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 2 푝 푝 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 - 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.