGIN OF SOUND

As the particles are moved from their rest position, they exert a force on the adjacent particles

Then transfer their kinetic energy

Thus sound energy travels outward from the source.

Mechanical waveMechanical waves require a medium in order to transport their energy

Sound waves are incapable of traveling through a vacuum

Examples:

Slinky waves, water waves, stadium waves and sound waves

creating a pressure disturbance consisting of an alternating pattern of

compressions and rarefactions

Wave propagation

Wave propagation

For example:

a vibrating guitar string forces surrounding air molecules to be compressed and expanded

The amount of energy transferred to the medium is equal to the amplitude squared of the wave

Pressure Wave?In terms of pressure what is a sound wave composed of?

Illustration of the pressure-time fluctuations

Do not conclude that sound is a transverse wave which has crests and troughs

Longitudinal WaveDefinition?

Longitudinal Wave parts

RAREFACTION a disturbance in air (or matter) in which the

pressure is lowered The transfer of a low pressure air front

Simulated guitar string

Longitudinal Wave parts

COMPRESSION A pulse of compressed air a disturbance in air (or matter) in which the

pressure is increased The transfer of a high pressure air

front

Whitney quickly and forcefully opens the door. Predict what will happen to the papers

The air in the car will be rarefied and the papers will go flying.

Bobby sees this, gets very upset. He forcefully closes the door. Now, predict what will happen to the papers

The air in the Hummer will be compressed, and the papers will go flying again!

Bobby’s hummer is a mess! There are papers piled up on the seats. Whitney, decides to straighten up.

Why are sound waves longitudinal waves?

How is wavelength measured?

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MEDIA THAT TRANSMIT SOUND

Sound can travel through: Air Solids Liquids

Sound cannot travel through… A vacuum There is nothing to compress & expand

The two main factors affecting the speed of sound…

The medium

The temperature

Minor factors Humidity Air pressure

Density & the Speed of sound

The speed of sound is not always the same

It is easier for sound waves to go through solids than through liquids because…

The closer the molecules are to each other the tighter their bonds,

the less time it takes for them to pass the sound to each other

Density & the Speed of sound

Density affects the speed of sound in a material

Density describes the mass of a substance per volume

Material Speed of Sound

Rubber 60 m/s

Air at 40oC 355 m/s

Air at 20oC 343 m/s

Lead 1210 m/sGold 3240 m/s Glass 4540 m/s

Copper 4600 m/sAluminum 6320 m/s

Temperature and the speed of sound

Heat, like sound, is a form of kinetic energy

higher temperatures have more energy, therefore they vibrate faster

molecules vibrating faster means, sound waves can travel more quickly

If no temperature is stated we will use 340 m/s

Speed of Sound

The speed of sound in dry air is given approximately by

V sound in air = (331 + 0.6 * Tc ) m/s

@ 0o C speed of sound is approx. 331 m/s or 742 mi/hr

@ 20oC speed of sound is approx. 343 m/s or 769 mi/hr

If no temperature is stated we will use 340 m/s

THE SPEED OF SOUND

The speed of sound is slower than the speed of light

Speed of Light = 3.00 x 108 m/s

How far away is the storm?

If you see a lightning flash and five seconds later you hear the thunder how far away is the storm?

If v = d / t then d = v * t

d = 340 m/sec * 5.0 sec

d = 1700 meters away

Assume speed of sound to be 340 m/sec or .21 miles/sec

d = 0.21 miles/sec * 5.0 sec

d = 1.1 miles awayExtra practice: Suppose the delay is 8 seconds.

Frequency vs. Speed Frequency refers to the… number of vibrations that an

individual particle makes in a specific period of time

Frequency refers to how often a wave passes through a certain point

Speed refers to how fast a wave passes through the point.

PITCH & FREQUENCY

Pitch – impression of frequency

Some might say the two are synonymous

High-pitched vs. Low-Pitched

NATURAL FREQUENCY

All objects have a natural frequency or set of frequencies at which they vibrate

A natural frequency is one at which minimum energy is required to force vibrations

Natural frequency depends on factors such as the density, elasticity and shape of an object

FORCED VIBRATION

Forced vibration is when one object is connected to a larger one,

forcing its sound vibrations on the larger surface

The resulting sound is relatively louder

RESONANCE

Resonance – when the frequency of a forced vibration of an object matches the object’s natural frequency, there is a dramatic increase in amplitude.

To resonate, objects need a force to pull them back to their starting position

For ex: pumping a swing in rhythm with its natural frequency produces larger amplitudes.

Energy Transport and the Amplitude of a WaveA wave transports energy along a medium without transporting matter

The amount of energy carried by a wave is related to the amplitude of the wave

Intensity

The amount of energy which is transported past a given area of the medium per unit of time

The greater the amplitude,

the greater the rate at which energy is transported, and therefore the more intense the sound wave

LOUDNESS Loudness is a physiological sensation sensed

in the brain It is subjective, but relates to sound intensity Intensity of sound is measured with the

decibel (dB) 0 dB is the threshold of hearing for a normal

ear An increase of 10 dB means that it increase by a

factor of 10

10 dB = 10 x 1 dB 20 dB = 10 x 10 dB 30 dB = 10 x 10 x 10 dB

Decibel quiz

2. The decibel level that can break bones in the ear 2. (150)dB

1. (130)dB

3. (30)dB

1. The decibel level that causes pain

3. The decibel level that is 1000 times more powerful than 10 dB

Common

dB

levels

Human Hearing Thresholds

Infrasonic frequencies below 20 hertzUltrasonic frequencies above 20,000 hertzWe cannot hear these sounds

Decibel HistoryA bel (symbol B) is a unit of measure of ratios

Invented by engineers of the Bell Telephone Laboratory, it was originally called the transmission unit or TU

Renamed in 1923 in honor of the laboratory's founder and telecommunications pioneer Alexander Graham Bell

Deci (symbol d) is the metric prefix meaning ten

Reflection of Sound

Sound reflects or bounces from all surfaces-walls, ceiling, floor, furniture, and people

An echo is reflected sound

Sound and light follow the same rules about reflections

Reflection of Sound Acoustics

the study of the reflective properties upon surfaces

Reverberations Persistence of a sound, as in a echo ,

due to multiple reflections

Reflection of Sound

Reflection of sound in a room makes it sound lively and full

In a concert hall, highly reflective surfaces are often placed behind and above the stage to direct sound out to an audience

Reflection of Sound

When sound reflects off a special curved surface called a parabola

it will bounce out in a straight line no matter where it originally hits

Many stages are designed as parabolas so the sound will go directly into the audience, instead of bouncing around on stage

Example: Parabolic Stage

Location: HOLLYWOOD BOWL

Reflection of Sound

If the parabola is closed off by another curved surface, it is called an ellipse

Sound will travel from one focus to the other, no matter where it strikes the wall

A whispering gallery is designed as an ellipse

In here if you and a friend stand in opposite corners, as if being punished, you can carry on a whispered conversation that others can’t hear

Located in Grand Central Station

Example: Ellipse Design

Refraction or (bending) of Sound

Sound waves are refracted or bent when parts of a wave front travel at different speeds

this happens in uneven winds or…

when sound is traveling through air of uneven temperatures

Refraction of Sound

On a warm day the waves tend to bend away from the warm ground…

making it appear that the sound does not carry well

On a cold day the speed of sound is reduced on the ground…

causing the waves to bend towards the earth, carrying the sound longer distances

Pop Quiz

How does sound reach you when are in a different room than the source?

Refraction and Reflection!

Pop Quiz

http://www.ewart.org.uk/science/Waves/wav2.htm

INTERFERENCE

Waves can be made to interfere with each other.

When the crests of one wave overlap those of another wave, there is a constructive interference and increase in amplitude.

When the crests of one wave overlap the overlap the troughs of another, there is a destructive interference and a decrease in amplitude.

INTERFERENCE AND SOUND

Interference effects loudness If one is equally distant from two

speakers simultaneously triggering identical sound waves, the sound is louder because the waves add.

Destructive interference is usually not a problem, and there is enough reflection to fill in canceled spots.

“Dead spots” are present in poorly designed theaters and gyms.

. .

BEATS Periodic variation in the loudness of

sound is called a beat.

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

Beats can occur with any kind of wave.