chapter 19 vibrations & waves

Apr 22, 2023 Physics 1 (Garcia) SJSU

Chapter 19 Vibrations & Waves

Part IV: Sound


Vibrations

Vibrations are a common phenomenon.

Oscillating motion of a mass on a spring


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.


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


Waves

Concept of vibrations extends into the phenomenon of wave motion.

Water waves

Light waves

Sound

Radio

String


Wave Motion

Transverse Waves

For transverse waves the wave’s amplitude is perpendicular to the wave’s motion.

Amplitude

Amplitude


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.


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


Wavelength

Wavelength is distance between crests or between troughs of waves.

Longitudinal

Transverse


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.


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


Wave Relations

Wave speed, wavelength & frequency related.

(Wave speed) = (Wavelength) x (Frequency)

(Wave length) =

(Frequency) =

(Wave speed)(Frequency)

(Wave speed)(Wavelength)


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?


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.


Constructive Interference

Two waves in phase add together, which is called constructive interference.


Destructive Interference

Two waves out of phase cancel each other out, which is destructive interference.

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 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


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.


Standing Waves and Phase

Standing waves are another example of constructive and destructive interference.


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.


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

chapter 19 vibrations & waves

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