Acoustics Lesson 1

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Acoustics lesson 1

Steve VanlanduitDepartement of Mechanical Engineering

u ng , roomE-mail: [email protected]

1-10-2012 1Herhaling titel van presentatie


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Consult

For any remark/question/complaint/etc.Concerning the Bruface program

Electromechanical Engineering During the Acoustics and vibrations

lectures

By e-mail: [email protected] After appointment (room ZW113)


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Acoustics and vibrations Courseoverview

,S. Vanlanduit,4 lectures,

Tuesday 10-12AM weeks 1-4

Vibrations part 1,A. Preumont9 lectures

Thursday 2-4PM, weeks 1-8

,P. Guillaume,

5 lectures, Tuesday 10-12AM week 5-7and Friday 8-10h weeks 8+9

Labs

Excercices


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Acoustics course content

Overview: Ph sics of sound Hearing Acoustics in enclosed spaces

European legislation


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Environmental noise


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Noise on the workfloor

c i t a t e d

f i n c a p a

e r s o n s

u m b e r

p

Source: Belgian Fund for Professional

Vibrations Noise Asbetos OtherSilicosis


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Introduction


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Acoustics course content

Overview: Ph sics of sound Hearing Acoustics in enclosed spaces

European legislation


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Basic physics of sound

Definition of sound: Longitudinal waves in a medium (air, water)

Pressure variations (sound pressure in Pa,Bar,atm) Frequencies: 20Hz-20kHz


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The dB scale oun pressure eve :


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Pressure dB conversion factors:


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Logaritmic frequency division

Tertz bands


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Propagation of sound

Simplified sound sources: Plane source plane sound wave

Point source spherical wave


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The plane wave equation:

Solution:


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The speed of sound: Gasses

Solids

helium 0 972air 0 331air 20 340water 0 1402water 20 1482sea water 20 1522iron 0 5130brass 0 4700copper 0 3560gold 0 3240


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Types of sound signals:

Periodic

Impulse


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Spectral analysis:


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Spectral analysis:


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The acoustic impedance:

Acoustic impedance

MKS unity:

In air: z=4 Ray


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Definities geluid: impedantieBasic physics of sound

Use of impedance:

Acoustic absorption: 14

z

za

Resonators

See lesson 2


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RMS value:

Equivalent sound level:

Ldn :


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Correlated uncorrelated sources:

. o eren source:RMS 2 (totaal sound) RMS 2(source1) + RMS 2 (source2)

2. Incoherent sources:RMS 2 (totaal sound) = RMS 2(source1) + RMS 2 (source2)


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Sound Power in Watt : = energy flux through a closed surface Sound power level L W in dB:

Lw

= 10log(W / W0) with W0 = 10 -12 Watt

Sound Intensit I in Watt m2:

energy flux through 1m2 surface Sound intensity level L I in dB:

S

IdS W

= og w t = - att m


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Sound intensity:

For p ane waves:

I is a vector quantity!


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Relation between power and pressure:


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The hearing system


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The hearing system


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The hearing system


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The hearing system


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Measurement of sound

Type 1

Type 3


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Measurement microphones

Principles: ezoe ec r c Inductive

Condenser E ectret


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f d


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M f d


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M f d


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Octaves and Tertz band analysis25 31,5 40 50 63 80 100 125 160

200 250 315 400 500 630 800 1000 1250

1600 2000 2500 3150 4000 5000 6300 8000 10000

12500 16000 20000



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n x r l v l:



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Sound intensity measurement:



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Limitation of the frequency range

Sound power measurement


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Different possible techniques:

. s ng measurements:

2.Using sound intensity measurements

ISO 9614 series



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Sound power measurementISO 3741 to 3747Procedure: Choice of a surrounding surface

SPL measurements at discrete points on the surface A weighted In octave or tertz bands

Variants depending on: Type of surface Acoustical environment (anechoic room?) Required accuracy

Background noise A lication area re uired information



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ISO 3744 Semi-anechoic, outdoor, large

room

< All types of sound In frequency bands Surrounding surface:

Parallellepipedum

Sphere

1 210logwS

L L K K 0.1110 log 10 pi N

L p L

0 1i



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Comparison method: ISO 3747 Accuracy = 4 to 5dB Fixed non-mobile machines

- No limitation on the volume of the source All stationary sound Method:

Power of the

reference source

Avg. SPL referencesource

Avg. SPL of

unknown source



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Using sound intensity: : e erm na on o soun power eve s o

noise sources using sound intensity

Definition of an arbitrary surrounding surface ur u y r

surface

i ni iW I S 10log iW L W



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p

Elimination of background noise

Nearfield measurements



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p

Disadvantages: Limite requency range

Directivity Cost of the instrumentation



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p

ISO 9614-2 :

Sweep method

Example exam questions


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p q

Ma or uestion: Explain the working principle of the human

hearing system. Discuss the working principle of a sonometer and

an intensity meter. Explain how one can measure sound power in

practice.

Minor questions: Calculate dB level (by heart) Give order of magnitude of acoustical quantities

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Acoustics Lesson 1