ABSORPTION OF SOUND
Transcript of ABSORPTION OF SOUND
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ABSORPTION OF SOUND MASTER-CLASS ACOUSTIC FUNDAMENTALS DUTCH ACOUSTICAL SOCIETYY.H.WIJNANT
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ABSORPTION OF SOUND
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Absorption
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ABSORPTION OF SOUND
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ABSORPTION OF SOUND
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Fundamentals Wave equation - Helmholtz equation (complex notation) Impedance Intensity/Power
Reflection / Transmission Absorption Demonstration
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ABSORPTION OF SOUND
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FUNDAMENTALS
Wave equation
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Helmholtz equation (time harmonic solution)
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FUNDAMENTALS
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Helmholtz equation (time harmonic)
equivalent (complex) notation
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FUNDAMENTALS
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FUNDAMENTALS
= phasor
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Helmholtz equation (time harmonic)
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FUNDAMENTALS
wave number [1/m]
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Particle velocity
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FUNDAMENTALS
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FUNDAMENTALS
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Plane wave solution
Purely propagating wave of amplitude A traveling in positive x-direction (no reflection)
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FUNDAMENTALS
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Characteristic specific acoustic impedance = pressure over particle velocity ratio for the
purely propagating plane wave
Air: Z0 = 415 [Rayl] Water: Z0 = 1.5 106 [Rayl]
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FUNDAMENTALS
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‘Incoming’ wave of amplitude A traveling in positive x-direction ‘Reflected’ wave of amplitude B traveling in the negative x-direction
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FUNDAMENTALS
p2p1
BA Z
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Specific acoustic impedance = ratio (complex!) pressure over (complex!) particle velocity Z = phasor
Note: only a frequency domain definition!
Normal impedance16
FUNDAMENTALS
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FUNDAMENTALS
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Relative specific acoustic impedance relative to the characteristic impedance Z0 = ρ0 c0
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FUNDAMENTALS
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Particle displacement / expansion Changing ζr (ζi = 0)
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FUNDAMENTALS
ζr = 1 ζr = 2 ζr = 5ζr = 1/2ζr = 1/5
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Particle displacement / expansion Changing ζi (ζr = 1)
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FUNDAMENTALS
ζi = 0 ζi = 1 ζi = 5ζi = -1ζi = -5
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Instantaneous acoustic intensity = energy flux [Watt/m2]
Active acoustic intensity = averaged over time
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FUNDAMENTALS
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Active intensity for harmonic signals averaged over one period
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FUNDAMENTALS
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Intensity of the incident plane wave
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FUNDAMENTALS
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Active intensity = intensity of the incident plane wave
minus the intensity of the reflected wave
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FUNDAMENTALS
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Active power = spatial integration of the intensity [Watt]
FUNDAMENTALS
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Absorption
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ABSORPTION OF SOUND
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normal incident plane waves constant cross-sectional area change in impedance
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REFLECTION AND TRANSMISSION
transmitted
incident
reflectedZ1 Z2
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reflection coefficient:
transmission coefficient:
... at the interface continuity of pressure: 1 + R = T continuity of mass: 1 – R = Z1/Z2 T
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REFLECTION AND TRANSMISSION
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solving for R and T yields: reflection coefficient:
transmission coefficient:
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REFLECTION AND TRANSMISSION
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sound power reflection coefficient r
using
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REFLECTION AND TRANSMISSION
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sound power transmission coefficient τ
which yields:
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REFLECTION AND TRANSMISSION
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in terms of impedance ...
and
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REFLECTION AND TRANSMISSION
conservation of energy
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Z2 >> Z1 (Rigid wall) yields R = 1, r = 1 T = 2, τ = 0 pressure doubling
... all power reflected
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REFLECTION AND TRANSMISSION
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Z2 << Z1 (Pressure release) R = -1, r = 1 T = 0, τ = 0 pressure zero
... all power reflected
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REFLECTION AND TRANSMISSION
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Z2 = Z1 (Matched impedance) R = 0, r = 0 T = 1, τ = 1 pressure undisturbed
... all power transmitted
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REFLECTION AND TRANSMISSION
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quarter-wave resonators
resonator = low impedance
surface = high impedance
total = matched impedance
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ABSORPTION OF SOUND (RESONATORS)
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Absorption v. impedance (normal incidence)
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ABSORPTION OF SOUND
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origin coordinate system at p1
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ABSORPTION OF SOUND (MEASUREMENT)
p2p1
microphonesspeaker
absorbing sample
BA
x1x2
Z
s s = distance between microphones
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Non-iso standard!
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ABSORPTION OF SOUND (MEASUREMENT)
Note: independent of position!
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Absorption depends does not only dependent on the impedance but also on e.g. the angle of incidence (or, more general, the incident sound field)
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ABSORPTION OF OBLIQUE SOUND (MEASUREMENT)
plane wave
normal impedance
θ
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Constant impedance (Zn=ρ0 c0; ζ=1 + 0i)
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ABSORPTION OF OBLIQUE SOUND
Zn = 1.15 ρ0 c0
Zn = 2 ρ0 c0
Zn = ρ0 c0 Zn = 1.04 ρ0 c0
Zn = 1.41 ρ0 c0 Zn = 3.86 ρ0 c0
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ABSORPTION OF SOUND (IN-SITU MEASUREMENT)
Louvre door
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SPL Source at A SPL Source at B
Louvre door/window
ABSORPTION OF SOUND (IN-SITU MEASUREMENT)
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LOCAL plane wave assumption for in-situ measurement of absorption Determine the amplitudes of the incident and reflected wave at the
measurement position (like in an impedance tube)
ABSORPTION OF SOUND (IN-SITU MEASUREMENT)
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Determine the incident intensity at every location and the associated incident power
= LPW-method
ABSORPTION OF SOUND (IN-SITU MEASUREMENT)
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Source at ASPL, Iin and Irefl
Source at BSPL, Iin and Irefl
Louvre door
ABSORPTION OF SOUND (IN-SITU MEASUREMENT)
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Source at A Source at B
Louvre door
ABSORPTION OF SOUND (IN-SITU MEASUREMENT)
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Impedance tube LPW method
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DEMONSTRATION