CHAPTER 16 Waves and Sound
Objectives: After completion of this module, you should be able to: Demonstrate your understanding of transverse and longitudinal waves. Define, relate and apply the concepts of frequency, wavelength, and wave speed. Define sound.
Mechanical Waves A mechanical wave is a physical disturbance in an elastic medium. Consider a stone dropped into a lake. Energy is transferred from stone to floating log, but only the disturbance travels. Actual motion of any individual water particle is small. Energy propagation via such a disturbance is known as mechanical wave motion.
Periodic Motion Simple periodic motion is that motion in which a body moves back and forth over a fixed path, returning to each position and velocity after a definite interval of time. f 1 T Period, T, is the time for one complete oscillation. (seconds,s) Amplitude, A Frequency, f, is the number of complete oscillations per second. Hertz (s -1 )
Example: The suspended mass makes 30 complete oscillations in 15 s. What is the period and frequency of the motion? T 15 s 30 cycles 0.50 s Period: T = 0.500 s f 1 T 1 0.500 s Frequency: f = 2.00 Hz
A Transverse Wave In a transverse wave, the vibration of the individual particles of the medium is perpendicular to the direction of wave propagation. Motion of particles Motion of wave
Longitudinal Waves In a longitudinal wave, the vibration of the individual particles is parallel to the direction of wave propagation. v Motion of particles Motion of wave
Water Waves An ocean wave is a combination of transverse and longitudinal. The individual particles move in ellipses as the wave disturbance moves toward the shore.
Periodic Wave Motion A vibrating metal plate produces a transverse continuous wave as shown. For one complete vibration, the wave moves a distance of one wavelength, λ as illustrated. A l B Wavelength, l, is distance between two particles that are in phase.
Velocity and Wave Frequency. The period, T, is the time to move a distance of one wavelength. Therefore, the wave speed is: l 1 v but T so v f l T f The frequency, f, is in something/sec, or s -1 or hertz (Hz). The something can be cycles, revolutions, beat, ect. The velocity of any wave is the product of the frequency and the wavelength: v fl
Production of a Longitudinal Wave l l An oscillating pendulum produces condensations and rarefactions that travel down the spring. The wave length, λ, is the distance between adjacent condensations or rarefactions.
Velocity, Wavelength, Speed l Frequency f = waves per second (Hz) Wavelength l (m) v s t v fl Velocity v (m/s) Wave equation
Example: An electromagnetic vibrator sends waves down a string. The vibrator makes 600 complete cycles in 5 s. For one complete vibration, the wave moves a distance of 20 cm. What are the frequency, wavelength, and velocity of the wave? f 600 cycles ; 5 s f = 120 Hz The distance moved during a time of one cycle is the wavelength; therefore: l = 0.020 m v = fl v = (120 Hz)(0.02 m) v = 2.40 m/s
Example A harmonic wave is traveling along a rope. It is observed that the oscillator that generates the wave completes 40.0 vibrations in 30.0 s. Also, a given maximum travels 425 cm along a rope in 10.0 s. What is the wavelength? f v v cycles 40 1.33 Hz sec 30 x 0.425 0.425 m / t 10 0.425 lf l 1.33 wave s 0.0319 m/s
Definition of Sound Sound is a longitudinal mechanical wave that travels through an elastic medium. Many things vibrate in air, producing a sound wave. Source of sound: a tuning fork.
Is there sound in the forest when a tree falls? Sound is a physical disturbance in an elastic medium. Based on our definition, there IS sound in the forest, whether a human is there to hear it or not! The elastic medium (air) is required!
Sound Requires a Medium The sound of a ringing bell diminishes as air leaves the jar. No sound exists without air molecules. Batteries Vacuum pump Evacuated Bell Jar
Graphing a Sound Wave. Sound as a pressure wave The sinusoidal variation of pressure with distance is a useful way to represent a sound wave graphically. Note the wavelengths, λ, defined by the figure. Sound Waves Demonstration
Sound Waves The production of sound involves setting up a wave in air. To set up a CONTINUOUS sound you will need to set a standing wave pattern. Three LARGE CLASSES of instruments Stringed - standing wave is set up in a tightly stretched string Percussion - standing wave is produced by the vibration of solid objects Wind - standing wave is set up in a column of air that is either OPEN or CLOSED Factors that influence the speed of sound are density of solids or liquid, and TEMPERATURE. See the chart on page 481 for the speed of sound in different substances. This will help you with a couple of problems.
Sound Waves Audible waves- longitudinal waves with a frequency range of 20 to 20,000 Hz. Infrasonic waves- Below the audible range. Those sounds below 20 Hertz. Ultrasonic waves- Above" the audible range. Those sounds above 20,000 Hertz. http://www.youtube.com/watch?v=s 8QVe7d3q8U&feature=kp
DOPPLER EFFECT This property is named for the Austrian physicist Christian Doppler. This property is a variation in the perceived sound frequency due to the motion of the sound source, the observer, or both. In general, the frequency is higher as you approach a source and less when you move away from it.
Assignment Ch 16, Pages 500 502, #1, 3, 4, 6, 9, 39, 41, 43