Basics of Acoustics - Siemens Digital Industries Software Fundamentals... · Basics Acoustics...

Basics of Acoustics

2019.01.30

Unrestricted © Siemens AG 2019 Siemens PLM Software

Agenda .

Basics Acoustics Theory

Acoustic Hardware: Microphones

Analysis and Processing

Siemens Solutions

2019.01.30

Unrestricted © Siemens AG 2019 Siemens PLM Software

.




Siemens Solutions

Agenda

2019.01.30

Unrestricted © Siemens AG 2019Page 5 Siemens PLM Software

Basics Acoustics TheoryWhat is sound?

Sound is a pressure fluctuation which propagatethrough gases, liquids or solids.

• A vibrating surface moves the particles of themedium.

• When a sound wave acts upon a particle, thatparticle is temporarily disturbed from its restposition.

• The particles transfer momentum from one particle toanother.

• Areas of compressions and rarefactions travelthrough the medium with a Speed of Sound.

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Basics Acoustics TheorySpeed of sound

The speed of sound determines how fast thecompressions and rarefactions travel throughthe medium. It depends on the physicalproperties of the elastic medium.

It’s dependent of:

Medium (gaseous/liquid/solid)

Temperature

𝑐𝑠𝑜𝑙𝑖𝑑 > 𝑐𝑙𝑖𝑞𝑢𝑖𝑑 > 𝑐𝑔𝑎𝑠𝑒𝑜𝑢𝑠

Medium Temp [⁰C]

Speed [m/s]

Air 0 331

Air 20 343

Ethanol 20 1162

Water 20 1482

Steel - 5960

c = 20.05 ∙ 𝑇[𝐾]

𝑇 𝐾 = 𝑇 ℃ + 273.1

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Basics Acoustics TheoryFrequency of sine waves

• The period T [s] is the time of one completesinusoidal, vibrational cycle.

• The frequency f [Hz] is the reciprocal of the period:

• Frequency range of human hearing is between20Hz and 20,000 Hz (20kHz)

• Frequencies lower than 20 Hz are perceived asvibrations, frequencies above 20,000 Hz arereferred to as ultrasonic.

Period T [s]

𝑓 =1

𝑇

freq Play me

125 Hz

250 Hz

500 Hz

1000 Hz

3500 Hz

5000 Hz

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Basics Acoustics TheoryWavelength λ

• The wavelength [m] is defined as the distance a pure-tone wave travels during a full period.

• is significant in a number of phenomena such as absorption and diffraction.

• is related to the frequency f and the speed of sound c through:

Why bother about ? It’s often important when thinkingabout boundary conditions - a 20Hz pure tone will not fitin a 5x5m room!

𝜆 = 𝑐 ∙ 𝑇 =𝑐

𝑓

Frequency Wavelength

10Hz 34m

34Hz 10m

340Hz 1m

3400Hz 10cm

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Basics Acoustics TheoryComplex Waves

Speech and music waveforms are far more complex than simple sine waves. However, no matter how complex the waveform is, it can be reduced to sine components

+

+(…)=

+

=

500 Hz

1200 Hz

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Basics Acoustics TheoryHow is sound measured?

Sound is measured as pressure fluctuations.

• The magnitude of pressure fluctuations is very small, generally in the rangefrom 0.00002 Pa (20 μPa) to 20 Pa as compared with the atmosphericpressure of 100 kPa.

• The brain does not respond to the instantaneous pressure, it behaves likean integrator. Therefore, the RMS (Root Mean Square) sound pressurelevel has been introduced.

𝑝 =1

𝑇∙ 0

𝑇

𝑝2 𝑡 𝑑𝑡

𝑝 =𝐴

2= 0.707 ∙ 𝐴

Linear time-averaging

Special case: RMS pressure of a pure tone

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Basics Acoustics TheoryDecibel scale

• The Bel scale is a logarithmic way of describing a ratio. It represents themeasured level as a ratio of what you hear to the typical threshold ofperception of an average human. Decibel, or dB, is 1/10th of a Bel.

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

• SPL = 0 dB = 0.00002 Pa is the threshold of hearing.• SPL = 94 dB = 1Pa• SPL = 120 dB = 20 Pa is the threshold of pain.

• Symbol used for SPL (e.g. in displays): L, L(dB), L dB.

Breathing

Soft whisper

Conversation

Niagara Falls

Rock concert

Jet takeoff

𝑆𝑃𝐿 = 20 ∙ log10 𝑝

𝑝𝑟𝑒𝑓= 10 ∙ log10

𝑝2

𝑝2𝑟𝑒𝑓

reference pressure pref = 2.10-5 (20 μPa) is minimum audible pressure at 1000 Hz

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Jet Taking Off

Inside Compact Car

Bedroom at Night

Heavy Truck

Average Classroom

Soft Whisper

Painful

Very Noisy

Noisy

Moderate

Quiet

Barely Audible

“The sound measured today in the office was around 84500 μPa”

Basics Acoustics TheoryDecibel scale - Sample sound levels

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Basics Acoustics TheoryHow do we hear?

• Sound waves travel into the ear canaluntil they reach the eardrum.

• The eardrum passes the vibrationsthrough the middle ear bones orossicles into the inner ear.

• The inner ear is shaped like a snailand is also called the cochlea. Insidethe cochlea, there are thousands oftiny hair cells.

• Hair cells change the vibrations intoelectrical signals that are sent to thebrain through the hearing nerve.

(eardrum)

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Human hearing system

Acoustic Wave

Sensation of hearing

Vibration

Electric signals

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Basics Acoustics TheoryHuman audible Range

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

0

102030405060708090

100110120

L dB

HEARING THRESHOLD

PAIN THRESHOLD

HEARING DOMAIN

MUSIC

SPEECH

130

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Basics Acoustics TheoryInterference

What if we have more than 1 sound source?

• Interference occurs when sounds from two or more sources come together.

• It refers primarily to combination effects associated with sound waves of thesame frequency.

Con

stru

ctiv

ein

terfe

renc

e

Des

truct

ive

inte

rfere

nce

+

=

+

=

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Basics Acoustics TheorySumming SPL – coherent sinusoidal sources

94 dB (1 Pa) at 1000 Hz

+

* in phase!

=

100 dB (2 Pa) Overall

94 dB (1 Pa) at 1000 Hz*

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Basics Acoustics TheorySumming SPL - incoherent sinusoidal sources

94 dB (1 Pa) at 1000 Hz

94 dB (1 Pa) at 2000 Hz

+

=

97 dB (1.42 Pa) Overall

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20 copyright LMS International - 2010

Basics Acoustics TheorySumming SPL - incoherent random sources

94 dB Overall Level

94 dB Overall Level

+

=

97 dB Overall Level

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Basics Acoustics TheorySound Fields

• On a distance from the sound source that is smaller than the wavelength of the highest frequency of interest.

• Significant variations in SPL with distance to source.

Location at which we measure has an important role in understanding the obtained results.

• Source can be considered as a point source.

• Consists of two parts: free field and reverberant field.

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Sound Source

Mic

Sound Source

Basics Acoustics TheorySound Fields - Diffuse field vs. free field

Diffuse Field

Uniform sound field regardless of microphone position

Sound Source

Free Field

Sound propagates without reflection, sound level decreases with distance

- microphone

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Basics Acoustics Theory Sound Fields - Near field vs. far field

Near Field Close to source Circulating & Propagating No predictable relationship

between distance and pressure

Far Field Far from source, source

appears as point source Plane wave approximation Linear relationship between Lp

& distance

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Basics Acoustics TheorySound reflection

Incident sound wave on a surface: (a) part of it is reflected, (b) part is absorbed and (c) part is transmitted:

The amount of reflection is dependent upon the dissimilarity of the two media (e.g. medium_1 – air, medium_2 – concrete wall).

The listener in a room with a source of sound. First, direct sound reaches the listener, then early reflections and finally late reflections or reverberation.

Dry speech Speech in a reverberant room

reflected energy

incident energy

transmitted energy

absorbing material

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Basics Acoustics TheoryAnechoic 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

The lowest frequency at which an anechoic room can be useddepends on the room volume and the depth of the wedges.

Rule of thumb:

h

ℎ ≅λ

2

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Basics Acoustics TheorySemi-anechoic Room

• Flat, reflecting floor• Sound-absorptive walls and ceiling• Optional: chassis dynamometer/ roller bench

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

Typical applications:

• Sound Power • TPA• ASQ• In-room Pass-by noise

semi-anechoic room with roller bench

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Basics Acoustics TheoryReverberation 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

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

Sound path

Sound Source

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Basics Acoustics TheoryRefraction

1) effects of wind:

2) temperature gradients:

Refraction is the bending of a sound wave due to changes in the medium. Inopen spaces, the wind field and temperature gradients play an important role.

c1

c2

increasing temperature with heightdecreasing temperature with height

c1

c2

c1

c2

vwind

wind coming from the right

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Basics Acoustics TheoryDiffraction

• Diffraction is the bending of a soundwave around the edges ofobstructions (barrier, opening,…) inthe path of the wave

• Bending due to diffraction is highlyselective with respect to frequency

• Long wavelength, low frequencysounds are less affected by barriersand openings than short wavelength

Example:

• Highway barriers fail in reducing lowfrequency truck noise

effects of diffraction at low frequencies : (a) behind a barrier, (b) through an opening

effects of diffraction at high frequencies : (a) behind a barrier, (b) through an opening

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Siemens Solutions

Agenda

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Acoustic Hardware: MicrophonesPrinciple of microphone

Condenser microphones operate on a capacitivedesign and utilize basic transduction principles:

sound pressure ↓

capacitance variation ↓

electrical voltage

In the presence of oscillating pressure, the gapbetween the diaphragm and backplate changes,which changes the capacitance.

To measure the change, a voltage is applied to thebackplate to form a transducer.

The charge applied to the back-plate can be either supplied externally (noPre-polarization) or from an electret layer on the back-plate (pre-polarization).

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Acoustic Hardware: MicrophonesMicrophones & preamplifiers, selection criteria

• Microphone is only the top part

• The very weak signal is pre-amplifiedbefore being sent over a cable to adata acquisition system

• There are 3 main criteria which haveto be taken into account when selectinga microphone:

• Dynamic Range• Frequency Response• Field Response

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Acoustic Hardware: MicrophonesDynamic range

Dynamic range - Range between the lowest level and the highest level that the microphone can handle.

Large microphone & loose diaphragm → high sensitivitySmall microphone & stiff diaphragm → low sensitivity

The sensitivity of a microphone is determined by the size of themicrophone and the tension of its diaphragm.

High sensitivity → measure very low levelsLow sensitivity → measure very high levels

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Acoustic Hardware: MicrophonesDynamic range

1”½”

¼”1/8”

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Acoustic Hardware: MicrophonesFrequency response

Frequency response refers to the way a microphone responds to differentfrequencies.

Ideally, the frequency response should be as flat as possible in the frequencybandwidth of interest.

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Acoustic Hardware: MicrophonesField response

There are three response types for precision condenser microphones, which are:Free Field, Pressure, and Random Incidence responses.

Their characteristics are similar at lower frequencies, but differ significantly athigh frequencies.

Free Field Pressure Random Incidence

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Acoustic Hardware: MicrophonesField response – “free field” type

free-field microphone:

• minimal (zero) interference with sound field

• designed essentially to measure the sound pressure as itexisted before placing the mic

• localized, not negligible disturbances of sound field at higherfrequencies

• accurate when measuring sound pressure levels that radiatefrom a single direction and source, which is pointed directly(0°incidence angle) at the microphone diaphragm, andoperated in an area that minimizes sound reflections (e.g.anechoic room).

Free Field

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Acoustic Hardware: MicrophonesField response – ”pressure” type

Pressure microphone:

• measuring actual sound pressure on the surface of the diaphragm

• typical measurement in a closed coupler or at a boundary or wall

• microphone as part of the wall and measures the sound pressure on the wall itself.

• sound pressure exerted on walls, exerted on airplane wings, or inside structures such as tubes, housings or cavities.

Pressure

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Acoustic Hardware: MicrophonesField response – ”random incidence” type

Random incidence microphone:

• Designed to be omnidirectional and measure soundpressure coming from multiple directions, multiple sourcesand multiple reflections.

• Designed and calibrated by manufacturer to compensate forits own presence in the field.

• When taking sound measurements in a reverb chamber,church or in an area with hard, reflective walls, a RandomIncidence microphone should be used to accurately measurethe sound from multiple sources.

Random Incidence

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Siemens Solutions

Agenda

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Analysis & ProcessingFrequency spectrum

It is a property of all real waveforms that they can be made up of a number ofsine waves of certain amplitudes and frequencies.

Each sine wave in the time domain is represented by one spectral line in thefrequency domain. The conversion of a time signal to the frequency domain (andits inverse) is achieved using the Fourier Transform.

The digital computation of the Fourier Transform is called the Discrete FourierTransform (DFT). A dedicated algorithm to compute the DFT is the Fast FourierTransform (FFT).

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The term “octave” is borrowed from music theory • 8 whole tones between notes of the same name

A4: 440 Hz(standard pitch)

A5: 880 Hz

Human hearing: frequency

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Human hearing: frequency

A4: 440 Hz(standard pitch)

A5: 880 Hz A6: 1760 Hz A7: 3520 Hz

440 Hz Span 880 Hz

Span 1760 Hz Span

The term “octave” is borrowed from music theory • 8 whole tones between notes of the same name

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Analysis & ProcessingOctaves

Octave bands group energy in standardized frequencybands.

Reference octave band: 1000 Hz as center frequency

is used to calculate the other bands which cover thewhole bandwidth. Each next center frequency is thedouble of the previous one.

Lowercutoff frequency

Center frequency

Upper cutofffrequency

11 16 22

22 31.5 44

44 63 88

88 125 177

177 250 355

355 500 710

710 1000 1420

1420 2000 2840

2840 4000 5680

5680 8000 11360

11360 16000 22720

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Analysis & ProcessingFractional Octave bands

For finer analysis, other octave band typeswere introduced.• 1/3 octaves – each octave band is dividedinto 3 separate bands

• 1/12 octaves • 1/24 octaves

16 31.5 63 125 250 500 1000 2000 4000Frequency [Hz]

20

30

40

50

60

70

80

Pres

sure

dB/

2e-0

05 [P

a]

Octaves Traces: 2/2

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Analysis & ProcessingOctave Bands calculations

There are two ways to calculate the center and boundary frequencies of bands, Base2 and Base 10 method:

Base 2 1/1 Octaves𝑓𝑐 = 1000 ∙ 2

𝑁

𝑓 𝑙𝑜𝑤𝑒𝑟 = 2−12 ∙ 𝑓𝑐

𝑓 𝑢𝑝𝑝𝑒𝑟 = 212 ∙ 𝑓𝑐

Base 2 1/3 Octaves

𝑓𝑐 = 1000 ∙ 2𝑁3

𝑓 𝑙𝑜𝑤𝑒𝑟 = 2−16 ∙ 𝑓𝑐


Base 10 1/1 Octaves

𝑓𝑐 = 1000 ∙ 103𝑁10

𝑓 𝑙𝑜𝑤𝑒𝑟 = 10−320 ∙ 𝑓𝑐


Base 10 1/3 Octaves

𝑓𝑐 = 1000 ∙ 10𝑁10

𝑓 𝑙𝑜𝑤𝑒𝑟 = 10−120 ∙ 𝑓𝑐


𝑓𝑜𝑟 𝑁 = ⋯ ,−2,−1,0,1,2…

IEC

612

60, A

NSI

S1.

11-2

004

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Analysis & ProcessingA-weighting

• Human hearing is not equally sensitive to all frequencies.

• Most sensitive between 3000 and 6000Hz.

• 1000Hz pure tone at 40dB = 40Hz at 70dB.

• A-weighting is a correction to account for perception: unit label: dB(A)

• dB(A) as a “noise label” for i.e. household equipment, environmental noise, tools, etc.

• Used for analysis, not for replay!

𝒇𝟎 [Hz] 63 125 250 500 1000 2000 4000 8000

𝐾𝐴𝑖 -26.2 -16.1 -8.6 -3.2 0 +1.2 +1 -1.1

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Analysis & ProcessingA-,B-,C-, D- and Z-weighting

• Based on Loudness curves:equal perceived loudness,expressed on phones

• A-weighting = 40-phone curveIs mostly used

• B- and C-weighting = 70- and100-phone equal loudnesscontours

• D-weighting for aircraft noise:1-10 kHz region

• Z-weighting: no weighting or“linear” weighting

frequency (Hz)

SPL

(dB

)

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Analysis & ProcessingTime weighting

Very often what we measure is not stationary - we can calculate a single SPL, but what about transient sounds? Duration of time over which we calculate the SPL starts to play a role.

𝑝 =1

𝑇∙ 0

𝑇

𝑝2 𝑡 𝑑𝑡

𝑆𝑃𝐿 = 20 ∙ log10 𝑝

𝑝𝑟𝑒𝑓

• Sound level meter & Integrating Sound Level Meter according IEC 61672-1 (class 1)

• Sound Pressure Level, A-weighted, Fast (1/8 sec), Slow (1 sec), User defined

• Leq: Equivalent Sound Pressure Level

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Analysis & ProcessingEquivalent Sound Pressure Level Leq

Equivalent Sound Pressure Level Leq – a widely used noise parameter thatcalculates a constant level of noise with the same energy content as the varyingacoustic noise signal being measured.

\

LAeqT A-weighted equivalent SPL over time T - first to current tracking pointLAeqt A-weighted equivalent SPL over time t - last to current tracking point

0.00 3600.00sTime

40.00

70.00

dB(A

)Pa

0.00

1.00

Ampli

tude

3600.00

47.55

F Overall level - LAeqt Point1 (A) 76.1 dBF Overall level - LAeqT Point1 (A) 81.5 dB

0.00 3600.00sTime

40.00

70.00

dB(A

)Pa

0.00

1.00

Ampli

tude

3600.00

47.55

F Overall level - LAeqt Point1 (A) 76.1 dBF Overall level - LAeqT Point1 (A) 81.5 dB

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Analysis & ProcessingLevel Calculation Presets

Type Level integration Description

Leq, LAeq Linear Continuous noise level, A-weight.

LF, LAF Fast 125ms averaging, A-weighted

LS, LAS Slow 1s averaging, A-weighted

LI, LAI Impulsive 35ms averaging, A-weighted

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Siemens Solutions

Agenda

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Siemens solutionsThe 6 boxes of Acoustic Testing

Do I meet quality objectives?

Sound PressureAcoustic Analyzer

Sound Material & Component Testing

What material should I use to

reduce the levels?Do I meet

standards?

Sound Power Pass-by Noise

Does it sound right?Why is it annoying?

Sound Quality

Where is the sound coming from?

Sound Source Localization

What is the root cause?

Source? Path?TPA

Source-Path-Receiver

Thank you.

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