gert sablon, lms international fundamentals of acoustics

Gert Sablon, LMS International

Fundamentals of Acoustics

2 copyright LMS International - 2010

Overview

BasicAcoustics

Theory

AcousticHardware:

Microphones

Applications&

Processing

The 6 S’s ofAcousticTesting

The Real world!You’re either meeting the requirements or you’re not

Sound Power Requirement Sound Quality

Source Identification

You have not metyour requirementsand need to solve

them fast

You’ve met yourrequirements and

now want toimprove the

sound quality

Source Ranking

Solution


Sound – Why is it important?

Cannot sell your product

EU-US Regulations, Passby Regulations

Example: Construction Equipment (Bulldozers, etc), Printers, Cars

Warranty Costs

Often driven by perceived issues via sound from customers

Competitive Advantage

Distinguish your product from competition

Example: Quietest Washing Machine on Market

Increase Sales

Example: Ice machine that can be in hospital patient rooms, rather than only nursingstation if it is quiet enough not to disturb patients

Military

Helicopter, Submarines, Military Vehicle – Avoid detection by enemy


Sound – Why is it important?

Cannot sell your product

EU-US Regulations, Passby Regulations

Example: Construction Equipment (Bulldozers, etc), Printers, Cars

Warranty Costs

Often driven by perceived issues via sound from customers

Competitive Advantage

Distinguish your product from competition

Example: BMW and Lexus are using their sound interior as a brand image



History

The word "acoustic" is derived from the ancient Greek wordακουστο�ς, meaning able to be heard.

Pythagoras

Aristotle

Leonardo Da Vinci

Vitruvius

Galileo

Marin Mersenne (father of acoustics)

Newton

Helmholtz

Lord Rayleigh

Sabine …


Effects of Noise

Pleasant vs. discomfort

Perceived product image

Perceived product quality

Purchasing criteria

Environmental noise

Comfort & Perception Safety HealthFatigue

Reduced sensitivity

Risk for accidents

Reduced balance

Abdominal & chest pain

Lower back pains

Reduced concentration White finger syndrome

Hearing loss

Effects of Noise (2)

Pregnant woman, infants and children may be seriously harmed by exposure to high noise.

Effects of Noise - hearing loss, sleep deprivation, chronic fatigue, anxiety, hostility, depression &hypertension. Increased Noise levels trigger Adrenaline resulting in narrowing of blood vessels

Vehicular Noise is known to constrict arterial blood flow and lead to elevated blood pressure..high noise levels are known to produce medical stress, another risk associated withcardiovascular problems

Other proven effects of high noise levels are increased frequency of headaches, fatigue,stomach ulcers and head-rush

Unlike Physical stress, Medical stress has series of health effects

Low birth weights and birth disorders are also associated with Increased Noise pollution.



Noise Control – Reduction of Levels ?

Disturbance

(Source)

Propagation

(Path)

Perception

(Receiver)

Sound Power - Cause


Sound Intensity - Flow Sound Pressure - Effect

Acoustics in a Nutshell – Source-Path-Receiver


Sound Descriptors

Amplitude (Sound Pressure)

Frequency

Wavelength

Phase

Freq Wavelength

34 Hz 10 meter

340 Hz 1.0 meter

3400 Hz 0.1 meter

What is Sound? Physics

pressure fluctuations which propagatethrough an elastic medium (air, liquid, gas,solid)

Any vibrating action which moves theparticles of the medium (vibrating plate,loudspeaker,…) may act as a sound source(=structure born noise <-> airborne noise)

The pressure fluctuations are propagatedthrough the air with the speed of sound (c =343 m/s @ 20ºC)

Frequency (Hz) & period (s):

20 Hz to 20,000 Hz

Wavelength (m):


c

fcT

1

Tf

c

fcT


Basic Acoustics TheoryInstantaneous Pressure and RMS pressure

The sound pressure p (Pa) varies in time and space

The magnitude of pressure fluctuations is very small, generally in the range from 0.00002 Pa(=20 μPa) to 20 Pa as compared with the atmospheric pressure of 100 kPa

The brain does not respond to the instantaneous pressure, it behaves like an integrator.Therefore, the RMS (Root Mean Square) pressure has been introduced. It is defined as:

p T

p0

2(t )dt1

T

“linear time-averaging”

Equal weighting is given to all parts of the signal that fall in theperiod T. This period can be chosen at will to suit the occasionand needs to provide the information required

Special case:RMS pressureof a pure tone:

A

20.707 Ap


Sound Pressure – Pa or dB ?

Basic unit of Sound Pressure isPascal (Pa). However, SoundPressures are normally representedin dB !!!

Why decibel?

Noise level if expressed in Pascal,the range is

For whisper - 10-5 Pa

For Airplane - 10 Pa

Ratio is – 100000 !!!

Sound Pressure in dB ?

Refrigerator ≈ 0.006 Pa / Diesel generator ≈ 6 Pa in LINEAR SCALE Ooops!!!Where isits top?




Refrigerator ≈ 0.006 Pa / Diesel generator ≈ 6 Pa in LOGARITHMIC SCALE



The human ear responds logarithmically rather than linearly to stimuli (pressuredisturbance)


Sample Sound Levels


Basic Acoustics TheoryPressure and Decibels

The Bel scale is a logarithmic scale well suited to human hearing,which is rather logarithmic than linear in its behaviour. deciBel, ordB, is 1/10th of a Bel.

The Sound Pressure Level SPL (dB) is defined as:

SPL = 0 dB is the threshold of hearingSPL = 120 dB is the threshold of painSPL = 94 dB = 1PaSymbol used for SPL (e.g. in displays): L, L(dB), L dB

ref ref

reference pressure pref = 2.10-5 (20 μPa) is minimumaudible pressure at 1 kHz

RMS pressure mean square pressure


Sound Pressure and Sound Pressure Level

The decibel (dB) is a logarithmic unit of measurementthat expresses the magnitude of a physical quantityrelative to a specified or implied reference level.


20 Hz 50 100 200 500 1 k 2 k 5 k 10 k 20 kHz

Basic Acoustics TheoryHuman Auditory Range

L dBPAIN THRESHOLD

HEARING DOMAIN

MUSIC

SPEECH

HEARING THRESHOLD

130

120

110

100

90

80

70

60

50

40

30

20

10

0


Decibel “Funny” Math

6 dB = Double Sound Pressure Level

Example 1 – Decibel “Funny” Math

Sound Source 1

2 Pa = 100 dB

?Sound Source 2

2 Pa = 100 dB

Source 1 + Source 2*?

* Assume coherent sources



Sound Source 1

2 Pa = 100 dB

Sound Source 2

2 Pa = 100 dB

Source 1 + Source 2

4 Pa = 106 dB

100 dB + 100 dB = 106 dB!

* Assume coherent sources



Adding Sources Together

Coherent

vs

Incoherent

Makes a Difference!

Pa

Amplitude

Pa

Amplitude


0.00 2000.00

2.00

60.0e-6

Summation of Coherent Sinusoidal Sound Sources

1000 Hz - Overall Level: 100 dB

100 dB (2 Pa) at 1000 Hz *

100 dB (2 Pa) at 1000 Hz

+

* In Phase

0.00 2000.00Hz

2.00

60.0e-6

Hz


Pa

Amplitude

Pa

Amplitude

Pa

Amplitude

0.00 2000.00

2.00

Summation of Coherent Sinusoidal Sound Sources


0.00 2000.00Hz

0.00

2.00

Hz


100 dB (2 Pa) at 1000 Hz

+0.00

* In Phase

0.00


2000.00Hz

4.01

0.00

Summation of Coherent Sources: 106 dB

100 dB (2 Pa) at 1000 Hz *

=

106 dB (4 Pa) Overall

6 dB Increase

Pa

Amplitude

Pa

Amplitude

0.00 3000.00Hz

2.00

Summation of Incoherent Sinusoidal Sound Sources


0.00


3000.00Hz


2.00

0.00

100 dB (2 Pa) at 1000 Hz

100 dB (2 Pa) at 2000 Hz

+0.00

Pa

Amplitude

Pa

Amplitude

Pa

Amplitude

0.00 3000.00Hz

2.00

Summation of Incoherent Sinusoidal Sound Sources


0.00 3000.00Hz

2.00

0.00


100 dB (2 Pa) at 1000 Hz

100 dB (2 Pa) at 2000 Hz

+0.00

=

103 dB (2.82 Pa) Overall

0.00


3000.00Hz

2.00

89.9e-6

Summation of Incoherent Sources: 103 dB

3 dB Increase

Pa

Amplitude

Pa

Amplitude


100 dB Overall Level


+

0.20

0.11

Summation of Incoherent Random Sound Sources

Overall Level of Random Signal1: 100 dB

0.00

0.00

24000.00

24000.00

0.20

0.11

Hz


Hz

Pa

Amplitude

Pa

Amplitude

Pa

Amplitude

0.00


24000.00Hz

0.28

0.11

Summation of Random Signals Overall Level: 103 dB

0.00 24000.00Hz

0.20

0.11

Summation of Incoherent Random Sound Sources



+


=

103 dB Overall Level0.00 24000.00

100 dB100 dB

Hz

Overall Level of Random Signal1:Overall Signal of Random Signal2:

0.20

0.11


3 dB Increase


Decibel “Funny” Math

6 dB increase when adding coherent sources

3 dB increase when adding incoherent sources


Sound Source 1 10 Pa = 113 dB

Sound Source 22 Pa = 100 dB

Sound Source 32 Pa = 100 dB

Source 1 + Source 2 + Source 3*10.39 Pa (RMS sum) = 114.3 dB

113 dB + 100 dB + 100 dB = 114.3 dB* Assume incoherent sources


Sum is almost identical to largest source

1) One source can dominate, affecting others does not help

2) If all sources equal, must address all


Addition of Sound Pressure Levels


Subtraction of Noise Levels

When we measure Noise from the source of interest we also measure Background Noise.This needs to be compensated to determine the true noise from the source.


Background Noise Criterion

If ΔL is less than 3 dB, the background noise is too high for anaccurate measurement and the correct noise level cannot befound until the background noise has been reduced. If, on theother hand, the difference is more than 10 dB, the backgroundnoise can be ignored.

The Measured Noise should be at least 10 dB more than theambient noise or background noise

ISO 11202

ISO 3740 (series), ISO 9614 (series) ….


Free field – Double distance

When doubling distance from source

6 dB decrease for a point source

3 dB decrease for a line source


Human Hearing

Complex system

Sensor with amplifier, filter

Behaves differently for different frequencies or levels of sound


Equal Loudness Curves (Fletcher & Munson Curves)

An equal-loudness contour is a measure of sound pressure (dB SPL), over the frequency spectrum, forwhich a listener perceives a constant loudness when presented with pure steady tones.


A-,B-,C- and D-weighting

Filter with similar attributes to ear

Simple curve shape, attenuates low frequencies

1000 Hz - no gain/attenuation, used for microphone cals

dB(A) maps on 40dBdB(B) maps on 70dB

gain(dB)


Basic AcousticsA-,B-,C- and D-weighting

A-weighting corresponds to the 40-phone curve, i.e. the equal loudness contour which passes 40 dB at 1kHz. Is most often used

B- and C-weighting are similar to A-weighting in concept, but correspond to the 70- and 100-phone equalloudness contours

D-weighting has been introduced for the purpose of measuring aircraft noise. It attributes more significanceto the 1-10 kHz region

frequency (Hz)

1/1 octave

Center Frequency

(Hz)

50

100

200

400

800

1600

3150

6300

12600

63

125

250

500

1000

2000

4000

8000

16000

80

160

315

630

1250

2500

5000

10000

20160

1/n OCTAVE filter 1/1 1/3 1/12

Filter bandwidth

(about % of the center frequency)

70% 23% 6%

PressuredB/2e-005[Pa]


Logarithmic

frequency scale63 125 250 500 1000 2000 4000Frequency [Hz]

20

Octaves

16 31.5

Traces: 2/2

X2

2

1

Octave analysis – Constant % bandwidth

380

70

60

50

40

30

Other Terms

Near-field

Close to source

Far-field

Far from source, source appears as point source

Diffuse Field

SoundSource

Diffuse Field

Mic

Uniform levels of sound surround microphone

Free Field

Sound is in front, propagating without reflectionSound

Source

Mic

Free Field


Propagation of SoundAnechoic Room

Highly absorbing surfaces

Source radiates as in a free field

Almost no reverberation

To measure:

sound power of source

directivity pattern of radiating source

Anechoic rooms are more effective at high than at low frequencies. The lowestfrequency at which an anechoic room can be used depends on the room volumeand the depth of the wedges

A very large room (several m’s) with 1-2 m wedges is effective down to 100 Hz


anechoic room


Propagation of SoundSemi-anechoic Room

Flat, reflecting floor

Sound-absorptive walls and ceiling

To test sources that are normally mounted on or operate in the presence ofa reflecting surface (e.g. cars,…)

Roller bench inside:

Sound power

Transfer Path Analysis

Airborne Source Quantification

Pass-by noise

…semi-anechoic room with roller bench

Propagation of SoundReverberation Room

High-reflecting, non-parallel walls

Diffuse field: nearly uniform sound intensity

To measure:

sound power of sources

Sound absorptive properties of materials

Sound transmission through building elements

At low frequencies, the frequency response to wide-band noise shows severalpeaks corresponding to the room modes. At higher frequencies, the individualmodes begin to overlap and are less obvious

To make the room response more uniform at lower frequencies, low-frequencysound absorptive elements and rotating diffusers are often used


Sound path

Sound Source

typical reverberation room


Intensity

Sound is a vector quantity that has both magnitude and direction, because the energywill flow in some directions but not others

Intensity is dependent on the source’s properties and the distance from the source


Watts

m2 PowerArea

Energy

Area Time

Force Distance

Area Time

What is Intensity?

Sound intensity is the average rate of sound energy transmitted in the specifieddirection through a unit area (1 m2) normal to this direction at the point considered.

Intensity Pressure x Particle Velocity


W W

d S = WIS

I +

d S = 0IS

Sound power can bemeasured to anaccuracy of 1dB fromsources as much as10dB lower than thebackground noise.

Stationary backgroundnoise has no effect onmeasured power.

• ISO Standard

I +

I +

I -

I -

I -

I +

I +

I +

I +

Sound Intensity:Effect of Background Noise


Intensity: p-p probe

Measurement Equipment

SpacersMic Signal Conditioning

Card for SCADAS III

Mic Signal Conditioning Card for SCADAS Mobile


USB Remote forTest.Lab

Measurement ConsiderationsFrequency Limitations

Lower Frequency LimitPhase mismatch between microphones

Typically < 0.3˚ degrees for matchedprobe microphones

Ex: At 63 Hz the wavelength isapproximately 5.5 m and the change ofphase over a 12mm spacer is only 0.8°so a phase mismatch of ± 0.3° willcause a significant error in the intensity


Upper frequency Limit

Spacer distance :

< 1/6th of the wavelength

Grade of Accuracy Bias ErrorFactor

(dB)

Precision (Grade 1) 10

Engineering (Grade 2) 10

Survey (Grade 3) 7

Pressure Residual Intensity Index (PRII)

Pressure Residual Intensity Index (PRII) – quantifies the phase mismatch

Defined as a “noise floor” below which measurements cannot be made

Caused by a small phase difference between the 2 signals, which is interpreted asintensity along the spacer

Measured in an intensity calibrator which gives the same signal to both mics (0 intensity)

Phase change in degrees along the spacer distance must be over 5 times the phasemismatch for accuracy within 1dB

Dynamic Capability Index (Ld) – gives a limit to the Pressure Intensity Index that can bemeasured with accuracy, applying a bias error correction factor (K)



PRII Measurement

Notice the PRII Index is afunction of frequency and

ranges typically between 15dB to 25 dB

dBLp LI

LdPIo

React. index

Residual

Intensitylevel

frequencyResidual intensity level < Sound intensity level < Sound pressure level


Acquisition module

Lp

Sound Intensity:PRII, RI and Reactivity Index


Source localization using SI:Intensity measurement at 35 cm from washing machine


Source localization using SI:Intensity measurement at 65 cm from washing machine

ISO Quality Checks

ISO 9614 Field Indicators ensure the quality of the measurements and meshes

F2: Surface pressure-intensity indicator

Examines the difference between the pressure and the absolute values of intensity,which tells how diffuse or reactive a field is

• Small value = good measurement conditions

• Large value = probe not aligned well or measuring in a diffuse field

F3: Negative partial power indicator

Examines the difference between measured intensity and pressure while taking thedirection of the intensities into account, which gives amount of extraneous noise

• Positive direction = intensity from source under investigation

• Negative direction = intensity from extraneous sources

F4: Non-uniformity indicator

Indicates the measure of spatial variability that exists in the field, which verifies themesh adequacy



Intensity: p-U probe

Pressure velocity probe

Pros:Direct velocity measurementIntensity = simple spectral productNo frequency range limitsNear-field measurementsEasy-to-useSizeDirect input for VL-Acoustics

Contra:All ISO-standards relate to 2-mic probesHard to calibrate (frequency dependant)


velocity probe

1 D intensity probes 3D


Acoustic Hardware: Microphones


Principle of a microphone

Most microphones incorporate a thin diaphragm as the primary transducer which issensitive to the air (sound wave) acting against it. The mechanical movement ofthe diaphragm is converted to an electric output by means a secondarytransducer (Capacitor or Condenser type, piezoelectric crystal type, Electrodynamictype and Carbon type) that provides an analogous electrical signal.


Microphone Types


Condenser type – Externally Polarized

The condenser microphone cartridge converts sound pressure to capacitancevariations. This variation in capacitance is converted to an electrical voltageby using a constant electrical charge, which is either permanently built into themicrophone cartridge or applied to it via an external voltage source referred toas preamplifier. The backplate is polarised, either from an external voltagesource as the case with the externally polarized microphones or from apermanently charged polymer known as electret, as employed in theprepolarized condenser microphones .


Microphones

Electret type – Pre or Permanently Polarised

Prepolarized microphones contain anelectret, consisting of a speciallyselected and stabilized, hightemperature polymer material whichis applied to the top of the backplate.The electret contains trapped or

“frozen” electrical charges whichproduce the necessary electrical fieldin the air gap. The frozen chargeremains inside the electret and staysstable for thousands of years.


Electret Microphones referred as Pre orPermanently polarized mics are invented byGerhard M Sessler and James E West in1962 at Bell Labs


Microphone types – Cartridge diaphragm

ParametersSensitivity

1”110 mV/Pa

½”50 mV/Pa

¼”4 mV/Pa

Frequency Range 12.5 Hz – 4 kHz 3.15 Hz – 20 kHz 10 Hz – 10 kHzDynamic Range -2 dB to 110 dB 15 – 146 dB 40 – 168 dB


Special Microphones

Measurement microphones are sensitive to environmental factors such as wind, rainand snow. These problems have been reduced in the special Outdoor andEnvironmental microphones. The construction of these microphones protects themicrophone diaphragms from the influence of the environment. The outdoormicrophones are designed for permanent installation in the field, for example airportnoise monitoring or industrial plant noise monitoring.


Sound Level Meter


Noise Measurement Considerations

Time Weighting

Fast (125 m sec) / Slow (1 sec) / Impulse (35 m sec)

Frequency Weighting

Weighting Network: A, B, C, D or Lin


Calibration

Mic./SLM Performance Check

Periodic calibration – Reciprocity or Comparison Method

Field Check - Before & After Measurements

Environmental Influence

Sound Field

Background Noise

Indoor/Outdoor

Environmental Influence


Gert Sablon, LMS International

Thank You

gert sablon, lms international fundamentals of acoustics

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