Pearson Prentice Hall Physical Science: Concepts in Action Chapter 17 Mechanical Waves and Sound

1. Explain what causes mechanical waves Objectives: 1. Explain what causes mechanical waves 2. Name and describe the three main types of mechanical waves

What Causes Mechanical Waves Def: A wave is a disturbance that carries energy through space or matter Def: What a wave travels through is the medium (sound-air, earthquake-ground) Def: Waves that require a medium to travel are called mechanical waves (majority of waves) Mechanical waves carry energy from one place to another by using matter (a medium) Mechanical waves are started with a vibration

Types of Mechanical Waves The three main types of mechanical waves are transverse waves, longitudinal waves and surface waves Particles in a medium can vibrate up and down or back and forth as a wave moves by Def: If the particles move up and down they will move perpendicular to the direction of the wave-called a transverse wave

Transverse waves take the shape of sine curves (looks like an s on its side) Def: High points are crests Def: Low points are troughs Def: Difference between high and low is called amplitude Bigger amplitudes mean more energy Def: in a longitudinal wave the medium moves in the same direction as the wave

Def: In longitudinal waves the bunched area is a compression (think of a slinky) Def: The spaced out areas are called rarefactions Def: Amplitude on a longitudinal wave is maximum deviation from normal density or pressure At the boundary between two mediums (on the ocean for example) surface waves develop Def: Surface waves are combinations of both types of waves The particles in the medium of a surface wave move back and forth and up and down resulting in a circle, but they end up where they started

17.2 Properties of Mechanical Waves Objectives: 1. Explain what determines the frequency of a wave 2. Solve problems for frequency, wavelength and speed 3. Describe how amplitude and energy are related

Frequency Def: periodic motion is any motion that repeats at regular time intervals Def: Frequency is how many waves pass by in a given time Frequency = 1/period = 1 / T = f It is measure in hertz (Hz) which is 1/s where s is seconds Humans can hear 20Hz to 20000Hz A wave’s frequency equals the frequency of the vibrating source producing the wave

Problems involving f, λ (wavelength) & speed Def: Wavelengths of waves are measured from one crest to the next (crest to crest OR trough to trough) or from one compression to the next It is represented by the Greek letter lambda whose symbol is λ The period of a wave is how long it takes for a complete wave to go by a spot, symbol is T Increasing the frequency (f) of a wave decreases the wavelength (λ)

Recall that speed = distance / time For waves we can use wavelength for distance (in meters, m) and period for time (s for seconds) Speed = wavelength X frequency v = λ * f The speed of a wave depends on what it travels through (medium) If the medium particles are closer together the energy from the wave can make vibrations easier So waves travels best in solids, then liquids, and worse in air

Amplitude and Energy Def: amplitude is the difference between crest and the rest position or point of origin Def: the rest position or point of origin is an imaginary line through the middle of the wave that separates the crest from the trough The more energy a wave has, the greater its amplitude

17.3 Behavior of Waves Objectives: 1. Explain reflection and refraction and how they affect waves 2. Identify several factors that affect the amount of wave diffraction 3. Discuss two types of interference 4. Explain what a standing wave is and detail the wavelengths that produce it

Reflection and Refraction Def: Reflection occurs when a wave meets a boundary and bounces off The wave cannot pass through the surface Reflection does not change the speed or frequency of a wave, but the wave can be flipped upside down Def: Refraction is the bending of a wave as it travels through different mediums When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side (ex: pencil in water)

Diffraction and Interference Def: Diffraction is the bending of a wave around an obstacle A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle The two types of interference are constructive interference and destructive interference

The combo of two or more waves at the same place at the same time causes interference Def: When two crests meet the interference is constructive-the wave becomes more energetic-the amplitude increases Def: When a crest and a trough meet they cancel each other and decrease the amplitude –destructive

Standing Waves Interference may cause standing waves- Def: Standing waves appear not to move along the medium Has areas of destructive interference where there is no vibration called nodes Def: a node is a point on a standing wave that has no displacement from the rest position

Areas of maximum interference called antinodes Def: an antinode is a point where a crest & trough meet midway between 2 nodes A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord

17.4 Sound and Hearing Objectives: 1. Describe the properties of sound waves and explain how sound is produced and reproduced 2. Describe how sound waves behave in applications such as ultrasound and music 3. Explain how relative motion determines the frequency of sound as the observer hears

Properties of Sound Waves Sound waves are longitudinal waves They have compressions are rarefactions Many behaviors can be explained by the properties of speed, intensity, loudness, frequency and pitch At 20°C in dry air, the speed of sound is 342 m/s Sound waves travel fastest in solids, slower in liquids and slowest in gases

Def: intensity is the rate at which a wave’s energy flow through a given area The decibel (dB) compares the intensity of different sounds Def: loudness is a physical response to the intensity of sound modified by physical factors As intensity increases, loudness increases Loudness also depends on the health of your ears and how your brain interpret sounds Def: pitch is the frequency of a sound as you perceive it

How Sound Waves Behave & Relative Motion Ultrasound is used in a variety of applications, including sonar and ultrasound imaging Def: Sonar is a technique for determining the distance to an object under water The pitch of a sound is determined by the frequency Higher pitch means faster frequency As the source of the waves moves it changes the frequency (this is the Doppler Effect) As it moves toward you the pitch rises and away from you the pitch lowers = Doppler Effect

For us to hear, the outer ear gathers & focuses sound into the middle ear where the vibrations are received and amplified The inner ear uses nerve endings to sense vibrations and send signals to the brain Sound is recorded by converting sound waves into electronic signals that can be processed and stored Sound is reproduced by converting electronic signals back to sound waves

Most musical instruments vary pitch by changing the frequency of standing waves Def: resonance is the response of a standing wave to another wave of the same frequency Musical instruments often use resonance to amplify sound