Mechanical Waves and Sound

advertisement

Mechanical Waves and Sound

Ch. 17

Physical Science

Mechanical Waves

&

Properties of Mechanical

Waves

17.1 / 17.2

Physical Science CIA

What are mechanical waves?

What do you think waves carry?

Can you see waves?

Examples?

Mechanical Waves

• Mechanical waves are disturbances in matter that carry energy from one place to another.

– Usually require matter through which to travel

– The matter a wave travels through is called a medium .

• Medium can be a solid, liquid, or gas

• Some waves can travel through space…with no medium!

How are mechanical waves created?

Creation of mechanical waves:

• Need a source of energy!

• That energy causes a vibration to travel through the medium

Types of Mechanical Waves

• Transverse:

– A wave that causes the medium to vibrate at right angles to the direction of the wave

Parts of a transverse wave:

Types of Mechanical Waves, ctd.

• Transverse

• Longitudinal:

– A wave in which the vibration of the medium is parallel to the direction the wave travels

Parts of a longitudinal wave:

Remember !

• A wave doesn’t move the medium…it’s just energy traveling through the medium!

Transverse and Longitudinal Wave

Which is which?

Types of Mechanical Waves, ctd.

• Transverse

• Longitudinal

• Surface:

– A wave that travels along a surface separating two media

Period vs Frequency

• Period – (T) – The time it takes for one cycle (= how long?)

– Measured in seconds

• Frequency – (f) – The number of cycles in a given time (= how many?)

– Measured in Hertz (Hz)

• Frequency is the inverse of the Period f = 1 / T

Speed of a wave

• We find speed by distance divided by time.

– The same holds true for waves!

• Speed of a wave = Wavelength/Period

• Speed of a wave =

Wavelength*Frequency

Speed of a Wave

• The speed of a wave is constant within a medium.

• The speed can change when a wave enters a new medium

• All waves of the same type travel at the same speed

– This means wavelength is inversely proportional to frequency!

• If the wavelength increases the frequency has to decrease!

• If the wavelength decreases the frequency has to increase!

Practice Problem #1

A wave in a spring has a wavelength of 0.1 meters and a period of 0.2 seconds. What is the speed of the wave?

• L

• G

• E

• P

• S

Practice Problem #2

Find the wavelength of a wave in a rope that has a frequency of 2.0 Hz and a speed of

0.4 meters/second.

• L

• G

• E

• P

• S

Behavior of Waves

17.3 Physical Science CIA

What happens when…

• A wave meets a hard surface like a wall?

• A wave enters a new medium?

• A wave moves around an obstacle?

• A wave meets another wave?

A wave meets a hard surface like a wall?

• Reflection

– A wave bounces off a surface that it can not pass through

• Reflection does not change the speed or frequency of the wave, BUT the wave can be flipped upside down!

How?

A wave enters a new medium?

• Refraction

– The bending of a wave as it enters a new medium

• The wave bends because as it enters a new medium it either speeds up or slows down!

Refraction

Refraction

A wave moves around an obstacle?

• Diffraction – the bending of a wave as it moves around an obstacle or passes through a narrow opening

Diffraction

A wave meets another wave?

• Interference – when two or more waves overlap and combine together.

– Constructive

– Destructive

Constructive Interference

• Two or more waves combine to produce a wave with a larger displacement

Destructive Interference

• Two or more waves combine to produce a wave with a smaller displacement

Standing Waves

• Occur because of interference!

• A standing wave is a wave that appears to stay in one place.

Parts of a Standing wave

• Node – Where there is no displacement of the medium in a standing wave

• Antinode – Where there is maximum displacement of the medium in a standing wave

How do standing waves fit on a string?

• Standing waves only form if a half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating string

Sound and Hearing

17.4 Physical Science CIA

Properties of Sound Waves

• What type of waves are sound waves?

Properties, ctd.

Sounds behave in certain ways because of the properties of sound waves. Some properties are:

• Speed

• Intensity & Loudness

• Frequency & Pitch

Speed

• Speed of sound = 342 m/s

…THAT’S 765 mph!

• Sound speed varies, depending on medium

– Travel fastest in solids, slowest in gases

– Travel fastest in the most dense media

– Speed of Sound animation

Intensity

• Intensity: the rate at which a wave’s energy flows through an area

• Sound intensity depends on

– Amplitude

– Distance from source

• Measured in decibels (dB)

Loudness

• Subjective ! (This means it depends on the person who is hearing it.)

• Loudness is a personal, physical response to the intensity of sound.

• As intensity increases, so does loudness, but loudness also depends on the listener’s ears and brain.

Frequency & Pitch

• Frequency of a sound wave depends on how fast the source of the sound is vibrating.

• Pitch is how we hear frequency of sound waves

• Pitch depends on frequency…high frequency sounds are high pitched, and low frequency sounds are low pitched.

• Pitch also depends on age and health

Frequency and

Pitch animation

Ultrasound

• Most people hear sounds between 20 and

20,000 Hz.

– Infrasound – sound at frequencies lower than people usually hear

– Ultrasound – sound at frequencies higher than people usually hear

• Used in technologies such as sonar and ultrasound imaging

Ultrasound, ctd.

• Sonar – a technique used to determine the distance to an object under water.

• Ultrasound – medical technique used to take pictures of different organs (or a fetus!)

The Doppler Effect

• Where have you heard of the term

“Doppler”?

• Doppler Effect : a change in sound frequency (pitch) caused by the motion of the sound source, the listener, or both

• Why do we observe this?

• Doppler Effect animation

Hearing and the Ear

• Your ear has a membrane that vibrates when sound waves hit it…what is this membrane?

• The Ear animation

Hearing & the Ear

Ear consists of 3 main parts

• Outer Ear – gathers and focuses sound

• Middle Ear – receives and amplifies vibrations

• Inner Ear – uses nerve endings to sense vibrations and send signals to the brain

Reproduction of Sound

• What are some ways that sound is stored?

• How do you think sound is stored, and then played back?

Reproduction of Sound

• To record: sound waves must be converted into electronic signals that can be stored

• To reproduce: electronic signals are converted back into sound waves

Music

• Musical instruments change their pitch by changing the frequency of the waves they produce… how do they do this?

• Reflection animation

Download