Chapter 19 Vibrations & Waves Part IV: Sound Chapter 19 Vibrations & Waves 24-Apr-17 Physics 1 (Garcia) SJSU

Vibrations Vibrations are a common phenomenon. Oscillating motion of a mass on a spring 24-Apr-17 Physics 1 (Garcia) SJSU

Period Time required for a full oscillation (one round trip) is called the period of oscillation. Pendulum that is about one meter long has a period of two seconds per oscillation. Note: Measure the period of a pendulum in lab. 24-Apr-17 Physics 1 (Garcia) SJSU

Frequency Frequency is the inverse of the period, (Frequency) = 1 For example, for a period of 2 seconds per oscillation, the frequency is ½ oscillation per second or ½ Hertz. 1 Hertz = 1 oscillation per second 1 (Period) 24-Apr-17 Physics 1 (Garcia) SJSU

Amplitude The distance from the rest position is the amplitude of oscillation. Amplitude

Waves Concept of vibrations extends into the phenomenon of wave motion. String Sound Water waves Radio Light waves 24-Apr-17 Physics 1 (Garcia) SJSU

Transverse Waves For transverse waves the wave’s amplitude is perpendicular to the wave’s motion. Amplitude Wave Motion Amplitude 24-Apr-17 Physics 1 (Garcia) SJSU

Doing “The Wave” Synchronized standing and sitting by a stadium’s crowd is an example of a transverse wave. Video analysis indicates that it takes only a few dozen fans leaping to their feet with their arms up to trigger a wave. Once started, it usually rolls in a clockwise direction at a rate of about 40 feet per second, or about 20 seats per second. At any given time, the wave pulse is about 15 seats wide. 24-Apr-17 Physics 1 (Garcia) SJSU

Longitudinal Waves For longitudinal waves, amplitude and wave motion are parallel. Wave Motion Amplitude Amplitude 24-Apr-17 Physics 1 (Garcia) SJSU A crowd can do a longitudinal wave by moving side-to-side instead of up-and-down

Wavelength Wavelength is distance between crests or between troughs of waves. Longitudinal Transverse 24-Apr-17 Physics 1 (Garcia) SJSU

Water waves at the beach move a few miles per hour Wave Speed The speed at which waves travel is called the wave speed. Speed of sound = 330 m/s = 725 mi/hr Speed of light = 300,000,000 m/s Water waves at the beach move a few miles per hour 24-Apr-17 Physics 1 (Garcia) SJSU

Tsunami Waves Tsunamis are ordinary water waves, just like waves in your bathtub, but because they are typically generated by deep sea earthquakes they carry huge amounts of energy and momentum, traveling at almost 500 mph while in the deep ocean. The tsunami of 26 December 26th 2004 was produced by an earthquake whose epicenter was located off the coast of Indonesia in the Indian Ocean. The death toll is estimated at over a quarter of a million persons. Animation by Vasily V. Titov 24-Apr-17 Physics 1 (Garcia) SJSU

Wave Relations Wave speed, wavelength & frequency related. (Wave speed) = (Wavelength) x (Frequency) (Wave length) = (Frequency) = (Wave speed) (Frequency) (Wave speed) (Wavelength) 24-Apr-17 Physics 1 (Garcia) SJSU

Check Yourself What is the wave length? What is the wave speed? What is the wave’s frequency? What is the wave’s period? 6 m/s 2 m 24-Apr-17 Physics 1 (Garcia) SJSU

Demo: Hearing Sound Range of human hearing is roughly 20 Hertz to 20,000 Hertz. Wave speed for sound is 330 m/s Wavelength of 20 Hertz is 16 m (about 50 ft) Wavelength of 20,000 Hz is 1.6 cm (½ inch) 24-Apr-17 Physics 1 (Garcia) SJSU

Hearing in Animals Frequency range varies widely, depending on natural adaptation using sound to communicate, locate food, avoid predators, etc.

Constructive Interference Two waves in phase add together, which is called constructive interference. 24-Apr-17 Physics 1 (Garcia) SJSU

Destructive Interference Two waves out of phase cancel each other out, which is destructive interference. 24-Apr-17 Physics 1 (Garcia) SJSU

In & Out of Phase

Demo: In & Out of Phase Pair of speakers constructively interfere when they are in phase (oscillating together). When out of phase (reverse wires on one of the speakers) then they destructively interfere. Out of Phase 24-Apr-17 Physics 1 (Garcia) SJSU

Noise-Canceling Headphones Noise-canceling headphones use a microphone that listens for noise and a speaker that produces the same noise but out of phase (cancellation by destructive interference) External Noise Canceling Sound

Demo: Speaker Baffle Why are speakers mounted behind a baffle and inside an enclosure? 24-Apr-17 Physics 1 (Garcia) SJSU

Standing Waves When a wave interferes with its reflection, this may create a standing wave.

Standing Waves and Phase Standing waves are another example of constructive and destructive interference. 24-Apr-17 Physics 1 (Garcia) SJSU

Doppler Effect Sound coming from a moving object has a different wavelength and frequency than if it were stationary. If moving towards you, wavelength shorter and frequency higher. If moving away, wavelength longer and frequency lower. 24-Apr-17 Physics 1 (Garcia) SJSU

Demo: Doppler Shift Hear frequency as higher when buzzer is moving towards you and hear it as lower when moving away from you. Shorter Wavelength Higher Frequency Lower Frequency Longer Wavelength 24-Apr-17 Physics 1 (Garcia) SJSU