Sound

Sound Waves

o Longitudinal Disturbance

o Mechanical waves

o require a medium

Sound is a

Pressure Wave

plot of pressure versus time would appear as a sine curve.

High Pressure = compressions = peaks points of the sine curve

Low pressure = rarefactions = low points of sine curve

Amplitude describes pressure or LOUDNESS

Remember Amplitude describes Energy

Pulse of Sound

Science of sound animations -Resources

3 conditions necessary for transmission

1. a vibrating source to produce the sound

2. medium with particles to transmit the sound

3. receiver to detect the sound.

Bell Jar Animation

Science of sound animations –Resources

http://www.ltscotland.org.uk/resources/s/sound/belljar.asp

A sound detector (ear or machine)

detect fluctuations in pressure as the sound wave impinges upon the detecting device

Properties and Perception

How we hear

The ear collects pressure waves and

converts them to electrical impulses

which are sent to brain

Structure of the Human Ear

Outer ear collects and amplifies

Middle ear vibrates at same frequency of sound wave

Inner ear hair-like nerve cells (frequency specific) convert vibrations to electrical impulses

Hearing Loss

Conduction deafness: reduced mobility of ossicles due to vibrous tissue (i.e., from repeated otitis).

Nerve deafness: deterioration of hair cells or nerve (also age-related presbycusis).

Can be distinguished through bone conduction.

How do we perceive sound?

High and Low

Loud and Soft

High vs Low: PITCH

We perceive frequency as differences in pitch

Directly related

The greater the frequency, the

higher the pitch. (Ex. The squeak of

a shoe on the floor has a high pitch.

The drone of a semi truck is a low

pitch)http://fun.drno.de/flash/gecko.swf

The Sonic Spectrum

Includes sounds within the normal human range of hearing as well as those above and below this range.

_________: human ear detect sound waves 20 Hz to 20 000 Hz.

___________: sound below audible range (less than 20 Hz)

___________: sound above the audible range (more than 20,000 Hz)

Infrasonic

The Boy Who Could See Like A bat (Echolocation) - YouTube

Audible

Infrasonic

Ultrasonic

Loud vs Soft

We perceive loudness as differences in pressure (the wave’s amplitude or energy)

The greater the amplitude the louder the sound

Examples

The threshold of hearing (faintest sound) measures __________.

The threshold of pain measures ____________________

0 dB

120 dB

Perception of sound waves: Summary

The pitch of a sound is determined by the frequency of the wave and the amplitude (pressure or energy) of the wave determines the loudness of a sound wave.

Each of these features is independent of each other.

In other words a sound can have a high pitch( high frequency) and be either soft( low amplitude) or loud.

Virtual Oscilloscope

Speed of sound animations -Resources

Refer to the sound oscilloscope

readings to answer the following:

Which example is the loudest? Which example is the softest?Which example has the highest pitch?Which example has the lowest pitch?

Refer to the sound oscilloscope

readings to answer the following:

Which example is the loudest? 4Which example is the softest? 5Which example has the highest pitch? 5Which example has the lowest pitch? 1

Speed of Sound

• Depends on the __________ it’s traveling through

• ______> _________ > _______

• Within a ___________ it travels faster through the _____ _________ material

medium

v solids v gasesv liquids

phaseless dense

Elastic between Phase

Inertial with in phase “sluggishness”

Inertial properties are those properties related to the material's tendency to be sluggish to changes in its state of motion. The density of a medium is an example of an inertial property. The greater the inertia (i.e., mass density) of individual particles of the medium, the less responsive they will be to the interactions between neighboring particles and the slower that the wave will be. As stated above, sound waves travel faster in solids than they do in liquids than they do in gases. However, within a single phase of matter, the inertial property of density tends to be the property that has a greatest impact upon the speed of sound. A sound wave will travel faster in a less dense material than a more dense material. Thus, a sound wave will travel nearly three times faster in Helium than it will in air. This is mostly due to the lower mass of Helium particles as compared to air particles.

Speed of Sound in a Variety of Mediums

Medium Temp (oC) Velocity (m/s)

Gases

air 20 343

Liquids

water 20 1482

Solids

lead 1960

copper 5010

glass 5640

steel 5960

What will sound travel faster in at 20C?

Air or Helium?

Helium: Both gases, but Helium less dense

Steel or Water?

Steel: sound travels faster in solids than liquids

Water or Mercury?

Water: Both liquids, but water less dense than Mercury

MythBusters - Fun With Gas -YouTube

What will sound travel faster in at 20C?

MythBusters - Fun With Gas -YouTube

Affect of temperature on speed

At 0o C, sound travels at 330 m/s through air

The speed of sound follows the equation

vsound = 330 m/s + 0.6(°C)

Example 1

What is the speed of sound at 30o

Celsius?

Answer: 348 m/s

vsound = 330 m/s + 0.6(°C)

vsound = 330 m/s + 0.6(30°C)

What is difference between

Echolocation & Sonar

Sonar (SOund Navigation And Ranging) Manmade

Ex 2: A submarine emits a signal that is reflected from

the coral reef and returned to the sub 1.8 s after

transmission. How far away is the reef? Assume the

speed of sound in water is 1500 m/s.

Remember v = d/t

d = vt/2

Why?

Sound wave has to travel there and back

d = (1500 m/s) (1.8s)/2

d = 1350 m

What is difference between

Echolocation & Sonar

Echolocation is done by animals

Infrasonic

The Boy Who Could See Like A bat (Echolocation) - YouTube

Echolocation: animals do this

The boy who sees without eyes

Infrasonic waves and echolocation

Imagine sitting in your car. You hear a siren. What does it sound like as it approaches and passes?

Doppler Effect

The Doppler Effect is the apparent change in ______ of a sound due to the _______ of either a source or a receiver.

(Remember pitch is related to frequency)

pitch

motion

Doppler Effect

Moving towards : the pitch appears _______.

Moving away: the pitch appears ______.

higher

lower

Doppler Effect

Applies to all types of waves

Cont’d

Applet Thumbnails from upcoming Applets

DopplerEffect

Light waves also exhibit the Doppler

effect. The spectra of stars that are

receding from us is shifted toward the

longer wavelengths of light. This is

known as the red shift.

Measurement of the red shift allows

astrophysicists to calculate the speed at

which stars are moving away. Since

almost all stars and galaxies exhibit a

red shift, it is believed that the universe

is expanding.

The movie at left shows a stationary sound source. Sound waves are produced at a constant frequency and wave-fronts move symmetrically away from the source at a constant speed v. The observers at A and B, here the same pitched sound.

A B

In the movie below, the same sound source is radiating sound waves at a constant frequency in the same medium. However, now the sound source is moving to the right with a speed 100m/s

Notice listener B is receiving waves that are closer together and he hears a higher apparent frequency than before.

BA

Notice listener A is receiving waves that are further apart and he hears a lower apparent frequency than before.

DOPPLER EFFECT:The pitch heard by the listener is given by the following equation:

f fv v

v vS

o

S

'

Units: Hz

f' is the perceived frequency (sound heard by the

listener (observer),

fS is the actual frequency of the sound emitted by the

source,

v is the speed of sound in air, USE 340 m/s unless told

otherwise

vS is the velocity of the source, and

vo is the velocity of the listener (observer).Sign Convention:

(+) for approaching velocities and

(-) for receding velocities.

Ex 3

An ambulance travels west at a speed of 17.3 m/s. Its siren emits a sound of frequency 400.0 Hz. What frequency is heard by a passenger of a car traveling east and approaching the ambulance at a rate of 25.0 m/s?

452.43 Hz

Ex 4

What frequency is heard by the passenger as she passes and moves away from the blaring ambulance (which we already know is moving in the opposite direction)?

352.64 Hz

What’s the deal with the red sift and

blue shift terminology?

Red shift….perceived decreased frequency

Blue shift…. perceived increased frequency

Red light has a smaller frequency than blue light

What is Mach 1

Speed of sound but dependent on temperature and pressure.

Around 340 m/s or 761 mph depending on conditions

What do a sonic boom and snapping

your little brother with a towel have in

common

They both are sounds due to breaking the sound barrier!!!

Sonic Boom

http://www.youtube.com/watch?v=-d9A2oq1N38

Sonic Boom - YouTube

What is a sonic boom?

An airplane moving faster than the speed of sound:

causes a conical three-dimensional shock wave continuously swept behind the plane

sonic boom created when the conical shell of the compressed air swept behind the supersonic craft approaches an observer’s ear.

A sonic boom is similar to a boat wake

Supersonic Flight, Sonic Booms - YouTube

Shock wave

Natural frequency

Natural frequency is the frequency at which a system naturally vibrates once it has been set into motion

Demo

resonance

____________ is a form of ______________ interference which produces an increased loudness of a sound wave due to 2 objects vibrating sympathetically (in phase) with one another.

Matching Natural Frequencies

Wine glass Inquiring Minds United streaming/Lexus Kutv

Resonance boxes

Resonanceconstructive

Resonance and music

Piano

http://www.youtube.com/watch?v=3mclp9QmCGs

Tacoma Bridge – YouTube

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

breaking a wine glass using resonance - YouTube

Tacoma Bridge failure is NOT DUE to resonance:

Tacoma Bridge Vortex Shedding

Baptistery in Pisa Under the dome of the Baptistry

in Pisa a stunning acoustic effect can be heard. Notes sung here last so long, it’s actually possible to sing with yourself: new notes will harmonize with old ones still reverberating around the space. The Baptistry Guards will often demonstrate this beautiful effect.

The key to the remarkable acoustic is that there is very little soft material about to absorb the sound. Consequently, notes rattles around the space for a long time, some suggest for over 12 seconds, before the sound dies away and becomes inaudible.

Standing Waves

Created when periodic waves with equal amplitude and wavelength reflect and superimpose on one another.

http://www.walter-fendt.de/ph14e/stwaverefl.htm

Cont’d

Nodes: seem stationary (due to destructive interference)

Antinodes: vibrate back and forth (due to constructive interference)