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Sound Quality of Audio Systems

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Sound Quality of Audio Systems Small, Loud-Speakers: Taking Physics To The Limit 135th AES Convention NY by Wolfgang Klippel, KLIPPEL GmbH Dresden University of Technology Small, Loud-Speakers: Taking Physics To The Limit, 1 OUR TOPIC TODAY Small loudspeakers producing sufficient output at acceptable quality ...that‘s what most customers want ! How far away is this target ? Why is it so difficult to develop such products? How to cope with the physical limitations? Small, Loud-Speakers: Taking Physics To The Limit, 2 The Weakest Part of the Audio Reproduction Chain microphone Listening Environment transmission Recording Sound Environment Engineering source storage amplifier Listener media Loudspeaker because it • limits the acoustical output • causes significant linear and nonlinear distortion • varies with time due to fatigue and ageing • depends on climate condition • contributes to weight, size and cost • has low efficiency and produces heat Small, Loud-Speakers: Taking Physics To The Limit, 3 Electro-Acoustic Conversion Range of Operation Amplitude Overload Large signal performance Our fundamental problem with small Small signal loudspeakers: performance Power efficiency Small, Loud-Speakers: Taking Physics To The Limit, 4 Pass-Band Efficiency of direct-radiator loudspeaker in an infinite baffle Example: Micro-speaker 2 2 Pa (Bl) S 0 d η0 0.007 % 0 2 Re 7.80 Ohm Pe R M 2c e ms Bl 0.774 N/A for f >fs and ka<1, MMS 0.082 g radiation on one side considered Sd 1.03 cm² effective DC transduction moving radiation resistance parameter 2a < 휆/πmass (force factor) surface Sd Re ka < 1 MMS q=Sdv v diaphragm acoustic output electric inputi power Pa power Pe pole plate p=FL/Sd 2RAR(f) U Blv cBolil F=Bmliagnet FL=pSd Sd 7 μW 100 mW backplate microspeaker Small, Loud-Speakers: Taking Physics To The Limit, 5 What makes the efficiency so low ? force factor effective (Bl)2 S 2 radiation area Pass-Band Efficiency: 0 D 0 2 ReM MS 2c voice coil moving resistance mass Re MMS q=Sdv v ii Fi ≈F 2 p=FL/Sd 2RAR(f) U Blv Bl F=Bli FL=pSd Sd FL RMR Sd ZAR( f ) acoustical radiation impedance Re i > Blv • acoustical load depends on radiation area Sd • inertia Fi of the moving mass MMS is larger than force FL at the acoustical load • electric power is dissipated in the resistance Re Small, Loud-Speakers: Taking Physics To The Limit, 6 Alternative Transducer Principles ? volume voltage force velocity electro- mechano- sound mechanic acoustic field transducer transducer current velocity sound pressure higher electro-static magneto-strictive mass lower mass electro-dynamic higher lower peak (moving coil, ribbon, resistance displacement planar magnetics) electro-magnetic (balance armature, piezo-electric moving iron, moving higher magnet) force higher price others Small, Loud-Speakers: Taking Physics To The Limit, 7 Leverage – a mechanical transformator R e Re MMS MMS v2=v/r q=Sdv v v i i 2 2 p 2RAR(f) U U Blv Blv Bl BlF=Bli F=Bli FFL L=pSd RrMR=l1/lr2 Sd Z AR (Ff )2=rSFdL l1 FL Problem: electro-mechanical U Transducer v transducer increase of high moving mass l2 force F2 RMR lower v2 low v2 velocity force acoustical F2 source load higher velocity l F v r 1 2 impedance l2 FL v2 matching Small, Loud-Speakers: Taking Physics To The Limit, 8 Horn – an Acoustic Transformer to increase the passband sensitivity Re MMS q=Sdv qM v i Benefit: • strong acoustical load (F ≈ FL) • high efficiency (η> 50 %) 2 S 0c FL SD R (f)M RAR U Blv Bl F=Bli FL=pSd RSd pc=FL/Sd AR pM S MR 0 S M ST T Drawback: • large for bass reproduction S q p M M ST q pM ST FL SM >100 Hz U Transducer q pM p fx S(x) ST e qM x l > λ/2 Small, Loud-Speakers: Taking Physics To The Limit, 9 Efficiency at Low Frequencies below the fundamental resonance frequency fs efficiency versus frequency wavelength λ 20log () 10 efficiency versus frequency compressionbaffle driverof airin baffle 12dB/oct. ΔV= 1 liter F 95 dB SPL transducer 3 m distance f 20 Hz f s f T ka 1 bass pass-band 20log () Benefits: 10 efficiency versus frequency • minimal enclosure volume • constant displacement for f< f F T • equalization possible for 1-2 octaves Drawbacks: • low efficiency f f s f T ka 1 Small, Loud-Speakers: Taking Physics To The Limit, 10 Compressing Air by an Additional Resonator extending the bandwidth to lower frequencies efficiency versus frequency 20log10() efficiency versus frequency using a box volume V Helmholtz F 24 dB/oct. (air compliance Resonator CAB) vented box + System bass air mass M AP ff f s ka 1 smaller box Benefits: V < V 2 lower compliance • increases efficiency at fs C F F AB F Drawbacks: higher mass MAP • system alignment longer port passive radiator • air noise (port) • cost (passive radiator) Small, Loud-Speakers: Taking Physics To The Limit, 11 Efficiency at High Frequencies Exploiting Modal Vibration break-up modes increases efficiency 2020loglog1010(()) efficiency versus frequency Full Band rigid radiator 6 dB/oct. Loudspeaker Slim (TV Speaker) high frequencies ff ff ss kaka11 Flat (Automotive) speaker surround geometry break-uprigidmodes piston deformed Distributed Mode Loudspeaker (flat panel) other surfaces used as radiator (enclosure, window, post card) Small, Loud-Speakers: Taking Physics To The Limit, 12 Assessing the Mechanical Vibration stimulus (input) output mechanical vibration Simulation (FEM) Measurement (Laser) electro-mechanical mechano-acoustical efficiency efficiency Mechanical power (AAL) electrical power Electro-acoustical sound power efficiency Small, Loud-Speakers: Taking Physics To The Limit, 13 Assessing the Mechanical Power by using the Accumulated Acceleration Level (AAL) 850 Hz 90 3.8 kHz AAL 11 kHz 85 6.4 kHz rigid body mode 80 peaks in AAL show the natural 75 frequencies of the modal resonances 70 65 60 Sound Power 55 dB for 1.00V, 0.4 m 0.4 1.00V, for dB 50 45 40 35 20 Hz - 600 Hz break-up modes 30 0.1 1 10 f [kHz] Small, Loud-Speakers: Taking Physics To The Limit, 14 Mechano-Acoustical Efficiency 90 AAL 1078,1 Hz q2 dB 80 70 60 50 q Sound 1 40 Power 30 0.1 f [kHz] 1 10 q1+q2=0 acoustical cancellation Problem: • sufficient mechanical vibration generates low sound power output • node divides radiator in two areas producing a positive and negative volume velocity generating a dip in the power response Small, Loud-Speakers: Taking Physics To The Limit, 15 Sound Distribution in the 3D Space Range of Operation Amplitude Overload Large signal performance Generating the desired Small signal performance Sound pressure field Small, Loud-Speakers: Taking Physics To The Limit, 16 Directivity of the Loudspeaker assessing the radiated direct sound in the far field distance r = 0.4 m, Input voltage u= 1Vrms dB 90 KLIPPEL 85 Omni-directional behavior (like a point source) 80 75 SPL on-axis 70 Power 65 60 SPL 30 degree 55 270° SPL 60 degree Directivity 50 Index 180° SPL 90 degree 0° 45 90° 40 35 30 0.1 1 10 f [Hz] Example: woofer Small, Loud-Speakers: Taking Physics To The Limit, 17 Sound Pressure Distribution on a sphere in the far field SPL 4.1 kHz at distance r=4m 6.1 kHz at distance r=4m 90° on-axis 270° azimutal angle angle azimutal 180° 0° frequency 90° -90° Balloon Plot Beam Pattern Distance r >> dimensions d of the loudspeaker Distance r >> wavelength Small, Loud-Speakers: Taking Physics To The Limit, 18 Complete 3D Information Required Sound Pressure at 7.6 kHz In the following application the listerner is far field data closely located to the source: are less important • personal audio equipment (smart phone) • multimedia (tablet, notebook) • studio-monitor Near Field • car audio loudspeaker Small, Loud-Speakers: Taking Physics To The Limit, 19 Example: Evaluation of a Notebook Using nearfield Acoustical Holography far3. fieldExtrapolation of the sound pressure at any 1. Measurement of the sound point2. Expansionoutside the intoscanningsphericalsurfacewaves pressure distribution near field r r0 r s scanning surface close to the source Small, Loud-Speakers: Taking Physics To The Limit, 20 The Loudspeaker at Higher Amplitudes Range of Operation Amplitude Overload Thermal Maximal Output Large signal and performance Nonlinear Distortion Model Compression Stability Small signal Linear performance Model Small, Loud-Speakers: Taking Physics To The Limit, 21 Compression of SPL Output SPL output at maximal permissable input Sound Pressure Response 130 Linear response 125 linear response +20dB dB predicted from a 120 small signal measurement 115 (-20dB) 110 105 Long Term Response (1 min) Long term response 100 measured after 95 amplitude compression caused applying the by nonlinearities and voice coil 90 sinusoidal chirp for 1 heating min 85 80 20 50 200 500 2k Frequency [Hz] Small, Loud-Speakers: Taking Physics To The Limit, 22 Compression of SPL Output SPL output at maximal permissable input Sound Pressure Response 130 Linear response 125 linear response +20dB dB predicted from a 120 small signal Short Term Response (1 s) measurement 115 (-20 dB) 110 105 Long Term Response (1 min) Short term response 100 measured within 1 s 95 (without voice coil heating) 90 amplitude compression caused 85 by nonlinearities only 80 20 50 200 500 2k Frequency [Hz] Small, Loud-Speakers: Taking Physics To The Limit, 23 Nonlinear Symptom: New Spectral Components spectrum of twoResponse-tone 1 Stimulus Frequency Domain Response 1 spectrum of reproducedFrequency Domain stimulus 20 10 output 20 0 input 10 -10 0 -20 -10 -30dBu (Uo = 1V) -20 -40 -30 -50 dBu (Uo = 1V) 101 102 103 -40 f [Hz] -50 101 102 103 f [Hz] Nonlinear System Amplitude sound pressure spectrum Intermodulation nd 2nd 2 2nd Distortion 3rd 3rd 3rd nth nth nth harmonics difference tones summed tones frequency nf f2 (n 1) f1 f2 f1 f2 f1 f (n 1) f 2 f1 1 2 1 f1 f2 “bass component” “voice component” Small, Loud-Speakers: Taking Physics To The Limit, 24 Example: Visible Nonlinear Symptoms Generated by a Loudspeaker stroboscope Generator tone at f scale pointer Resonance frequency fs 1.
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