Apr 22, 2023 Physics 1 (Garcia) SJSU
Chapter 19 Vibrations & Waves
Part IV: Sound
Apr 22, 2023 Physics 1 (Garcia) SJSU
Vibrations
Vibrations are a common phenomenon.
Oscillating motion of a mass on a spring
Apr 22, 2023 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.
Apr 22, 2023 Physics 1 (Garcia) SJSU
Frequency
Frequency is the inverse of the period,
(Frequency) =
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
(Period)
1
Amplitude
The distance from the rest position is the amplitude of oscillation.
Amplitude
Apr 22, 2023 Physics 1 (Garcia) SJSU
Waves
Concept of vibrations extends into the phenomenon of wave motion.
Water waves
Light waves
Sound
Radio
String
Apr 22, 2023 Physics 1 (Garcia) SJSU
Wave Motion
Transverse Waves
For transverse waves the wave’s amplitude is perpendicular to the wave’s motion.
Amplitude
Amplitude
Apr 22, 2023 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.
Apr 22, 2023 Physics 1 (Garcia) SJSU
Longitudinal WavesFor longitudinal waves, amplitude and wave
motion are parallel.
Wave Motion
Amplitude
Amplitude
A crowd can do a longitudinal wave by moving side-to-side instead of up-and-down
Apr 22, 2023 Physics 1 (Garcia) SJSU
Wavelength
Wavelength is distance between crests or between troughs of waves.
Longitudinal
Transverse
Apr 22, 2023 Physics 1 (Garcia) SJSU
Wave Speed
The speed at which waves travel is called the wave speed.
Speed of sound = 330 m/s = 725 mi/hrSpeed of light = 300,000,000 m/s
Water waves at the beach move a few miles per hour
Tsunami WavesTsunamis 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.
Apr 22, 2023 Physics 1 (Garcia) SJSU
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
Apr 22, 2023 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)
Apr 22, 2023 Physics 1 (Garcia) SJSU
Check Yourself
6 m/s 2 m
What is the wave length? What is the wave speed? What is the wave’s frequency? What is the wave’s period?
Apr 22, 2023 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/sWavelength of 20 Hertz is 16 m (about 50 ft)Wavelength of 20,000 Hz is 1.6 cm (½ inch)
Hearing in Animals
Frequency range varies widely, depending on natural adaptation using sound to communicate, locate food, avoid predators, etc.
Apr 22, 2023 Physics 1 (Garcia) SJSU
Constructive Interference
Two waves in phase add together, which is called constructive interference.
Apr 22, 2023 Physics 1 (Garcia) SJSU
Destructive Interference
Two waves out of phase cancel each other out, which is destructive interference.
In & Out of Phase
Apr 22, 2023 Physics 1 (Garcia) SJSU
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 ofPhase
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
Apr 22, 2023 Physics 1 (Garcia) SJSU
Demo: Speaker BaffleWhy are speakers mounted behind a baffle
and inside an enclosure?
Standing Waves
When a wave interferes with its reflection, this may create a standing wave.
Apr 22, 2023 Physics 1 (Garcia) SJSU
Standing Waves and Phase
Standing waves are another example of constructive and destructive interference.
Apr 22, 2023 Physics 1 (Garcia) SJSU
Doppler EffectSound 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.
Apr 22, 2023 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.
HigherFrequency
LowerFrequency Longer Wavelength
Shorter Wavelength