Propagation of Sound In this presentation you will: explore how sound is propagated ClassAct SRS...

Propagation of Sound

In this presentation you will: explore how sound is propagated

ClassAct SRS enabled.


Sound travels in waves. How do these waves travel through the air and other materials?

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In this presentation, you will explore how waves travel.


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Types of Waves

A wave is a disturbance that travels from one place to another.

There are many kinds of waves. Water waves, light waves, and radio waves are just a few that come to mind.

There are two main types of waves:

Some of these waves can travel through a vacuum. Others need a medium in which to travel.

Electromagnetic waves - these can travel through a vacuum. Mechanical waves - these need a medium through which to travel.


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Electromagnetic Waves

Electromagnetic waves travel as a wave of electrical and magnetic energy. They can travel through a vacuum.

Electromagnetic waves include light waves, infra-red, ultra-violet and gamma radiation, as well as radio and television signals.


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Mechanical Waves

Mechanical waves travel as a disturbance in a material.

Mechanical waves include sound waves and water waves.

The medium itself does not move. It oscillates about a mean position. Only the energy of the wave is propagated.

It follows that mechanical waves must have a medium through which to travel. They cannot travel through a vacuum.


1

Which of the following statements is true?

Question

A) All waves can travel through a vacuum.

B) Electromagnetic waves require a medium to travel through.

C) Mechanical waves can travel through a vacuum.

D) Electromagnetic waves can travel through a vacuum.


2

Which of the following is NOT an electromagnetic wave?

Question

A) Water wave

B) Light wave

C) Radio wave

D) Gamma wave


3

Which of the following cannot travel through a vacuum?

Question

A) A sound wave

B) A light wave

C) A radio wave

D) A gamma wave


Distance

Properties of Waves

If we examine a wave over a distance, at a single moment in time, we can define the following properties:

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Wavelength - length of one wave cycle

Amplitude - maximum displacement from the mean position

AmplitudeWavelength

Displacement

However, the wave is also moving. The distance the wave travels in one second is its speed.


Properties of Waves

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In addition to sharing all of these properties, all waves undergo reflection, refraction, interference and diffraction. These terms will be explained later in this presentation.

Frequency - the number of cycles passing the point per second

Period - the time for one cycle to pass a given point

1 sFrequency = 2 cycles/sec (2 Hz)

Time

PeriodDisplacement

If we consider a single point that is disturbed by the wave’s travel, and examine its movement over time, we can also define:


Direction of motion

Displacement

Distance

AC

DB

A1C1

D1

B1

Distance

λλ

λ Distance

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Transverse Waves

In a transverse wave, the disturbance caused by the wave is at right angles to the direction of motion.

Each element is displaced in exactly the same way, but at a different time.

Two adjacent points that are in phase will be exactly one wavelength (λ) apart.

Electromagnetic waves are transverse waves.

Places on a wave that are undergoing exactly the same motion at the same time are said to be in phase.


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Transverse Waves

We have already seen that all electromagnetic waves are transverse waves. However, not all transverse waves are electromagnetic waves.

One example of a transverse mechanical wave can be formed by moving one end of a taut rope up and down.


4

Which two points on the diagram are in phase?

Question

A) A and A1

B) B and B1

C) C and C1

D) D and D1

B

A

C

DB1

A1 C1

D1


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Longitudinal Waves

Mechanical waves can be transverse waves or longitudinal waves.

One example of a mechanical longitudinal wave is a sound wave. Sound waves that are of higher frequency than the audible range are called ultrasonic waves.

A longitudinal wave has particles or elements that oscillate in the same direction as the direction of motion. This is quite different from transverse waves.

Longitudinal wave

Transverse wave


5

Which of the following is a longitudinal wave?

Question

A) Light wave

B) Ultra-violet wave

C) Sound wave

D) Radio wave


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Sound Waves

A sound wave travels through a material as a pressure wave. Sound can travel through solids, liquids and gases.

The speed of sound differs in different media. The table shows some of these values.

Sound cannot travel through a vacuum.

Velocity in km/s

3.7

4-5

5.1

1.51

1.5

0.343

1.284

Materials

Copper

Glass

Steel

Sea water at 20 °C

Oil

Air at 20 °C

Hydrogen

In general, the denser the material, the faster sound travels through it.


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Speed of Sound Waves

The speed of sound is also dependent on temperature and, in the case of a gas, on the pressure.

As the temperature increases, the density of the medium decreases, resulting in an increasing separation between the particles of the substance.

So, an increase in temperature causes a decrease in the speed at which sound travels through the medium.


6

The speed of sound increases in materials of increasing density. Is this true or false?

Answer True or False.

Question


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Properties of Sound Waves - Reflection

If a sound wave encounters an obstruction, then all or part of the wave will be reflected.

This reflection obeys the law of reflection: The angle of incidence is equal to the angle of reflection.

When the distance between the observer and the obstruction is large, the sound may be bounced back and heard as a separate sound – an echo.

Normal

Incident wave Reflected wave

Obstruction

Angle of q

Angle of q


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Properties of Sound Waves - Refraction

At a boundary between one medium and another, some of the wave will be reflected and some will pass into the new medium.

If the incident wave meets the boundary at 90°, then the wave passing into the new medium will continue in the same direction.

Sound wave Different medium

90°

Its frequency will always remain the same, but its speed and wavelength will change.


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Properties of Sound Waves - Refraction

If the wave is incident at an angle other than 90°, then the direction of the wave will change. This is called refraction.

The angle of the refracted wave will depend upon the angle of the incident wave, and the ratio of the speeds of the waves in the two media.

Incident wave Reflected wave

Refracted wave

AirGlass


7

A sound wave travels from air into water. Which of the following could NOT occur?

Question

A) The wave could speed up

B) The wavelength could increase

C) The frequency could increase

D) A portion of the wave could be reflected


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Properties of Sound Waves - Diffraction

A third effect that can occur when a wave hits a barrier is called diffraction.

Diffraction is the bending of the wave around the obstruction.

If the obstruction is small enough, the wave will recombine on the other side and it will appear as if the obstruction is not there.

Similarly, if a wave passes through a gap, it can spread out from the gap as if the gap was the source of the wave.


8

What is the bending of waves around a barrier called?

Question

A) Reflection

B) Refraction

C) Diffraction

D) Interference


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Combining Sound Waves - Interference

If two sound waves of exactly the same amplitude and frequency meet, then different outcomes are possible.

If the waves are exactly in phase, the result will be a wave of double the amplitude.

In phase means that the crest of one wave coincides with the crest of the other wave.

A

B

A+B


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If they are totally out of phase, the result will be a wave of zero amplitude.

Totally out of phase means that the crest of one wave coincides with the trough of the other wave.

Two waves that are in phase are said to have a phase difference of 0°, while two waves that are totally out of phase are said to have a phase difference of 180°.

A

B

A+B


9

Which picture shows two waves that are totally out of phase?

Question

A) B)

C) D)


S1 S2

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If there are two (or more) sources of sound waves, then the waves can interfere with each other.

Interference patterns can be heard if the sound is a constant note, such as is created by two loudspeakers connected to a signal generator.

If a person moves through the wave pattern, they will hear the sound intensity rise and fall.

Walking path


Summary

After completing this presentation you should be able to:

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show knowledge and understanding of the similarities and differences between mechanical and electromagnetic waves.

show knowledge and understanding of the similarities and differences between longitudinal and transverse waves.

show knowledge and understanding of some wave properties.

Propagation of Sound In this presentation you will: explore how sound is propagated ClassAct SRS...

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