PowerPoint Presentation

Electroacoustic absorbers

low frequency absorption by hybrid sensor/shunt-based impedance controlDr. Herv Lissek, Dr. Sami Karkar, Etienne Rivet (EPFL)WARNING: all sounds in this slideshow should be listened to with a subwoofer or (good) headphones.OutlineContext: room modes in the low-frequency range

Presentation of the Electroacoustic Absorber prototype

Design Methodology for room modal damping

Experimental assessment: modal damping in roomsAES Swiss Section / SSA joint meeting - 09.03.20152ContextAES Swiss Section / SSA joint meeting - 09.03.2015ContextRoom modes characterization: standing waves uneven spatial distribution of sound energy

AES Swiss Section / SSA joint meeting - 09.03.2015

4ContextRoom modes characterization: frequency responsedominated by strong peaks (resonances) and dips (anti-resonances)

AES Swiss Section / SSA joint meeting - 09.03.2015

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Im(pk): resonance frequencyRe(pk): dampingContextRoom modes characterization: modal decay time sustain at the various resonance frequencies once the source stops

definition of MT60 may strongly vary with frequency

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Problem(s)Low-frequency noise issue

AES Swiss Section / SSA joint meeting - 09.03.20157Problem(s)Music rendering issue in the low-frequency range

AES Swiss Section / SSA joint meeting - 09.03.20158

Problem(s)State of the art sound absorberstotally inefficient in the LF range (see /4 rule)

Porous materialsHelmholtz resonatorsPanel absorbersabsorptionBFAES Swiss Section / SSA joint meeting - 09.03.20159Problem(s)State of the art sound absorbersbass-traps (membrane absorbers)Efficient LF narrow-band (1 resonance frequency) sound absorbers

e-traps (active bass traps)Efficient on 2 (adjustable) resonance frequencies

E-trapAES Swiss Section / SSA joint meeting - 09.03.201510Electroacoustic AbsorberspresentationAES Swiss Section / SSA joint meeting - 09.03.2015Electroacoustic absorbersDevelopment of an active sound absorption solution with >0.83 along the range [20 - 200Hz]

Based on actuated membranes (loudspeakers) used as a membrane absorbers

Evaluation in reverberant conditions, focusing on low-frequency sound equalization

H. Lissek, R. Boulandet, and R. Fleury, Electroacoustic absorbers: bridging the gap between shunt loudspeakers and active sound absorption, J. Acoust. Soc. Am., 129(5), 2968-2978, (2011).AES Swiss Section / SSA joint meeting - 09.03.201512Electroacoustic absorbersParameter DescriptionValueUnitMmsMoving mass14.7gRmsMechanical resistance1.31N.s.m-1CmsMechanical compliance242.3mm.N-1SdMembrane surface151cm2BlForce factor6.85N.A-1VbCabinet volume10dm3rAir mass density1.2kg/m3cSound celerity in air344m.s-1

ptvAES Swiss Section / SSA joint meeting - 09.03.2015pi13

Electroacoustic absorbersThe desired acoustic impedance Zat can be assigned by identifying the controller transfer function:

ptv

AES Swiss Section / SSA joint meeting - 09.03.2015piQp(w)Objective: force the membrane to behave as a given acoustic impedance ZatEffective acoustic impedance of the membrane14Electroacoustic absorbersImplementation in the prototype

Voltage-driven current amplifier

AES Swiss Section / SSA joint meeting - 09.03.2015Filter implemented on a DSP platformDSP15

Electroacoustic absorbers

ISO 10534-2 : acoustic impedance measurement technique

Acoustic impedanceAbsorption coefficient

AES Swiss Section / SSA joint meeting - 09.03.2015DSP16Design Methodology for room modal dampingAES Swiss Section / SSA joint meeting - 09.03.2015

1. FEM simulationEigenfrequency study with Comsol MultiphysicsRoom CAD of the targeted room Define a given uniform real acoustic resistance Rwall for walls to fit with measurementsAbsorbersSame acoustic resistance Ra on all diaphragms

result: eigenfrequencies in [20 100 Hz]Eigenvalues modal decay times

AES Swiss Section / SSA joint meeting - 09.03.201518RwallRa2. Optimal acoustic resistanceChanging the value of resistance Rafor each mode, find the minimal value of MT60translate as a frequency-dependant resistance19AES Swiss Section / SSA joint meeting - 09.03.2015

zoom2. Optimal acoustic resistanceChanging the value of resistance Rafor each mode, find the minimal value of MT60translate as a frequency-dependant resistance20AES Swiss Section / SSA joint meeting - 09.03.2015

2. Optimal acoustic resistanceChanging the value of resistance Rafor each mode, find the minimal value of MT60translate as a frequency-dependant resistance Ra(f)

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3. Impedance specificationThe function Ra(f) serve as a specification for the electroacoustic absorbers impedance Zat(f)=Rat(f)+iwXat(f)

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4. ImplementationImplementation in the prototype

Voltage-driven current amplifier

AES Swiss Section / SSA joint meeting - 09.03.2015Filter implemented on a DSP platformDSP23

Experimental validation of room modes dampingAES Swiss Section / SSA joint meeting - 09.03.2015Experimental assessment4x4 electroacoustic absorbers prototypes(total surface = 16x151 cm2 = 0.24 m2)In a reverberant chamber of 226.9 m2 evaluation of room modal damping

AES Swiss Section / SSA joint meeting - 09.03.2015

25Experimental assessment Frequency response without and with absorbers identify individual room modes assess damping performance on peaks and dips amplitudes

Modal decay times without and with absorbers assess damping performance in the time domain

Recording of music rendering, without and with absorbers listen to the effect on music rendering

Recording of kick drum, without and with absorbers listen to the effect on acoustic music playingAES Swiss Section / SSA joint meeting - 09.03.201526Experimental setupHardwareRecorders/analyzersB&K Pulse (frequency responses)frequency resolution: 31.5 mHzM-Audio M-Track 8 soundcard (recordings)Microphones

Sources

Facility (reverberant chamber, V=215.6 m3, S=226.9 m2)

Subwoofer

Kick drum(Pearl Export)

PCB 130D20(frequency responses)Beyerdynamic M101 N(recordings)

Supplementary panel absorber4 electroacoustic absorbersat the 4 room corners 7 microphone positionsAES Swiss Section / SSA joint meeting - 09.03.2015271. Frequency response(blue: Hardwalls, red: Absorbers)

AES Swiss Section / SSA joint meeting - 09.03.2015281. Frequency responseModal damping (dB)

AES Swiss Section / SSA joint meeting - 09.03.201529Mode 8 waveforms(hardwalls : 52.78 Hz - absorbers : 52.72 Hz)

Mode 8 - echograms(hardwalls : 52.78 Hz - absorbers : 52.72 Hz)

2. Modal decay timeAES Swiss Section / SSA joint meeting - 09.03.2015302. Modal decay time

1/6MT101/3MT201/2MT30AES Swiss Section / SSA joint meeting - 09.03.2015312. Modal decay time

AES Swiss Section / SSA joint meeting - 09.03.2015322. Modal decay time

AES Swiss Section / SSA joint meeting - 09.03.201533Peggy Lee Fever

White Stripes 7 nation army

3. Music renderingHardwalls (mic 5)Absorbers (mic 5)Hardwalls (mic 5)Absorbers (mic 5)Whats this tune?Whats this tune?AES Swiss Section / SSA joint meeting - 09.03.201534

Waveforms

Echograms

4. Kick drumHardwalls (mic 1)Absorbers (mic 1)AES Swiss Section / SSA joint meeting - 09.03.201535

Conclusions4 electroacoustic absorbers prototypes achieve efficient room mode damping in the reverberant chamberMax damping: 12.2 dB @ 58 Hz, Global damping: 8 dB over [20 100 Hz],Max modal decay time reduction: 85% @ 58 Hz (from 20 s down to 3 s),

for a total absorber surface representing only 0.1% of the whole room walls surfaceAES Swiss Section / SSA joint meeting - 09.03.201536ConclusionsMost recent results (August 2015)AES Swiss Section / SSA joint meeting - 09.03.2015

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as per December 201420 dB8dB70 dB30 dB40 dB65 dBThank you for your attentionThis work has been supported by the Swiss Commission for Technology and Innovation (CTI), under the project INTERACTS, agreement number: 14220.1 PFNM-NM.

e-mail: herve.lissek@epfl.ch

AES Swiss Section / SSA joint meeting - 09.03.201538Other20192.646Other20035.908Other20035.908Other7889.003Other3160.82