Wave Energy Transfer & Sound Wave Energy

If a vibrational disturbance occurs, energy travels out in all directions from the vibrational source.

Ripple demo

All points on a wave that are in phase comprise a wave front.

Wavefronts join points in phase.

Rays – a ray is an arrow sketched through the wave fronts (perpendicular) to show direction of wave propagation.

Rays

Waves transfer energyRate of E transfer is proportionalto the A .

Less energy

More energy

For light increased amplitude increases brightness.

For sound: increased amplitude increases volume.

What does wave frequency (f) determine?

• Wave type for EM waves.

• Color for light.

• Pitch for sound

Hear different frequencies

• http://www.fearofphysics.com/Sound/sounds.html

Sound

Sound is a Mechanical Longitudinal/Compressional Wave

solid

liquid

gas

In gas

hot faster.

cold slower.

Increasing velocity

Sound velocity

The Doppler Effect

Stationary Source Emitting Waves all Directions. Circular wavefronts have = & f.

Doppler Effect from Moving Source

In front of source is less, behind is longer.

In front of source -short higher f:

hear higher pitch sound-see shorter light (blue).

behind source - longer lower f:

hear lower pitch soundsee longer light (red).

When objects are in relative motion:

a) Toward each other, f received increases.

b) Away from each other, f received decreases.

Doppler Effect

Doppler Clip 3.1 minutes

• http://www.youtube.com/watch?v=Kg9F5pN5tlI

• https://www.youtube.com/watch?v=h4OnBYrbCjY

Resonance & Sympathetic Vibration

All objects have a naturalAll objects have a natural

frequency of vibration.frequency of vibration.

ResonanceResonance - the inducing- the inducingof vibrations of a naturalof vibrations of a naturalrate by a vibrating sourcerate by a vibrating source

having the same frequencyhaving the same frequency

““sympathetic vibrations”sympathetic vibrations”

Push at natural frequency, amplitude increases

Resonance:• An oscillatory system that is driven by a force

with a f = to its natural f.

• System will resonate – amplitude will increase.

• The natural frequency of objects is the frequency that produces a standing wave in the object.

• Can occur from reflection causing a standing wave. Amplitude is increased.

Sympathetic Vibration

when a wave is near an object & is vibrating at the natural frequency of the object. Object vibrates sympathetically at same frequency – they resonate.

Causes the amplitude to increase.

Break the glass 20 sec.

http://www.youtube.com/watch?v=17tqXgvCN0E

Standing Waves on Guitar String

½ .

hwk Rd Text 13 – 1 and 13-2Wksht Review concepts sound.

hwk Rd 13 – 1 & pg 491 – 493

p.507 #1, 3, 6-8, 11-13, 15, 16

The three components of sound are:

• Pitch (how high or low)

• Loudness (volume)

• Timbre (tone color)

Pitch• Vibration patterns are also called waveforms.

• Each repetition of a waveform is called a cycle.

• We can hear frequencies between 20 hertz to 20 kHz i.e. 20,000 Hz.

• When the frequency of a sound doubles we say that the pitch goes up an octave.

• We can hear a range of pitches of about ten octaves.

• Many animals can make sounds and hear frequencies that are beyond what we can hear.

Loudness• To create vibration

energy is used.

• The greater amount of energy used the louder the sound.

• The strength of the changes in air pressure made by the vibrating object determines loudness.

• As the distance from the source increases the amount of power is spread over a greater area.

• The amount of power per square meter is called the intensity of the sound.

Humans do not perceive sound intensity linearly.

• For us to perceive a sound as twice as loud its intensity must be ten times greater.

• The perceived intensity level of sound is measured in a logarithmic scale using a unit called the decibel

(dB).

The scale begins (0 dB) on the softest

sound that a person can hear. This is

called the threshold of hearing.

The scale ends at the volume that causes pain (120 dB) and is therefore called the threshold of pain.

Tacoma Narrows Bridge

mechanical universe resonance

“Timbre” (TAM-ber) or tone color

is the specific property of sound that enables us to determine the difference between a piano and a harp.

A broad variety of tone colors exist because most sounds we perceive as pitch contain many frequencies.

• The predominant pitch The predominant pitch is called the is called the fundamental fundamental frequency. It is the frequency. It is the longest longest that forms a that forms a standing wave.standing wave.

Standing Wave patterns form notes.

Each string or pipe vibrates with particular frequencies of standing waves.

Other frequencies tend to die out.

Although we would perceive a string vibrating as a whole,

it vibrates in a pattern that appears erratic producing many different overtone pitches. What results are particular tone colors or timbres of instruments and voices.

Waveform with overtones.

Frequencies which occur along with the primary note are called the harmonic or overtone series.

When C is the fundamental the pitches below represent its first 15 overtones.

There are several standing waves which can be produced by vibrations on a string, or rope. Each pattern corresponds to vibrations which occur at a particular frequency and is known as a harmonic.

Harmonics

The lowest possible frequency at which a string could vibrate to form a standing wave pattern is known as the fundamental frequency or the first harmonic.

2nd Harmonic

Which One??

String Length L, & Harmonics

Standing waves can form on a string of length L, when the can = ½ L, or 2/2 L, or 3/2L etc.

Standing waves are the overtones or harmonics.

L = nn. n = 1, 2, 3, 4 harmonics. 2

Harmonic Frequencies

form where ½ can fit the string exactly.To calculate f:

L

nvf

2

f

vfv

2

nL

Substitute v/f for .

f

nvL

2

1st standing wave forms when = 2LFirst harmonic frequency is when n = 1 as below.

L

nvf

21

When n = 1 f is fundamental frequency or 1st harmonic.

L

nvf

22 For second harmonic n = 2.

f2 = v/L

Other standing waves with smaller wavelengths form other frequencies that ring out along with the fundamental.

In general,

The harmonic frequencies can be found where n = 1,2,3… and n corresponds to the harmonic. v is the velocity of the wave on the string. L is the string length.

L

nvfn 2

It is helpful to note that the distance between nodes on a standing wave is ½ .

½

Pipes and Air Columns

A resonant air column isA resonant air column issimply a standing simply a standing

longitudinallongitudinalwave system, much likewave system, much like

standing waves on a standing waves on a string.string. closed-pipe resonatorclosed-pipe resonator tube in which one end is tube in which one end is

openopenand the other end is closedand the other end is closed

open-pipe resonatoropen-pipe resonatortube in which both endstube in which both ends

are openare open

Open Pipe – open end has antinode.

Standing Waves in Open PipeBoth ends must be antinodes.

How much of the wavelength is the fundamental?

The 1st harmonic or fundamental can fit ½ into the tube.

Just like the string L = n 2

fn = nv2L

Where n, the harmonic is an integer.

Closed pipes must have a node at closed end and an antinode at the open end.

How many wavelengths??

Here is the next harmonic.How many ’s?

There are only odd harmonics possible.

L = 1/4.L = 3/4.L = 5/4

fn = nv where n = 1,3,5 … 4L

Beats – caused by constructive & destructive interference from 2

frequency sounds interacting.

• Beat Frequency heard is the difference between 2 frequencies.

• If a 50 Hz wave and a 60 Hz wave overlap, you hear beat of 10 Hz.

• hear beat frequencies

Traveling Waves Beats

Holt read 13 - 3

pg 499 #1 – 4

Start in class finish for hwk.

Hwk read 491 – 503

do 499 and 503