# Electronics | Acoustics » Basics of Acoustics

Year, pagecount:2019, 52 page(s)
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Basics of Acoustics Agenda . Basics Acoustics Theory Acoustic Hardware: Microphones Analysis and Processing Siemens Solutions Unrestricted Siemens AG 2019 Page 2 2019.0130 Siemens PLM Software . Agenda Basics Acoustics Theory Acoustic Hardware: Microphones Analysis and Processing Siemens Solutions Unrestricted Siemens AG 2019 Page 3 2019.0130 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 2019.0130 Siemens PLM Software Basics Acoustics Theory Speed of sound The speed of sound determines how fast the

compressions and rarefactions travel through the medium. It depends on the physical properties of the elastic medium. It’s dependent of:  Medium (gaseous/liquid/solid) Medium Temp [⁰C] Speed [m/s] Air 0 331 Air 20 343 Ethanol 20 1162 Water 20 1482 Steel - 5960 ������ > ������� > ��������  Temperature � � = � ℃ + 273.1 c = 20.05 ∙ �[�] Unrestricted Siemens AG 2019 Page 6 2019.0130 Siemens PLM Software Basics Acoustics Theory Frequency of sine waves • The period T [s] is the time of one complete sinusoidal, vibrational cycle. Period T [s] • The frequency f [Hz] is the reciprocal of the period: 1 �= � • Frequency range of human hearing is between 20Hz and 20,000 Hz (20kHz) • Frequencies lower than 20 Hz are perceived as vibrations, frequencies above 20,000 Hz are referred to as ultrasonic. freq Play me 125 Hz 250 Hz 500 Hz 1000 Hz 3500 Hz 5000 Hz Unrestricted

Siemens AG 2019 Page 7 2019.0130 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. •  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: � =�∙� = Frequency Wavelength 10Hz 34m 34Hz 10m 340Hz 1m 3400Hz 10cm � � 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 2019.0130 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 2019.0130 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 ∙ � � �2 � �� 0 Special case: RMS pressure of a pure tone �= � 2 = 0.707 ∙ � Unrestricted Siemens AG 2019 Page 10 2019.0130 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 ��� = 20 ∙ log10 � ���� = 10 ∙

log10 �2 �2 ��� 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 • SPL = 120 dB = 20 Pa is the threshold of pain. Soft whisper Breathing • Symbol used for SPL (e.g in displays): L, L(dB), L dB Unrestricted Siemens AG 2019 Page 11 2019.0130 Siemens PLM Software Basics Acoustics Theory Decibel scale - Sample sound levels Painful Jet Taking Off Very Noisy Heavy Truck Noisy Inside Compact Car Moderate Average Classroom Quiet Bedroom at Night Barely Audible Soft Whisper “The sound measured today in the office was around 84500 μPa” Unrestricted Siemens AG 2019 Page 12 2019.0130 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 tiny hair cells. (eardrum) • Hair cells change the vibrations into electrical signals that are sent to the brain through the hearing nerve. Unrestricted Siemens AG 2019 Page 13 2019.0130 Siemens PLM Software Human hearing system Acoustic Wave Vibration Electric signals Sensation of hearing Unrestricted Siemens AG 2019 Page 14 2019.0130 Siemens PLM Software Basics Acoustics Theory Human audible Range L dB PAIN THRESHOLD 130 120 110 HEARING DOMAIN 100 90 80 MUSIC 70 60 50 SPEECH 40 30 20 10 0 HEARING THRESHOLD 20 Hz 50 100 200 500 1k 2k 5k 10 k 20 kHz Unrestricted Siemens AG 2019 Page 16 2019.0130 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. + = Destructive interference Constructive

interference • It refers primarily to combination effects associated with sound waves of the same frequency. + = Unrestricted Siemens AG 2019 Page 17 2019.0130 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 Page 18 2019.0130 * in phase! 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 2019.0130 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 Page 20 2019.0130 20 copyright LMS International - 2010 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 smaller than the wavelength of the highest frequency of interest. • Source can be considered as a point source. • Significant variations in SPL with distance to source. • Consists of two parts: free field and reverberant field. Unrestricted Siemens AG 2019 Page 21 2019.0130 Siemens PLM Software Basics Acoustics Theory Sound Fields - Diffuse field vs. free field - microphone Mic Sound Source Sound Source Sound Source Diffuse Field Uniform sound field regardless of microphone position Free Field Sound propagates without reflection, sound level decreases with distance Unrestricted Siemens AG 2019 Page 22 2019.0130 Siemens PLM Software 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  Unrestricted Siemens AG 2019 Page 23 2019.0130 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 2019.0130 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. Rule of thumb: ℎ≅ λ 2 Unrestricted Siemens AG 2019 Page 25 2019.0130 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 2019.0130 Siemens PLM Software Basics Acoustics Theory Reverberation 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 Sound path 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 2019.0130 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: c1 c2 decreasing temperature with height c2 c1 increasing temperature with height Unrestricted Siemens AG 2019 Page 29 2019.0130 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 frequency truck noise effects of diffraction at high frequencies : (a) behind a barrier, (b) through an opening Unrestricted Siemens AG 2019 Page 30 2019.0130 Siemens PLM Software . Agenda Basics Acoustics Theory Acoustic Hardware: Microphones Analysis and Processing Siemens Solutions Unrestricted Siemens AG 2019 Page 31 2019.0130 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 2019.0130 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 2019.0130 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 2019.0130 Siemens PLM Software Acoustic Hardware: Microphones Dynamic range 1/8” ¼” ½” 1” Unrestricted Siemens AG 2019 Page 35 2019.0130 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 2019.0130 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 2019.0130 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 2019.0130 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 on the wall itself. Pressure • sound pressure exerted on walls, exerted on airplane wings, or inside structures such as tubes, housings or cavities. Unrestricted Siemens AG 2019 Page 39 2019.0130 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, church or in an area with hard, reflective walls, a Random Incidence microphone should be used to accurately measure the sound from multiple sources. Random Incidence Unrestricted Siemens AG 2019 Page 40 2019.0130 Siemens PLM Software . Agenda Basics Acoustics Theory Acoustic Hardware: Microphones Analysis and Processing Siemens

Solutions Unrestricted Siemens AG 2019 Page 41 2019.0130 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 2019.0130 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 (standard pitch) A5: 880 Hz Unrestricted Siemens AG 2019 Page 43 2019.0130 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 (standard pitch) A5: 880 Hz 440 Hz Span A6: 1760 Hz 880 Hz Span A7: 3520 Hz 1760 Hz Span Unrestricted Siemens AG 2019 Page 44 2019.0130 Siemens PLM Software Analysis & Processing Octaves Octave bands group energy in standardized frequency bands. Reference octave band: 1000 Hz as center frequency is used to calculate the other bands which cover the whole bandwidth. Each next center frequency is the double of the previous one. Lower cutoff frequency Center frequency Upper cutoff frequency 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 Unrestricted Siemens AG 2019 Page 45 2019.0130 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 Pressure dB/2e-005 [Pa] 80 70 60 50 40 30 20 Octaves 16 31.5 Traces: 2/2 63 125 250 500 1000 2000 4000 Frequency [Hz] Unrestricted Siemens AG 2019 Page 46 2019.0130 Siemens PLM Software Analysis & Processing Octave Bands calculations IEC 61260, ANSI S1.11-2004 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 �� = 1000 ∙ 2� � ����� = �� = 1000 ∙ 1 −2 2 ∙ �� 1 22 ∙ �� � ����� = Base 2 1/3 Octaves �� = 1000 ∙ � ����� = � ����� = 3 − 10 20 ∙ �� 3 1020 ∙ �� � ����� = Base 10 1/3 Octaves � 23 1 −6 2 1 26 � ����� = �� = 1000 ∙ ∙ �� 3� 10 10 � ����� = ∙ �� � 1010 1 −20 10 ∙ �� 1 20 10 ∙ ��

� ����� = ��� � = ⋯ , −2, −1,0,1,2 Unrestricted Siemens AG 2019 Page 47 2019.0130 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! �� [Hz] 63 125 250 500 1000 2000 4000 8000 ��� -26.2 -16.1 -8.6 -3.2 0 +1.2 +1 -1.1 Unrestricted Siemens AG 2019 Page 48 2019.0130 Siemens PLM Software Analysis & Processing A-,B-,C-, D- and Z-weighting • A-weighting = 40-phone curve Is mostly used • B- and C-weighting = 70- and 100-phone equal loudness contours SPL (dB) • Based on Loudness curves: equal perceived loudness, expressed on phones frequency

(Hz) • D-weighting for aircraft noise: 1-10 kHz region • Z-weighting: no weighting or “linear” weighting Unrestricted Siemens AG 2019 Page 49 2019.0130 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 ∙ � � �2 � �� 0 ��� = 20 ∙ log10 � ���� • 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 2019.0130 Siemens PLM Software Amplitude Analysis & Processing Equivalent Sound Pressure Level Leq 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 F Overall level - LAeqt Point1 (A) 76.1 dB Overall level - LAeqT Point1 (A) 81.5 dB Pa dB(A) Amplitude 47.55 47.55 3600.00 000 40.00 0.00 s Time 3600.00 000 s Time 3600.00 3600.00 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 2019.0130 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 2019.0130 Siemens PLM Software . Agenda Basics Acoustics Theory Acoustic Hardware: Microphones Analysis and Processing Siemens Solutions Unrestricted Siemens AG 2019 Page 53

2019.0130 Siemens PLM Software Siemens solutions The 6 boxes of Acoustic Testing Do I meet standards? Sound Pressure Acoustic Analyzer What material should I use to reduce the levels? Sound Material & Component Testing Sound Power Pass-by Noise Does it sound right? Why is it annoying? Where is the sound coming from? What is the root cause? Source? Path? Sound Quality Sound Source Localization TPA Source-PathReceiver Do I meet quality objectives? Unrestricted Siemens AG 2019 Page 54 2019.0130 Siemens PLM Software Thank you.