Basics of Acoustics Agenda
.
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
+
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
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.
Unrestricted © Siemens AG 2019 Page 40 2019.01.30 Siemens PLM Software . Agenda
Basics Acoustics Theory
Acoustic Hardware: Microphones
Analysis and Processing
Siemens Solutions
Unrestricted © Siemens AG 2019 Page 41 2019.01.30 Siemens PLM Software Analysis & Processing Frequency spectrum
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).
Unrestricted © Siemens AG 2019 Page 42 2019.01.30 Siemens PLM Software Human hearing: frequency
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)
Unrestricted © Siemens AG 2019 Page 43 2019.01.30 Siemens PLM Software Human hearing: frequency
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
Unrestricted © Siemens AG 2019 Page 44 2019.01.30 Siemens PLM Software Analysis & Processing Octaves
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
Unrestricted © Siemens AG 2019 Page 45 2019.01.30 Siemens PLM Software Analysis & Processing Fractional Octave bands
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
80
70
60
50
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]
Unrestricted © Siemens AG 2019 Page 46 2019.01.30 Siemens PLM Software Analysis & Processing Octave Bands calculations
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 …
Unrestricted © Siemens AG 2019 Page 47 2019.01.30 Siemens PLM Software Analysis & Processing A-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!
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
Unrestricted © Siemens AG 2019 Page 49 2019.01.30 Siemens PLM Software Analysis & Processing Time 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
Unrestricted © Siemens AG 2019 Page 51 2019.01.30 Siemens PLM Software Analysis & Processing Level 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
Unrestricted © Siemens AG 2019 Page 52 2019.01.30 Siemens PLM Software . Agenda
Basics Acoustics Theory
Acoustic Hardware: Microphones
Analysis and Processing
Siemens Solutions
Unrestricted © Siemens AG 2019 Page 53 2019.01.30 Siemens PLM Software Siemens solutions The 6 boxes of Acoustic Testing
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
Unrestricted © Siemens AG 2019 Page 54 2019.01.30 Siemens PLM Software Thank you.