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1 © 2006 Nokia Acoustical measurements.ppt / / IJ Acoustical measurements Iiro Jantunen Nokia Research Center S Licentiate course in measurement science and technology
2 © 2006 Nokia Acoustical measurements.ppt / / IJ Contents Principles of acoustics Acoustics measurements Microphone Sound pressure level measurements Sound intensity measurements Calibration SoundField
3 © 2006 Nokia Acoustical measurements.ppt / / IJ Principles of acoustics Sound waves in gas or liquid No shear forces → no transverse waves → purely longitudinal waves Audible sound range 20 Hz – 20 kHz Fully described by 3 variables Pressure Particle velocity Density
4 © 2006 Nokia Acoustical measurements.ppt / / IJ Wave equations of sound Euler’s equation Newton’s 2nd law (F=ma) applied to fluid Continuity equation Bringing extra air to a volume increases density State equation Relates pressure changes to density
5 © 2006 Nokia Acoustical measurements.ppt / / IJ Wave equation of sound Previous wave equations used pressure, density and particle velocity Eliminating density and particle velocity the wave equation of sound is obtained Two basic solutions: Plane wave Spherical wave
6 © 2006 Nokia Acoustical measurements.ppt / / IJ Free field acoustics Sound propagates to all directions without diffraction, reflection or absorption Spherical waves In principle, infinite, empty space without reflections In practice, anechoic chamber, with near 100% absorptive walls
7 © 2006 Nokia Acoustical measurements.ppt / / IJ Free field microphone Intended to measure the sound pressure as it existed before the microphone was introduced Microphone pointed to source Microphone tip causes an increase in sound pressure Taken care of by internal acoustical damping to achieve flat frequency response
8 © 2006 Nokia Acoustical measurements.ppt / / IJ Diffuse field – random incidence microphone Sound reflects from many directions → sound comes to microphone from every direction In practice achieved in a reverberation room with 100% reflective and unparallel walls Microphone diffracts the sound waves from different directions in different ways Combined influence depends on directional distribution of sound waves Standard distribution based on statistical considerations used for random incidence microphone
9 © 2006 Nokia Acoustical measurements.ppt / / IJ Closed coupler Chamber with small dimensions compared to sound wavelength Special case: standing wave tube Diameter smaller than sound wavelength Source at the end Possible to calculate the sound field Used in calibration Used in microphone calibration
10 © 2006 Nokia Acoustical measurements.ppt / / IJ Pressure microphone Measuring the actual pressure on a wall Typically used in closed coupler for calibration
11 © 2006 Nokia Acoustical measurements.ppt / / IJ Microphone directionality Directionality indicates the sensitiveness of a microphone to sound coming from different directions No microphone is perfectly omnidirectional Cardioid or hypercardioid commonly used to record vocals Most ribbon microphones are bi-directional Shotgun directionality used outdoors for TV/film production and wildlife recordings
12 © 2006 Nokia Acoustical measurements.ppt / / IJ Parabolic microphone Parabolic reflector used to collect sound waves to microphone Very directional For eavesdropping in e.g. spying
13 © 2006 Nokia Acoustical measurements.ppt / / IJ Microphone transducers Condenser microphones Electret capacitor microphones Dynamic microphones Ribbon microphones Carbon microphones Piezoelectric microphones Laser microphones
14 © 2006 Nokia Acoustical measurements.ppt / / IJ Condenser microphone Diaphragm and backplate form a plate capacitor Charge kept constant → voltage varies as pressure actuates the diaphragm External voltage supply or pre-charged diaphragm Acoustical performance determined by physical dimensions
15 © 2006 Nokia Acoustical measurements.ppt / / IJ Condenser microphone – cont The larger the diaphragm, the more sensitive the microphone Upper limit is defined by diaphragm touching the backplate The smaller the microphone, the greater the frequency range Increasing tension extends range but decreases sensitivity Optimum size of a measurement microphone is (up to 20 kHz) is about 12.6 mm (1/2’’) Damping effect of air reduced by drilling holes in the backplate
16 © 2006 Nokia Acoustical measurements.ppt / / IJ Electret microphone Invented at Bell Labs in 1962 by Gerhard Sessler and Jim West Diaphragm permanently polarized the same way as permanent magnets magnetized (electrostatic magnet) Once considered low price and low quality Now most common microphone type
17 © 2006 Nokia Acoustical measurements.ppt / / IJ Dynamic microphone A movable coil is attached to the diaphragm An unmovable magnet produces a magnetic field Moving diaphragm moves the coil in the magnetic field, inducing a measurable current Exactly same principle as in loudspeakers, only reversed Poor low-frequency response → reduces handling noise Robust, relatively inexpensive and resistant to moisture → widely used on-stage
18 © 2006 Nokia Acoustical measurements.ppt / / IJ Ribbon microphones Revolutionized recording and broadcast industry in the 30’s Special type of dynamic microphones Thin metal ribbon between poles of magnet Voltage output typically low compared to normal dynamic microphones Bidirectional Very sensitive and accurate Generally delicate and expensive
19 © 2006 Nokia Acoustical measurements.ppt / / IJ Carbon microphones Invented by David Hughes in 1878 Very important in the history of telephone Sound pressure (AP) presses the diaphragm (2) to a bed of carbon granules (1). Contact resistance depends on the pressure → resitance R changes Also an amplifier Extremely low-quality sound reproduction Very limited frequency range Very robust
20 © 2006 Nokia Acoustical measurements.ppt / / IJ Piezo microphones Piezoelectric material Diaphragm moves the armature to bend piezoelectric crystal over a fulcrum Small size, cheap, low quality Have replaced carbon microphones Often used as contact microphones to sound instruments underwater or other unusual environments
21 © 2006 Nokia Acoustical measurements.ppt / / IJ Laser microphones Window of a room acting as diaphragm Reading with laser beam reflected from the window Two laser beams for common mode rejection of large window movements and path disturbances For eavesdropping Works best with one-glass windows
22 © 2006 Nokia Acoustical measurements.ppt / / IJ Sound level measurements Measurement of sound pressure filtered by frequency (A-weighting) time-domain (RMS) Mimics response of human ear to noise
23 © 2006 Nokia Acoustical measurements.ppt / / IJ Human hearing frequency response A-weighting curve For subjective responses in special cases there are B-, C- and D-weighting curves very high or low level special noise, e.g., of aircraft
24 © 2006 Nokia Acoustical measurements.ppt / / IJ Sound level measurements IEC International Standard 651 ”Sound Level Meters” Tolerances per frequency band defined for 4 classes of accuracy Type 0: precision laboratory use Type 1: general purpose Type 2: low price Type 3: not used in practice (too wide tolerances)
25 © 2006 Nokia Acoustical measurements.ppt / / IJ Sound intensity measurements no.x/ry/rz/r ISO Standard 3745 “Acoustics — Determination of sound power levels of noise sources — Precision method for anechoic and semi-anechoic rooms”
26 © 2006 Nokia Acoustical measurements.ppt / / IJ Two-microphone probe Measures the sound intensity in two directions Pressure is mean of the two measured pressures Air particle velocity calculated from the two pressures All intensity is in radial direction, no intensity in perpendicular Powerful tool to locate noise sources
27 © 2006 Nokia Acoustical measurements.ppt / / IJ Calibration techniques Reciprocity calibration method Comparison or substitution methods Pistonphone (closed coupler) Sound pressure calibrator Electrostatic actuation
28 © 2006 Nokia Acoustical measurements.ppt / / IJ Reciprocity calibration method Microphone can be used as a loudspeaker Three test microphones measured against each other alternating the function As a result a set of 3 equations with microphone sensitivities as unknowns Very accurate Rather tedious Requires well-controlled environment Seldom used in practical situations
29 © 2006 Nokia Acoustical measurements.ppt / / IJ Comparison/substitution methods Microphone measured related to a reference microphone Comparison method: microphone and reference at the same time Substitution method: microphone put in the lace of the reference Sound source stability
30 © 2006 Nokia Acoustical measurements.ppt / / IJ Pistonphone Closed coupler Well-defined sound pressure level Relatively simple mechanically, very stable Used often as the sound source in comparison/subsitution calibration Accuracy around 0.1 dB Depends on Volume of the coupler Volume displacement Barometric pressure Humidity Heat dissipation
31 © 2006 Nokia Acoustical measurements.ppt / / IJ Sound pressure calibrator Small, self-contained Comparison calibrator Closed coupler Small loudspeaker produces single-frequency signal Reference microphone gives feedback signal Well-defined, provided that reference microphone and feedback gain are stable For field-calibration of microphones Normally not for laboratory calibrations
32 © 2006 Nokia Acoustical measurements.ppt / / IJ Electrostatic calibration Direct use of electrostatic actuator to drive the diaphragm 800 V DC V AC signal Generally used to measure frequency response of microphones Widely used as a convenient and accurate test method For production and final calibration of measurement microphones
33 © 2006 Nokia Acoustical measurements.ppt / / IJ SoundField microphone 3D view of the sound with a single device 4-channel measurement of sound: B-format The spatial pattern can be decided later Mono, stereo, 5.1, … Fairly expensive, but replaces effectively a system of many microphones
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