Steve Vanlanduit Departement of Mechanical Engineering B ...mech.vub.ac.be/avrg/Courses/Acoustics/Acoustics lesson 1.pdf · Acoustics lesson 1 Steve Vanlanduit Departement of Mechanical

Acoustics lesson 1

Steve VanlanduitDepartement of Mechanical EngineeringB ildi Z ZW113Building Z, room ZW113E-mail: [email protected]

1-10-2012 1Herhaling titel van presentatie

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)

Acoustics and vibrations Course overview

• Acoustics • Acoustics, S. Vanlanduit, 4 lectures,

• Tuesday 10-12AM weeks 1-4

• Vibrations part 1, A. Preumont, ,9 lectures

• Thursday 2-4PM, weeks 1-8

• Vibrations part 2 • Vibrations part 2, P. Guillaume, 5 lectures,

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

• Labs• Excercices

Acoustics course content

Overview:– Physics of soundy– Hearing– Measurement of soundMeasurement of sound– Acoustics in enclosed spaces– Transmission of sound– Transmission of sound– European legislation

Environmental noise

Noise on the workfloorci

tate

dof

inca

pac

erso

nsN

umbe

ro peN

Source: Belgian Fund for Professionaldeseases (FBZ)deseases (FBZ)

Vibrations Noise Asbetos OtherSilicosis

Introduction

Sound sources Sound sources

Acoustics course content

Overview:– Physics of soundy– Hearing– Measurement of soundMeasurement of sound– Acoustics in enclosed spaces– Transmission of sound– Transmission of sound– European legislation

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


The ‘dB’ scaleS d l l (SPL)Sound pressure level (SPL):


Pressure – dB conversion factors:




Logaritmic frequency division• Octave bands• Octave bands• Tertz bands


Propagation of soundp g

Simplified sound sources:• Plane source plane sound wave• Line source cylindrical sound wave• Line source cylindrical sound wave• Point source spherical wave


The plane wave equation:

Solution:


The speed of sound:• Gasses

MEDIUM TEMPRATURE (C) SOUND SPEED (m/s)

• SolidsMEDIUM TEMPRATURE (C) SOUND SPEED (m/s)helium 0 972air 0 331air 20 340air 20 340water 0 1402water 20 1482sea water 20 1522iron 0 5130brass 0 4700copper 0 3560gold 0 3240


Wave equation solutions for simplified sources:

Plane wave:

Spherical wave:

Cylindrical wave:Cylindrical wave:



Types of sound signals:

• Periodic

• Stochastic• Stochastic

• Impulse


Spectral analysis:


Spectral analysis:


The acoustic impedance:

Acoustic impedance

MKS unity:

For plane waves:For plane waves:

I i 400R lIn air: z=400Rayl

Definities geluid: impedantieBasic physics of sound

Use of impedance:

– Acoustic absorption:2

14zza 2

– Resonators

See lesson 2


RMS value:

Equivalent sound level:

Ldn:


Adding two dB values:


Correlated – uncorrelated sources:

1 C h t 1. Coherent source: RMS2(totaal sound) ≠ RMS2(source1) + RMS2(source2)

2. Incoherent sources: RMS2(totaal sound) = RMS2(source1) + RMS2(source2)


Sound Power in Watt : • = ‘energy flux through a closed surface’gy g• Sound power level LW in dB:

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

Sound Intensity I in Watt/m2: Sound Intensity I in Watt/m2: • ‘energy flux through 1m2 surface’• Sound intensity level LI in dB:

LI 10l (I / I0) i h I0 10 12 W / 2

S

IdSW

LI = 10log(I / I0) with I0 = 10-12 Watt/m2


Sound intensity:

F l For plane waves:

I is a vector quantity!


Relation between power and pressure:

The hearing system

The hearing system

The hearing system

The hearing system

Measurement of sound

Simple instrument for sound tpressure measurement:

= ‘sonometer’

Microphone SignalAmplifier DisplayMicrophone Signal conditioning

Amplifier Display


IEC 651:IEC 651:• Type 1• Type 2• Type 2• Type 3


Measurement microphones

Principles:Pi l t i• Piezoelectric

• Inductive

• Condenserl• Electret









A, B, C en D filters (40, 70, 100, 120 dB respectively)IEC 60651 (1979 01)IEC 60651 (1979-01)



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



Sound exposure level: Sound exposure level:


Sound intensity measurement:


Limitation of the frequency range

Sound power measurement

Different possible techniques:

1 U i SPL t1.Using SPL measurements:

ISO 3740 series ISO 3740 series

2.Using sound intensity measurements

ISO 9614 series

Sound power measurementISO 3741 to 3747Procedure: Procedure: • 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• Size of the source• Size of the source• Background noise• Application area, required informationpp , q


ISO 3744• Semi-anechoic, outdoor, large

room• Largest source dimension < 15mLargest source dimension < 15m• All types of sound• In frequency bands• Surrounding surface:

• Parallellepipedum• Hemisphere• Hemisphere• Sphere

1 210logw pSL L K KS

0.1110log 10 piN

LpL

N

0w p S 1

piN


Comparison method: ISO 3747• Accuracy σ = 4 to 5dB• Fixed non-mobile machines• In-house and outside• In-house and outside• No limitation on the volume of the source• All stationary soundy• Method:

Power of the

reference source

Avg. SPL referencesource

Avg. SPL of

unknown source


Using sound intensity:ISO 9614 ‘D t i ti f d l l f ISO 9614 : ‘Determination of sound power levels of noise sources using sound intensity’

Procedure:Procedure:• Definition of an arbitrary surrounding surface

Meas e aco stic intensit at disc ete points on the • Measure acoustic intensity at discrete points on the surface

• Calculation of the sound power:• Calculation of the sound power:

N Wi ni iW I S

1 0

10logN

iW

i

WLW

1 0i W


Advantages:Advantages:• Elimination of background noise• In situ measurement possible• In situ measurement possible• Nearfield measurements• Arbitrary surfaces possible• Arbitrary surfaces possible


Disadvantages:Li i d f• Limited frequency range

• Directivity• Cost of the instrumentation


ISO 9614-2 :

Sweep methodp

Example exam questions

Major question:j q• 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

Documents

Steve Vanlanduit Departement of Mechanical Engineering B ...mech.vub.ac.be/avrg/Courses/Acoustics/Acoustics lesson 1.pdf · Acoustics lesson 1 Steve Vanlanduit Departement of Mechanical