Acoustics (VTAF05) - LTH fileAcoustics (VTAF05) JUAN NEGREIRA, ... D. Bard / Akustik VTAF05 / 3 Nov. 2015 Laboratories & Project task ... Wind turbine noise

Acoustics (VTAF05)

JUAN NEGREIRA, DELPHINE BARD

DIVISION OF ENGINEERING ACOUSTICS, LUND UNIVERSITY

J. Negreira, D. Bard / Akustik VTAF05 / 3 Nov. 2015

Outline

Course Information

Wave propagation

Introduction to Acoustics

Summary


Teachers

• Lectures:

‒ Delphine Bard, KC-building (3rd floor)

‒ Juan Negreira, KC-building (3rd floor)

‒ Anders Sjöström

• Exercises:

‒ Juan Negreira

• Laboratory:

‒ Juan Negreira

‒ Marie-Laure Divoux

• Administration:

– Christina Glans, KC-building (3rd floor)

Course responsibles


Course material

• Handed out material

– Lecture notes

– Exercise material

– Laboration instructions

– Project task

– Formulae

• Website (course material will be uploaded here):

http://www.akustik.lth.se/utbildning/kurser/

http://www.akustik.lth.se/utbildning/kurser/


Laboratories & Project taskLaboratories

• Three Lab sessions (in groups of 2-3 students)

1. Recording, calibration and evaluation of sound

2. Room acoustics

3. Wind turbine noise

• Approximately 2 hours on site

– Preparation and post-processing time needed

• Results presented in form of a report

– Either passed or returned for completion

Project Tasks

• Performed in groups of 2-3 students

• Presented Friday, December 18th at 13:00-15:00


Examination

The final grade will be obtained as follows…

• Written exam (50 %)

– Thursday 15/1 at 8.00-13.00 in MA:8C

– Theoretical questions and exercises

– Calculator and handed out formulae summary allowed

– Graded: u, 3, 4, 5

• Project task (50 %)

– Graded: u, 3, 4, 5

• Executed laborations with passed reports


The course

• Time:

– 6 h/week of lectures and exercises (Tuesdays and Fridays)

– 6 h laboratory exercises off-schedule

• Purpose


Outline

Course Information

Wave propagation


Summary


Why address sound issues?

• Noise affects people physiologically and psychologically

• At least 25 % of EU citizens are

exposed to noise in such extent that it

affects health and quality of life

• …

• Today, approximately 2 million people in

Sweden are exposed to a noise level

that exceeds the regulations set up by

the Swedish parliament


What is acoustics?• Acoustics: part of physics studying generation, transmission reception,

absorption, reproduction and control of sound

‒ Environmental ac., building ac., room ac., psychoac., musical ac…

• Sound: ondulatory movement produced in an elastic medium by a

vibratory source producing variations in the atmospheric pressure

‒ Characteristics: pitch, quality and loudness

‒ Noise: random (unwanted) sound

» Classified by ”colours”

Violet noise: +6 dB/octave

Blue noise: +3 dB/octave

White noise: flat power spectrum

Pink noise: -3 dB/octave

Brown noise: -6 dB/octave

https://www.youtube.com/watch?v=3vEDZ-_iLNU&feature=youtu.be

https://www.youtube.com/watch?v=3vEDZ-_iLNU&feature=youtu.be


Time & frequency domains (I)

Harmonic signal: y t = A sin ωt = A cos ωt + ∅ = A sin 2πf ∙ t

‒ Amplitude:

‒ Period [s]:

‒ Frequency [Hz]:

‒ Wavelength [m]:

‒ Propagation Speed [m/s]:

NOTE:

‒ Effective value (RMS):

FFT

T λ

A

c ≠ v

ARMS = A =1

∆t

t0

t0+∆t

y2 t dt , Aharmonicsignal

= A2

‒ Frequency domain

A

T = 1 f

f = 1 T

λ=cT= c f

c=f λ


Time & frequency domains (II)

• A more complex time signal (traffic load)

• Narrow band analyses

‒ Impractical, time-consuming

‒ Octave & 1/3 octave bands

FFT

NOTE: Spectrum (any magnitude plotted against frequency)

https://www.youtube.com/watch?v=r18Gi8lSkfM

https://www.youtube.com/watch?v=r18Gi8lSkfM


Octave and 1/3 octave bands

If fn is the cut-off lower frequency

and fn+1 the upper one, the ratio of

the band limits is given by:

where k=1 for full octave and k=1/3

for one-third octave band

fn+1

fn= 2k


Hearing process

• Pressure waves

• For a sound to be perceived

‒ Frequency: 20 Hz – 20 kHz

‒ Sound pressure level (SPL): frequency dependent

• Inner ear detects: ∆p ϵ [20 μPa, 200 Pa] wide range

‒ Use of logarithmic scale (in decibels)

Source Conveying medium Receptor


The decibel (dB) & SPL

• Logarithmic way of describing a ratio

‒ Ratio: velocity, voltage, acceleration…

‒ Need of a reference

• Sound pressure level (SPL / Lp)

‒ p measured with microphones

‒ Frequency response of human hearing changes with amplitude

Lp = 10 log p2

pref2 = 20 log

p

pref

p = p f = RMS pressurepref = 2·10

−5 Pa = 20 μPapatm = 101 300 Paptot(t) = patm ± p(t)


Sound (acoustic) intensity

• Sound power per unit area [W/m2]

‒ Vector quantity: energy flow and direction

– In a free field:

• Types of propagation

‒ Plane:

‒ Cylindrical:

‒ Spherical:

• In decibels…

I = pv =1

∆t

0

T

p t v t dt

I = p2

ρc; I ∝ p2

I r ∝1

r2;

I ≡ constant ;

LI = 10 logI

I0; I0 = 1 pW

m2= 10−12 W m2

I(r) =∏

4πr2

I(r) ∝1

r


Sound (acoustic) intensity – example

• Ex: In a rock concert, measurements are performed next to you

yielding a value of 90 dB. Which level will a person who is 5

times further away from the speakers perceive, assuming…

‒ … plave wave propagation?

‒ … cylindrical wave propagation?

‒ … spherical wave propagation?


Frequency weightings (I)

• Correlate objective sound measurements with subjective human response

‒ A-weighting [dB(A)/dBA]: designed to reflect the response of how the

human ear perceives noise, i.e. 20 Hz-20 kHz

Only really accurate for relatively quiet sounds and pure tones?

Low frequency noise is suppressed (wind turbine noise?)

‒ C-weighting [dB(C)/dBC]: developed for high level aircraft noise

‒ Z-weighting: zero frequency weighting (un-weighted values)

‒ B-weighting: covers the mid-range between the A- and C-weighting

‒ D-weighting: designed for use when measuring high level aircraft noise

________________________________*Filters are defined in the standard IEC 61672

Fallen into disuse


Frequency weightings (II)

• Filters and calculation

Lweighted = 10 log 10(Ln+weighting)

10


Summation of noise (I)

• Types of sources

‒ Correlated (or coherent)

Constant phase difference, same frequency

Interferences (constructive/destructive)

‒ Uncorrelated (or uncoherent)

The total RMS pressure:

Lp,tot = 20 log

n=1

N

10Lp,n20

Lp,tot = 10 log

n=1

N

10Lp,n10

For uncorrelated sources, the 3rd term vanishes

ptot2 = p1

2 + p22+2

∆𝑡

t0

t0+∆t

p1 t p2 t dt


Summation of noise (II)

• Graphical methods

‒ Adding equally loud incoherent sources

‒ Adding two different sources

e.g. L1=61 dB / L2=55 dB

‒ Substracting two different sources

e.g. LS+N=65 dB / LN=60 dB

Lt= 62 dB

Lt= 63.4 dB


Single event noise metrics

• Maximum sound level (Lmax):

‒ Accounts only for sound amplitude [dB/dBA…]

• Sound exposure level (SEL) & Single event noise exposure level (SENEL)

‒ Total “noisiness” of an event. It takes duration into account

‒ If SENEL is measured for the period when the level is within 10 dB of the

Lmax, it will be essentially the same as SEL


Cumulative exposure metrics

• Equivalent SPL during the measurement time T (units: dB, dBA…)

Ex: Calculate the Leq,8h that corresponds to 105 dBA for 15 min.

Leq,T = 10 log1

T 0

Tp2(t)

pref2 dt =10 log

1

T 0

T

10Lp(t)

10 dt


Other indicators / Measurement of SPL

• Day and night average sound level (DNL or Lden)

• Community noise equivalent level (CNEL)

• Time above threshold

• Effective perceived noise level (EPNL)

• …

• Measurement of SPL: Sound level meter

‒ Microphone measures acoustic levels omni-directionally

‒ Sampling: Fast (0.125 s), Slow (1 s), Peak (impulse value 35 ms)

‒ Weighting filters (A, C…) built-in

‒ Calculation of Leq,T, building acoustic indicators, traffic noise…

‒ Calibrated


Regulations – environmental noise

• Infrastrikturprop.

1996/97:53

• Noise-maps

Location Measure Road Track Flight

Indoors LAeq,24h 30 30 30

Indoors LAFmax 45 45 45

Outside (façade) LAeq,24h 55 60 55

Outside LAFmax 70 70 70


Malmö – actions for noise exposure 2014

• Citizens exposed to >30 dBA indoors: 48 000,

>55 dBA outdoors: 126 000.

• Estimated cost (incl. health care and loss of work): 1 100 MSEK

• Proposed long term measures (250 MSEK):

– Source: Lower speed limit, silent asphalt,

driving style and silent car/tires

– Sound reduction: Noise barriers,

allowance for improvement of

sound reduction at dwellings

– Focus on sensitive places, e.g.

schools, pre-schools and parks

http://malmo.se/Stadsplanering--trafik/Trafik--hallbart-resande/Trafikbuller.html

http://malmo.se/Stadsplanering--trafik/Trafik--hallbart-resande/Trafikbuller.html


Outline

Course Information

Wave propagation


Summary


Types of waves – classifications

• Depending on propagation media

‒ Mechanical waves (solids and fluids)

‒ Electromagnetical waves (vacuum)

• Propagation direction

– 1D, 2D and 3D

• Based on periodicity

– Periodics and non-periodics

• Based on particles’ movement in relation with propagation direction:

‒ Longitudinal waves (solids and fluids)

‒ Transverse waves (solids)

• … NOTE: waves do not transport mass, just energy


Types of waves in solid media

• Longitudinal waves (∞ medium ≈ beams)

– Quasi-longitudunal waves (finite ≈ plates)

• Shear waves

• Bending waves (dispersive)

cL =E

ρ

cqL =𝐸′

ρ=

E

ρ(1 − υ2)

csh =G

ρ=

E

2(1 + υ)ρ

cB = ω4 𝐵

𝑚B𝜕4vy

𝜕x4+m

𝜕2vy

𝜕t2= 0

G𝜕2vy

𝜕x2− ρ𝜕2vy

𝜕t2= 0

E′𝜕2vx𝜕x2

− ρ𝜕2vx𝜕t2

= 0

x

y

Plate: E, G, ρ, υ, h

m = ρh

Bplate =Eh3

12(1 − υ2)NOTE: torsional waves (beams and columns) are not address here

Bbeam = Ebh3

12


Waves in fluid media (I)

• Sound waves: longitudinal waves

‒ Pressure as field variable

‒ Velocity as field variable

Comparing both equations: (acoustic impedance)

𝜕2p

𝜕x2−1

c2𝜕2p

𝜕t2= 0

cair =γP0

ρ(T = 0°C)1 +

T

2 ∙ 273= 331.4 1 +

T

2 ∙ 273,cmedium =

D

ρ,

p x, t = p± cos(ωt ± kx) = p±e−i(ωt±kx)

𝜕2v

𝜕t2= c2

𝜕2v

𝜕x2v x, t =

1

ρc p± e

−i(ωt±kx)

Z ≡p±v±= ±ρc

k =2π

λ


Waves in fluid media (II)

Time and position dependency: p x, t = p+ cos ωt − kx = p+e−i(ωt−kx)


Other types…

• In reality, combinations of aforementioned waves can exist, e.g.

• Surface waves

Water waves

(long+transverse waves)

Particles in clockwise circles. The radius

of the circles decreases increasing depth

Pure shear waves don’t exist in fluids

• Body waves

Rayleigh waves

(long+transverse waves)

Particles in elliptical paths. Ellipses

width decreases with increasing depth

Change from depth>1/5 of λ


Wave phenomena

• Interferences: constructive / destructive

• Standing waves

p− x, t = p cos(ωt − kx)

p+ x, t = p cos(ωt + kx)p x, t = p− x, t + p+ x, t = 2 p sin(kx)cos ωt

Position-dependent amplitude

oscillating according to cos(ωt)Two travelling waves propagating in

opposite directions

p1 x, t = p cos(ωt − kx)

p2 x, t = p cos(ωt − kx + θ)p x, t = p1 x, t + p2 x, t = 2 pcos

θ

2sin(ωt − kx + θ)

Constructive/destructive

depending on Ф


Outline

Course Information

Wave propagation


Summary


Summary

• Course introduction

• Basics of acoustics

• Wave propagation

REFERENCES: Animations retrieved from Dan Russell’s website

http://www.acs.psu.edu/drussell/demos.html

Thank you for your attention!

[email protected]

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Acoustics (VTAF05) - LTH fileAcoustics (VTAF05) JUAN NEGREIRA, ... D. Bard / Akustik VTAF05 / 3 Nov. 2015 Laboratories & Project task ... Wind turbine noise