82168772 Acoustics Presentation JuL

7/29/2019 82168772 Acoustics Presentation JuL

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ACOUSTICS

For I know the plans I have

for, plan to prosper you andnot to harm you, plan to give

you hope and a future.

Jer. 29:11


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Acoustics

The science of sound, including itsproduction, propagation and effects

The objective study of the physical behavior

of sound in an enclosed space

Sound

A wave motion consisting of a series of

condensations and rarefactions in an elastic

medium produced by a vibrating body


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Audible Range: 20 20 000Hz

Infrasonic

frequencies below the audible range

Ultrasonic

frequencies above the audible range


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Human Voice

60 7000 Hz

85 155 Hz ( Adult Male )

165 225 Hz ( Adult Female )

Generation:

1. Lungs

2. Vocal folds within the larynx3. Articulators ( tongue, cheek, palate, lips, etc. )


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General Interpretation of Sound

1. Physical phenomenon consisting of wave

motion in a transmitting medium

(objective)

2. Sensation due to outside simulation

(subjective)


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Requirements to Produce Sounds

1. Presence of vibrating body

2. Presence of transmitting medium


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Physical Properties of Sound

1. Amplitude

2. Period

3. Frequency4. Wavelength

5. Velocity of Propagation


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Velocity of Sounds

A. Solids

Where:E = Youngs Modulus ofelasticity ( N/m2 )

d = density of the medium, ( kg/m3 )


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Velocity of Sounds

B. Liquids

Where:e = Bulks Modulus ofelasticity ( N/m2 )

d= density of the medium ( kg/m3 )


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Velocity of Sounds

C. Gases

Where:

k = ratio of the specific heat at constant volumePo = the steady pressure of the gas (N/m

2 )

o = the steady or average density of the gas ( kg/m3 )


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Velocity of Sounds

D. Dry Air/Air (for TC 200C)

where:TC = temperature in Celsius


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Velocity of Sounds

D. Dry Air/Air (for TC 200C)

where:

TK = temperature in Kelvin


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Velocity of Sounds

Notes

- Sounds travel more slowly in gasesthan in

liquids, and more slowly in liquids than solids.

- Sounds travels slower with an increased

altitude (elevation if you are on solid earth),

primarily as a result and humidity changes.


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Mach Number

Mach Number

- The speed of an object in air, or any fluid

substance, divided by the speed of sound as

it is in that substance.


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Mach Number

Six Categories

Subsonic: M


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Supersonic Transport (SST)

Concorde


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Supersonic Transport (SST)

Tupolev Tu-144


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Sonic Boom

Sonic Boom

This term is commonly used to refer to

the shocks caused by the supersonic flight of

an aircraft.It generates enormous amount of

sound energy, sounding much like an

explosion.


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Sonic Boom CLoud


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Possibilities when a Propagated

Sound is Obstructed

Sound is Reflected

Becomes apparent to the listener only when the distancefrom the source and the reflecting medium is great and the

difference between the original and reflected sound is greateror equal to 1/17 of a second.

Brought about by a series of rarefactions between twoparallel surfaces resulting to prolongation of sound

Creates listener fatigue

Reflection caused by two parallel surfaces, producingstanding waves


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Possibilities when a Propagated

Sound is Obstructed

Sound is absorbed

Conversion of sound energy to heat energy

Onward transmission through

obstruction


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Physiological Characteristics of

Wave Motion

1) Pitch

2) Tone

3) Loudness


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Wave Motion

Pitch

Number of cycles a wave goes through in a

definite interval

The higher the frequency the higher the pitch

Mel unit of pitch

1000 mels pitch of 1000Hz tone at 40dB

Octave pitch interval 2:1; frequency is twice thegiven tone


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Wave Motion

Tone

Timbre quality of sound

- The quality of sound related to harmonic structure

- tone quality or tone color

Pure Tone a sound composed of only one

frequency in which the sound pressure varies

sinusoidally with time.

Musical Sounds composed of the fundamental

frequency and its harmonics


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Wave Motion

Loudness

Fluctuation of air pressure created by sound waves

Observers auditory impression of the strength of a

sound and is associated with the rate at which energyis transmitted to the ear.

Depends on the amplitude of the sound

Sone unit of loudness

Loudness Level measured by the sound level of astandard pure tone or specified frequency which is

assessed by normal observers as being equally loud


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PHON

Phon is the unit of loudness level when:

The standard pure tone is produced by asensibly plane sinusoidal progressive

sound wave coming from directly in frontof the observer and having the frequencyof 1kHz

The sound pressure level in the freeprogressive wave is expressed in dBabove 2 x 10-5 N/m2


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Sound Levels

A. Sound Pressure (P) and

Sound Pressure Level (SPL)

Sound Pressure

The local pressure deviation from the ambient

(average, or equilibrium) pressure caused by a

soundwave.

The alternating component of the pressure at a

particular point in a sound field

Expressed in N/m2 or Pa


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Sound Levels

Sound Pressure Level Equal to 20 times the logarithm to the base 10 of the

ratio of the RMS sound pressure to the reference

sound pressure

SPL = 20 log (P/Po)Where:

P = rms sound pressurePo = reference sound pressure

Po = 2 x 10-5 N/m2 or Pa

Po = 0.0002 bar

Po = 2.089 lb/ft2


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Sound Pressure Levels

Sound Pressure Level (SPL) at any unit ofpressure in dB

SPL = 20log(P+N)

Where:

PN = rms sound pressure expressed in any of

pressure in dBN = SPL constant corresponding to the unit atwhich sound pressure is expressed


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SPL Constants

Unit of Sound Pressure DesignationSPL Constant

(N)

Microbar bar 74

Pascal N/m2 94

lb/ft2 psf 127.6

mmHg mmHg 136.5

torr torr 136.5lb/in2 psi 170.8

atm (technical) atm 193.8

atm (standard) atm 194.1


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Sound Levels

B. Sound Intensity (I) and

Sound Intensity Level (SIL)

Sound Intensity

Defined as the power per unit area

The basic units are W/m2

The average rate of transmission of sound energy

through a cross-sectional area of 1 m2

at rightangles to a particular motion.


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Sound Levels

Inverse Square Law

The sound intensity from a point source of

sound will obey the inverse square law if there

are no reflections or reverberation.Any physical law stating that a specified

physical quantity or strength is inversely

proportional to the square of the distance from

the source of that physical quantity.


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Sound Levels

Ground Source:


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Sound Levels

Isotropic Source:


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Sound Levels

Sound Intensity Level

Where:

I = sound intensityIo = threshold intensity, W/m2

Io = 10-12 W/m2 = 10-16 W/cm2


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Sound Levels

C. Sound Power (W) and

Sound Power Level (PWL)

Sound Power (W)

The total energy radiated per unit time.


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Sound Levels

Sound Power Level (PWL)

Where:

W = sound power , W

Wo = reference sound power

Wo = 10-12 W = 10-16 W


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Volume Unit


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Volume Unit Meter


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Sound Fields

Free fieldA region in space where sound maypropagate free from any form of obstruction.

Near fieldA region close to a source where

the sound pressure and acoustic particlevelocity are not in phase.

Far fieldIt begins where the near field endsand extends to infinity.


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Sound Fields

Direct fieldIt is the part of the sound field

which has not suffered any reflection from

any room surfaces or obstacles.

Reverberant fieldIt is the part of the soundfield which has experienced at least onereflection from a boundary of the room orenclosure containing the source.


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End

Thank you!!

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82168772 Acoustics Presentation JuL