Vibrations, Waves and Sound

Vibrations, Waves and Sound

Unit 7: Vibrations, Waves & SoundChapter 19: Waves • 19.1 Waves • 19.2 The Motion of Waves • 19.3 Wave Interference and Energy

Key Question: How do waves move? 19.1 Investigation: Waves in Motion Objectives: • Explain how waves move. • Compare and contrast transverse and longitudinal waves. • Use knowledge of longitudinal and transverse waves to describe water waves.

Waves • A waveis an oscillation that travels from one place to another. • If you poke a floating ball, it oscillates up and down. • The oscillation spreads outward from where it started.

Why learn about waves? Waves carry useful information and energy. Waves are all around us: light from the stoplight ripples in a puddle of electricity flowing in wires radio and television and cell phone transmissions

Recognizing waves around you Waves are present: when you see a vibration that moves. when something makes or responds to sound. when something makes or responds to light. when technology allows us to “see through” objects. when information travels through the air (or space) without wires.

Waves • Waves are a traveling form of energy because they can change motion. • Waves also carry information, such as sound, pictures, or even numbers.

Wave pulses • A wave pulseis a short ‘burst’ of a traveling wave. • It is sometimes easier to see the motion of wave pulses than it is to see long waves with many oscillations.

Transverse waves • A transverse wave has its oscillations perpendicular to the direction the wave moves. A • A wave pulse along a rope attached to a wall moves left to right, while the boys hand moves up and down.

Longitudinal waves • The oscillations of a longitudinal wave arein the same direction that the wave moves.

Longitudinal waves • A sharp push-pull on the end of the spring results in a traveling wave pulse as portions of the spring compress, then relax. • Sound waves are longitudinal waves. • Like a wave pulse on a spring, air molecules oscillate back and forth as sound travels.

Frequency, amplitude, and wavelength • You can think of a wave as a moving series of high points and low points. • A crestis the high point of the wave. • A troughis the low point.

Frequency • The frequencyof a wave is the rate at which every point on the wave moves up and down. • Frequency means “how often”.

Amplitude • The amplitude of a water wave is the maximum height the wave rises above the level surface.

Wavelength • Wavelength is the distance from any point on a wave to the same point on the next cycle of the wave. • The distance between one crest and the next crest is a wavelength.

The speed of waves • The speed of a water wave is how fast the wave spreads, NOT how fast the water surface moves up and down or how fast the dropped ball moves in the water. How do we measure the wave speed?

The speed of waves • A wave moves one wavelength in each cycle. • Since a cycle takes one period, the speed of the wave is the wavelength divided by the period.

The speed of waves • The speed is the distance traveled (one wavelength) divided by the time it takes (one period). • We usually calculate the speed of a wave by multiplying wavelength by frequency.

Calculating wave speed A wave has a wavelength of 0.5 meters, and its frequency is 40 hertz. What is the speed of the wave? • Looking for:…speed of the wave. • Given:…wavelength (0.5 m) and frequency (40 Hz). • Relationships: Use formula: speed = ƒ x  • Solution: …speed = 40 Hz × 0.5 m = 40 1/s× 0.5 m speed = 20 m/s

Cooking with waves • A microwave heats food by transferring wave energy to the food. • The magnetron is a device in a microwave oven that creates a wave with electricity. • The wave vibrates inside the cooking space at 2.5 gigahertz- the frequency best absorbed by water.

Standing waves on a string A wave that is confined between boundaries is called a standing wave. With all waves, resonance and natural frequency are dependent on reflections from boundaries of the system containing the wave.

Standing Waves and Harmonics The standing wave with the longest wavelength is called the fundamental. The fundamental has the lowest frequency in a series of standing waves called harmonics. The first five standing wave patterns of a vibrating string shows that patterns occur at multiples of the fundamental frequency.

Standing waves • Standing waves have nodes and antinodes. • A nodeis a point where the string stays at its equilibrium position. • An antinode is a point where the wave is as far as it gets from equilibrium.

Standing waves • It is easy to measure the wavelength of a standing wave on a string. • Two harmonics equals one wave!

Key Question: How do we make and control waves? 19.2 Investigation: Resonance and Standing Waves Objectives: • Describe how frequency, wavelength, and speed are related. • Measure the wavelength and frequency of a vibrating string. • Recognize and apply the concept of harmonics in resonant systems. • Define natural frequency and apply methods for changing the natural frequency of a system.

Waves propagation • Waves propagate, which means they spread out from where they begin. • When you drop a ball into water, some of the water is pushed aside and raised by the ball.

Wave motion • A wave front is the leading edge of a moving wave which is considered to be the crest for purposes of modeling. • The crests of a plane wave look like parallel lines. • The crests of a circular wave are circles.

Four wave interactions • reflection, • refraction, • diffraction, or • absorption. • When a wave encounters a surface, four interactions can occur:

Diffraction • Diffraction usually changes the direction and shape of the wave. • When a plane wave passes through a small hole diffraction turns it into a circular wave.

Key Question: How does changing the tension affect a vibrating string? 19.3 Investigation: Exploring Standing Wave Properties Objectives: • Apply an understanding of inertia and restoring force to describe the effects of increasing the tension of a vibrating string. • Explain how a string’s tension and mass affects its frequency and amplitude. • Describe how wave properties are applicable to musical instruments.

Superposition principle Interference happens when two or more waves mix together. When more than one wave is present, the total oscillation of any point is the sum of the oscillations from each individual wave.

Noise canceling headphones • Specialized headphones can create “anti-noise.” • A microphone in the headphone samples the noise and generates anti-noise, or sound that is 180 degrees out of phase with the noise. • The anti-noise uses superposition to reduce or muffle noise.

Constructive interference • Constructive interference happens when waves add up to make a larger amplitude. • Suppose you make two wave pulses on a stretched string. • One comes from the left and the other comes from the right. • When the waves meet, they combine to make a single large pulse.

Destructive interference • What happens when one pulse is on top of the string and the other is on the bottom? • When the pulses meet in the middle, they cancel each other out. • During destructive interference, waves add up to make a wave with smaller or zero amplitude.

Resonance and light • A wave has to be caught in a system with boundaries to show resonance. • Catch light between two perfect mirrors and we can get resonance of light waves. • This is exactly how a laser works!

Resonance and elastic string • Resonance elastic strings is created by adding new pulses so that each adds to the reflected pulse in constructive interference.

Waves and energy • The energy of a wave is proportional to its frequency. • Higher frequency means higher energy.

Waves and energy • The energy of a wave is also proportional to its amplitude. • Given two standing waves of the same frequency, the wave with the larger amplitude has more energy.

On January 12, 2010, a 7.0 magnitude earthquake struck the Caribbean nation of Haiti. Its capital, Port-au-Prince, was nearly destroyed. Many government buildings, schools, hospitals, and businesses collapsed. The powerful earthquake was caused by waves traveling through Earth. Waves that Shake the Ground

Vibrations, Waves and Sound