# Mechanical Waves and Sound Ch. 17 Physical Science.

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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

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 animationSpeed 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