Akustisches Kolloquium Electroacoustic absorption: towards room equalization in the low- frequency range Dr. Hervé Lissek EPFL STI IEL LTS2 [email protected]

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Electroacoustic absorption: towards room equalization in the low-frequency rangeDr. Herv LissekEPFL STI IEL [email protected] Akustisches KolloquiumWhat is this sound ?H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range2

10/28/2015

Akustisches KolloquiumOutlineContext: room modes in the low-frequency range

Presentation of the Electroacoustic Absorber concept

Practical realization

Application to room modes dampingH. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range310/28/2015Akustisches Kolloquium1. Context4H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range5Low-frequency in music1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Akustisches KolloquiumRoom modes characterization: standing wavesIn 1 dimension, with 2 ideally rigid walls10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range6Problems of LF room acoustics

1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Particle velocitySound pressure

nodesantinodesAkustisches KolloquiumRoom modes characterization: standing wavesIn 3 dimensions

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range7Problems of LF room acoustics

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingnodesantinodesAkustisches KolloquiumRoom modes characterization: standing waves uneven spatial distribution of sound energy, depending on frequency



Akustisches KolloquiumRoom modes characterization: frequency responsedominated by strong peaks (resonances) and dips (anti-resonances)

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range9Problems of LF room acoustics1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Corner #2Center of the room

SourceAkustisches KolloquiumRoom modes characterization: modal decay time sustain at the various resonance frequencies once the source stops definition of MT60 (see RT60) may strongly vary with frequency



Akustisches KolloquiumMusic rendering issue in the low-frequency range10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range11Problems of LF room acoustics


Akustisches KolloquiumLow-frequency noise issue10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range12Problems of LF room acoustics

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumOptimal placement of subwoofersSolutions of the state of the artSource: http://www.svsound.com/t/art-of-subwoofer-placement

1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range13Akustisches Kolloquium13In-line equalizationSolutions of the state of the art

1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range14Akustisches Kolloquium14Sound absorption: the room resonances are due to reflections on walls

Solutions of the state of the artprfluid (air)materialpi

1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range15provide absorption in the LF range?Akustisches Kolloquium15State of the art sound absorbersinefficient in the LF range (see /4 rule)10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range16Solutions of the state of the art

Porous materialsHelmholtz resonatorsPanel absorbersabsorptionLF1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Akustisches KolloquiumState of the art sound absorbersbass-traps (membrane absorbers)Efficient LF narrow-band (1 resonance frequency) sound absorbers

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range17

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingSolutions of the state of the art

Source: T. Cox and P. DAntonio, Acoustic absorbers and diffusers, Taylor & Francis Publ., 2009

Akustisches KolloquiumState 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

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range18

E-trap1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Solutions of the state of the art

Akustisches Kolloquium

Active modal control with multiple subwoofers Opposite phase, opposite directionControlled Acoustic Bass System (CABS)Active noise control solutionSource: S. B. Nielsen and A. Celestinos. "Low frequency sound field control in rectangular listening rooms using CABS (Controlled Acoustic Bass System) will also reduce sound transmission to neighbor rooms." Forum Acusticum 2011.

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range19Akustisches Kolloquium192. The concept of Electroacoustic Absorption20H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumDevelopment of an active sound absorption solution with >0.83 along the range [20 - 200Hz]

Based on actuated membranes (loudspeakers) used as membrane absorbers

The electric load of the loudspeaker acts as an additional acoustic impedance for the membrane10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range21Electroacoustic absorbers

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).1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumThe acoustic impedance at the diaphragm can be set as10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range22Passive electroacoustic absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Diaphragm acoustic impedance Akustisches Kolloquium10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range23Passive electroacoustic absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingEx1: open circuit (i=0)

Acoustic impedanceAkustisches Kolloquium10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range24Passive electroacoustic absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Normalized acoustic admittanceParameter DescriptionValueUnitMmsMoving mass13.0gRmsMechanical resistance0.8N.s.m-1CmsMechanical compliance120mm.N-1ReElectrical resistance5.6WLeElectrical inductance0.9mHSdMembrane surface133cm2BlForce factor6.9N.A-1VbCabinet volume10dm3rAir mass density1.2kg/m3cSound celerity in air343m.s-1Ex1: open circuit (i=0)


10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range25Passive electroacoustic absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Normalized acoustic admittanceEx1: open circuit (i=0)

Parameter DescriptionValueUnitMmsMoving mass13.0gRmsMechanical resistance0.8N.s.m-1CmsMechanical compliance120mm.N-1ReElectrical resistance5.6WLeElectrical inductance0.9mHSdMembrane surface133cm2BlForce factor6.9N.A-1VbCabinet volume10dm3rAir mass density1.2kg/m3cSound celerity in air343m.s-1Akustisches Kolloquium10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range26Passive electroacoustic absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Sound absorption coefficient

Parameter DescriptionValueUnitMmsMoving mass13.0gRmsMechanical resistance0.8N.s.m-1CmsMechanical compliance120mm.N-1ReElectrical resistance5.6WLeElectrical inductance0.9mHSdMembrane surface133cm2BlForce factor6.9N.A-1VbCabinet volume10dm3rAir mass density1.2kg/m3cSound celerity in air343m.s-1

Ex1: open circuit (i=0)

Akustisches Kolloquium10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range27Passive electroacoustic absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingSound absorption coefficientEx2: shunt RLC

s=jwAkustisches KolloquiumEx2: shunt RLC10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range28Passive electroacoustic absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Akustisches KolloquiumEx3: (p,v) feedback control (aka direct impedance control)10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range29Active electroacoustic absorbers

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumEx3: (p,v) feedback control (aka direct impedance control)10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range30Active electroacoustic absorbers

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumEx4: electric impedance synthesis approach10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range31Active electroacoustic absorbers

Characteristic equationsControl lawLissek et al., Electroacoustic absorbers: bridging the gap between shunt loudspeakers and active sound absorption, JASA 129 (5), 20111. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumAfter the duality between the acoustic and electric impedances, there exists:a unique acoustic impedance for each electric loada unique electric load for each acoustic impedancethe idea is to identify the electric load achieving a desired acoustic impedanceMethodology:Define a target acoustic impedance ZatIdentify the corresponding electric load Yeq

Active electroacoustic absorbersH. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range32

Target resistanceNeed to set reactive terms for stability

10/28/2015Ex4: electric impedance synthesis approach1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingNote: we need to implement Yeq rather than Zeq for sake of causalityAkustisches Kolloquium32Active electroacoustic absorbersH. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range3310/28/2015Ex4: electric impedance synthesis approach

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium33

Active electroacoustic absorbersEx4: electric impedance synthesis approachSuch loads are complex to realise with analog devices:Design of digital filters on FPGA platform to mimic a desired electrical admittance at the loudspeaker terminalsUse a voltage-current converter to deliver a current to the loudspeakerH. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range34

IIR filter generatingCompactRIO real-time controller10/28/20151. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium34Active electroacoustic absorbersEx5: specific feedforward approachThe synthetic electric admittance Yeq acts as a current command, based on the sensing of the emf e.

What if we substitue the pressure p for the emf e as the input?

This new technique has been recently developped in the lab, which shall improve the closed-loop stability ( bandwidth extension)H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range35

Source of instability

10/28/20151. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium

ptcontrollerEx5: specific feedforward approachSpecification of a target acoustic impedance:

Hybrid solution of the admittance synthesis and feedback controlUse of sensor (microphone) and digital controller : usefull: source of instabilityTransfer function (pi) p:

Always stable (only dependant on the coupling with the room)10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range36Active electroacoustic absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

specific feedforward


A desired acoustic impedance Zat can be assigned by identifying the controller transfer function:H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range37

ptv

10/28/20151. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingElectroacoustic absorbersEx5: specific feedforward approachAkustisches Kolloquium3. Practical realization38H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium

Eigenfrequency 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]

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range391. FEM simulationRwallRa1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium

Eigenvalues modal decay times

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range401. FEM simulation

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingKarkar et al., Electroacoustic absorbers for the low-frequency modal equalization of a room: what is the optimal target impedance for maximum modal damping, depending on the total area of absorbers?, Forum Acusticum 2014Akustisches KolloquiumChanging the value of resistance Rafor each mode, find the minimal value of MT60translate as a frequency-dependant resistance Ra(f)10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range412. Optimal acoustic resistance

zoom1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingKarkar et al., Electroacoustic absorbers for the low-frequency modal equalization of a room: what is the optimal target impedance for maximum modal damping, depending on the total area of absorbers?, Forum Acusticum 2014Akustisches KolloquiumChanging the value of resistance Rafor each mode, find the minimal value of MT60translate as a frequency-dependant resistance Ra(f)10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range422. Optimal acoustic resistance1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Karkar et al., Electroacoustic absorbers for the low-frequency modal equalization of a room: what is the optimal target impedance for maximum modal damping, depending on the total area of absorbers?, Forum Acusticum 2014Akustisches KolloquiumChanging the value of resistance Rafor each mode, find the minimal value of MT60translate as a frequency-dependant resistance Ra(f)

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range432. Optimal acoustic resistance1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Karkar et al., Electroacoustic absorbers for the low-frequency modal equalization of a room: what is the optimal target impedance for maximum modal damping, depending on the total area of absorbers?, Forum Acusticum 2014Akustisches KolloquiumThe function Ra(f) serve as a specification for the electroacoustic absorbers impedance Zat(f)=Rat(f)+iwXat(f)

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range443. Acoustic impedance specification

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumImplementation in the prototype10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range454. Implementation

Voltage-driven current amplifier

Filter implemented on a DSP platformDSP

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumImplementation in the prototype10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range46Electroacoustic absorbers

CompactRIO real-time controllerVoltage-driven current amplifier

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range47Electroacoustic absorbersISO 10534-2 : acoustic impedance measurement technique

Acoustic impedanceAbsorption coefficient

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium4. Room modes damping: experimental assessment48H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium4x4 electroacoustic absorbers prototypes(total surface = 16x151 cm2 = 0.24 m2)In a reverberant chamber of 226.9 m2 evaluation of room modal damping

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range49Experimental assessment


Akustisches KolloquiumFrequency 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 playing10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range50Experimental assessment 1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumHardwareRecorders/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 positions51H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range10/28/20151. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingExperimental assessment Akustisches Kolloquium10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range521. Frequency response(blue: Hardwalls, red: Absorbers)

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumModal damping (dB)10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range531. Frequency response

1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumMode 8 waveforms(hardwalls : 52.78 Hz - absorbers : 52.72 Hz)

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

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range542. Modal decay time1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumMode 8 waveforms(hardwalls : 52.78 Hz - absorbers : 52.72 Hz)

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

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range552. Modal decay time1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingMode 8 - HardwallsMode 8 - Absorbers

Akustisches Kolloquium10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range562. Modal decay time

1/6MT101/3MT201/2MT301. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range572. Modal decay time1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches Kolloquium

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range582. Modal decay time1. Context2. Electroacoustic absorber3. Realizations4. Room modes dampingAkustisches KolloquiumPeggy Lee Fever

White Stripes 7 nation army

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range593. Music renderingHardwallsAbsorbersHardwallsAbsorbersWhats this tune?Whats this tune?1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Akustisches KolloquiumWaveforms

Echograms

10/28/2015H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range604. Kick drumHardwalls Absorbers1. Context2. Electroacoustic absorber3. Realizations4. Room modes damping

Akustisches KolloquiumConclusions and perspectives61H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency rangeAkustisches KolloquiumConclusions and perspectives4 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 surface62H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range10/28/2015Akustisches KolloquiumConclusions and perspectivesEven better performances achieved recently63H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range10/28/2015AES Swiss Section / SSA joint meeting - 09.03.2015

20 dB70 dB30 dB40 dB65 dBAkustisches KolloquiumConclusions and perspectivesEven better performances achieved recently

The objective performances are under evaluation this week in a more conventional listening room (Goldmund Auditorium)

Subjective assessments should also be undertaken (to be done in the coming weeks)64H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range10/28/2015Akustisches KolloquiumConclusions and perspectivesNew product released by PSI Audio: AVAA (Active Velocity Acoustic Absorber)65H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range10/28/2015

http://www.psiaudio.com/en/our-products/avaa-c20/ Akustisches KolloquiumThank 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.Thanks to : Dr. Sami KarkarDr. Romain BoulandetEtienne Rivet

e-mail: [email protected]

66H. Lissek - Electroacoustic absorption: towards room equalization in the low-frequency range10/28/2015Akustisches Kolloquium

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Akustisches Kolloquium Electroacoustic absorption: towards room equalization in the low- frequency range Dr. Hervé Lissek EPFL STI IEL LTS2 [email protected]