Mechanical Waves And Sound Powerpoint Notes w4g72

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:

! • 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 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 es 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