Basics of Acoustics Agenda

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Basics Acoustics Theory

Acoustic Hardware: Microphones

Analysis and Processing

Siemens Solutions

Unrestricted © Siemens AG 2019 Page 2 2019.01.30 Siemens PLM Software . Agenda

Basics Acoustics Theory

Acoustic Hardware: Microphones

Analysis and Processing

Siemens Solutions

Unrestricted © Siemens AG 2019 Page 3 2019.01.30 Siemens PLM Software Basics Acoustics Theory What is sound?

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

• A vibrating surface moves the particles of the medium.

• When a sound wave acts upon a particle, that particle is temporarily disturbed from its rest position.

• The particles transfer momentum from one particle to another.

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

Unrestricted © Siemens AG 2019 Page 5 2019.01.30 Siemens PLM Software Basics Acoustics Theory Speed of sound

Temp Speed The speed of sound determines how fast the Medium [⁰C] [m/s] compressions and rarefactions travel through Air 0 331 the medium. It depends on the physical properties of the elastic medium. Air 20 343 Ethanol 20 1162

It’s dependent of: Water 20 1482

Steel - 5960 . Medium (gaseous/liquid/solid)

푐푠표푙푖푑 > 푐푙푖푞푢푖푑 > 푐푔푎푠푒표푢푠

. Temperature 푇 퐾 = 푇 ℃ + 273.1

c = 20.05 ∙ 푇[퐾]

Unrestricted © Siemens AG 2019 Page 6 2019.01.30 Siemens PLM Software Basics Acoustics Theory Frequency of sine waves

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

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

1 freq Play me 푓 = 푇 125 Hz • Frequency range of human hearing is between 20Hz and 20,000 Hz (20kHz) 250 Hz

• Frequencies lower than 20 Hz are perceived as 500 Hz vibrations, frequencies above 20,000 Hz are 1000 Hz referred to as ultrasonic.

3500 Hz

5000 Hz

Unrestricted © Siemens AG 2019 Page 7 2019.01.30 Siemens PLM Software Basics Acoustics Theory Wavelength λ

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

Frequency Wavelength •  is significant in a number of phenomena such as 10Hz absorption and diffraction. 34m 34Hz 10m

•  is related to the frequency f and the speed of 340Hz 1m c sound through: 3400Hz 10cm

푐 휆 = 푐 ∙ 푇 = 푓

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

Unrestricted © Siemens AG 2019 Page 8 2019.01.30 Siemens PLM Software Basics Acoustics Theory Complex 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 + = (…) =

Unrestricted © Siemens AG 2019 Page 9 2019.01.30 Siemens PLM Software Basics Acoustics Theory How is sound measured?

Sound is measured as pressure fluctuations.

• 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) sound pressure

level has been introduced. Linear time-averaging

1 푇 푝 = ∙ 푝2 푡 푑푡 푇 0

Special case: RMS pressure of a pure tone

퐴 푝 = = 0.707 ∙ 퐴 2

Unrestricted © Siemens AG 2019 Page 10 2019.01.30 Siemens PLM Software Basics Acoustics Theory Decibel scale

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

• The Sound Pressure Level SPL (dB) is defined as: Jet takeoff Rock concert 푝 푝 2 푆푃퐿 = 20 ∙ log10 = 10 ∙ log10 2 푝푟푒푓 푝 푟푒푓 Niagara Falls

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

Conversation

• SPL = 0 dB = 0.00002 Pa is the threshold of hearing. • SPL = 94 dB = 1Pa Soft whisper

• SPL = 120 dB = 20 Pa is the threshold of pain. Breathing

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

Unrestricted © Siemens AG 2019 Page 11 2019.01.30 Siemens PLM Software Basics Acoustics Theory Decibel scale - Sample sound levels

Painful Jet Taking Off

Very Noisy Heavy Truck

Noisy Inside Compact Car “The sound measured today in the office was around 84500 μPa” Moderate Average Classroom

Quiet Bedroom at Night

Barely Audible Soft Whisper

Unrestricted © Siemens AG 2019 Page 12 2019.01.30 Siemens PLM Software Basics Acoustics Theory How do we hear?

• Sound waves travel into the ear canal until they reach the eardrum.

• The eardrum passes the vibrations through the middle ear bones or ossicles into the inner ear.

• The inner ear is shaped like a snail and is also called the cochlea. Inside the cochlea, there are thousands of (eardrum) tiny hair cells.

• Hair cells change the vibrations into electrical signals that are sent to the brain through the hearing nerve.

Unrestricted © Siemens AG 2019 Page 13 2019.01.30 Siemens PLM Software Human hearing system

Acoustic Wave

Vibration

Electric signals

Sensation of hearing

Unrestricted © Siemens AG 2019 Page 14 2019.01.30 Siemens PLM Software Basics Acoustics Theory Human audible Range

L dB 130 PAIN THRESHOLD 120 110 HEARING DOMAIN 100 90

80 MUSIC 70 60 50 SPEECH 40 30 20 10

0 HEARING THRESHOLD

20 Hz 50 100 200 500 1 k 2 k 5 k 10 k 20 kHz Unrestricted © Siemens AG 2019 Page 16 2019.01.30 Siemens PLM Software Basics Acoustics Theory Interference

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 the same frequency.

+ + interference interference = = Destructive Constructive

Unrestricted © Siemens AG 2019 Page 17 2019.01.30 Siemens PLM Software Basics Acoustics Theory Summing SPL – coherent sinusoidal sources

94 dB (1 Pa) at 1000 Hz

94 dB (1 Pa) at 1000 Hz*

100 dB (2 Pa) Overall

Unrestricted © Siemens AG 2019 * in phase! Page 18 2019.01.30 Siemens PLM Software Basics Acoustics Theory Summing SPL - incoherent sinusoidal sources

94 dB (1 Pa) at 1000 Hz

94 dB (1 Pa) at 2000 Hz

97 dB (1.42 Pa) Overall

Unrestricted © Siemens AG 2019 Page 19 2019.01.30 Siemens PLM Software Basics Acoustics Theory Summing SPL - incoherent random sources

94 dB Overall Level

97 dB Overall Level

Unrestricted © Siemens AG 2019 20 copyright LMS International - 2010 Page 20 2019.01.30 Siemens PLM Software Basics Acoustics Theory Sound Fields

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

• On a distance from the sound source that is • Source can be considered as a smaller than the wavelength of the highest point source. frequency of interest.

• Significant variations in SPL with distance to • Consists of two parts: free field source. and reverberant field.

Unrestricted © Siemens AG 2019 Page 21 2019.01.30 Siemens PLM Software Basics Acoustics Theory Sound Fields - Diffuse field vs. free field

- microphone

Mic

Sound Sound Source Source

Sound Source

Diffuse Field Free Field Uniform sound field regardless of Sound propagates without reflection, microphone position sound level decreases with distance

Unrestricted © Siemens AG 2019 Page 22 2019.01.30 Siemens PLM Software Basics Acoustics Theory Sound Fields - Near field vs. far field

. Near Field . Far Field . Close to source . Far from source, source . Circulating & Propagating appears as point source . . No predictable relationship Plane wave approximation . between distance and pressure Linear relationship between Lp & distance

Unrestricted © Siemens AG 2019 Page 23 2019.01.30 Siemens PLM Software Basics Acoustics Theory Sound reflection

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

reflected energy transmitted energy

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

Dry speech Speech in a reverberant room

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. Unrestricted © Siemens AG 2019 Page 24 2019.01.30 Siemens PLM Software Basics Acoustics Theory Anechoic 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 h The lowest frequency at which an anechoic room can be used depends on the room volume and the depth of the wedges.

λ ℎ ≅ Rule of thumb: 2

Unrestricted © Siemens AG 2019 Page 25 2019.01.30 Siemens PLM Software Basics Acoustics Theory Semi-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 a reflecting surface (e.g. cars,…).

Typical applications:

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

semi-anechoic room with roller bench

Unrestricted © Siemens AG 2019 Page 26 2019.01.30 Siemens PLM Software Basics Acoustics Theory Reverberation Room

• High-reflecting, non-parallel walls • Diffuse field: nearly uniform sound intensity

Sound path To measure: • Sound power of sources • Sound absorptive properties of materials • Sound transmission through building elements Sound Source

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

Unrestricted © Siemens AG 2019 Page 27 2019.01.30 Siemens PLM Software Basics Acoustics Theory Refraction

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

1) effects of wind: vwind

c2 c1

wind coming from the right 2) temperature gradients:

c2 c1

c1 c2

decreasing temperature with height increasing temperature with height

Unrestricted © Siemens AG 2019 Page 29 2019.01.30 Siemens PLM Software Basics Acoustics Theory Diffraction

• Diffraction is the bending of a sound wave around the edges of obstructions (barrier, opening,…) in the path of the wave

• Bending due to diffraction is highly selective with respect to frequency effects of diffraction at low frequencies : (a) behind a barrier, (b) through an opening • Long wavelength, low frequency sounds are less affected by barriers and openings than short wavelength

Example:

• Highway barriers fail in reducing low effects of diffraction at high frequencies : (a) behind a frequency truck noise barrier, (b) through an opening Unrestricted © Siemens AG 2019 Page 30 2019.01.30 Siemens PLM Software . Agenda

Basics Acoustics Theory

Acoustic Hardware: Microphones

Analysis and Processing

Siemens Solutions

Unrestricted © Siemens AG 2019 Page 31 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Principle of microphone

Condenser microphones operate on a capacitive design and utilize basic transduction principles: sound pressure ↓ capacitance variation ↓ electrical voltage

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

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

The charge applied to the back-plate can be either supplied externally (no Pre-polarization) or from an electret layer on the back-plate (pre-polarization).

Unrestricted © Siemens AG 2019 Page 32 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Microphones & preamplifiers, selection criteria

• Microphone is only the top part

• The very weak signal is pre-amplified before being sent over a cable to a data acquisition system

• There are 3 main criteria which have to be taken into account when selecting a microphone:

• Dynamic Range • Frequency Response • Field Response

Unrestricted © Siemens AG 2019 Page 33 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Dynamic range

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

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

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

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

Unrestricted © Siemens AG 2019 Page 34 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Dynamic range

1/8” ¼”

½” 1”

Unrestricted © Siemens AG 2019 Page 35 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Frequency response

Frequency response refers to the way a microphone responds to different frequencies.

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

Unrestricted © Siemens AG 2019 Page 36 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Field response

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

Free Field Pressure Random Incidence

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

Unrestricted © Siemens AG 2019 Page 37 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Field response – “free field” type free-field microphone:

• minimal (zero) interference with sound field

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

• localized, not negligible disturbances of sound field at higher frequencies Free Field • accurate when measuring sound pressure levels that radiate from a single direction and source, which is pointed directly (0°incidence angle) at the microphone diaphragm, and operated in an area that minimizes sound reflections (e.g. anechoic room).

Unrestricted © Siemens AG 2019 Page 38 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Field 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 pressure on the wall itself.

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

Unrestricted © Siemens AG 2019 Page 39 2019.01.30 Siemens PLM Software Acoustic Hardware: Microphones Field response – ”random incidence” type

Random incidence microphone:

• Designed to be omnidirectional and measure sound pressure coming from multiple directions, multiple sources and multiple reflections.

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

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

Basics Acoustics Theory

Acoustic Hardware: Microphones

Analysis and Processing

Siemens Solutions

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

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

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

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

A4: 440 Hz A5: 880 Hz (standard pitch)

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

A4: 440 Hz A5: 880 Hz A6: 1760 Hz A7: 3520 Hz (standard pitch)

440 Hz Span 880 Hz Span 1760 Hz Span

Lower Center Upper Octave bands group energy in standardized frequency cutoff frequency cutoff bands. frequency frequency 11 16 22 22 31.5 44 Reference octave band: 1000 Hz as center frequency 44 63 88 is used to calculate the other bands which cover the 88 125 177 whole bandwidth. Each next center frequency is the 177 250 355 double of the previous one. 355 500 710 710 1000 1420 1420 2000 2840 2840 4000 5680 5680 8000 11360 11360 16000 22720

For finer analysis, other octave band types were introduced. • 1/3 octaves – each octave band is divided into 3 separate bands • 1/12 octaves • 1/24 octaves

40 Pressure dB/2e-005 [Pa] dB/2e-005 Pressure 30

20 16 31.5 63 125 250 500 1000 2000 4000 Octaves Traces: 2/2 Frequency [Hz]

There are two ways to calculate the center and boundary frequencies of bands, Base2 and Base 10 method: Base 2 1/1 Octaves Base 10 1/1 Octaves 3푁 푁 푓푐 = 1000 ∙ 2 푓푐 = 1000 ∙ 1010 1 − 3 2004 2 − - 푓 = 2 ∙ 푓 푙표푤푒푟 푐 푓 푙표푤푒푟 = 10 20 ∙ 푓푐 1 3 푓 = 22 ∙ 푓 푢푝푝푒푟 푐 푓 푢푝푝푒푟 = 1020 ∙ 푓푐

Base 2 1/3 Octaves Base 10 1/3 Octaves

푁 푁 푓푐 = 1000 ∙ 23 푓푐 = 1000 ∙ 1010 1 1 − − 푓 푙표푤푒푟 = 2 6 ∙ 푓푐 푓 푙표푤푒푟 = 10 20 ∙ 푓푐 1 1 IEC 61260, ANSI S1.11 ANSI IEC 61260, 푓 푢푝푝푒푟 = 26 ∙ 푓푐 푓 푢푝푝푒푟 = 1020 ∙ 푓푐

푓표푟 푁 = ⋯ , −2, −1,0,1,2 …

• 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!

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

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

Unrestricted © Siemens AG 2019 Page 48 2019.01.30 Siemens PLM Software Analysis & Processing A-,B-,C-, D- and Z-weighting

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

• A-weighting = 40-phone curve SPL (dB) SPL Is mostly used

• B- and C-weighting = 70- and 100-phone equal loudness contours frequency (Hz)

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

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

1 푇 푝 = ∙ 푝2 푡 푑푡 푇 0

푝 푆푃퐿 = 20 ∙ log10 푝푟푒푓

• 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

Unrestricted © Siemens AG 2019 Page 50 2019.01.30 Siemens PLM Software 70.00 1.00 F Overall level - LAeqt Point1 (A) 76.1 dB F Overall level - LAeqT Point1 (A) 81.5 dB

Analysis & Processing Equivalent Sound Pressure Level L Pa eq dB(A) Amplitude

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

70.00 1.00 F Overall level - LAeqt Point1 (A) 76.1 dB F Overall level - LAeqT Point1 (A) 81.5 dB

47.55 Pa dB(A) Amplitude

47.55

40.00 3600.00 0.00

0.00 s 3600.00 3600.00 40.00\ 0.00 0.00 s 3600.00 Time Time

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

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

Basics Acoustics Theory

Acoustic Hardware: Microphones

Analysis and Processing

Siemens Solutions

What material Do I meet quality Do I meet should I use to objectives? standards? reduce the levels? Sound Pressure Sound Material & Sound Power Acoustic Analyzer Component Testing Pass-by Noise

What is the root Does it sound right? Where is the sound cause? Why is it annoying? coming from? Source? Path? Sound Quality Sound Source TPA Localization Source-Path- Receiver