Sound Quality of Audio Systems

Loudspeaker Data – Reliable, Comprehensive, Interpretable

Introduction

Biography: 1977-1982 Study Electrical Engineering, TU Dresden 1982-1990 R&D Engineer VEB RFT, Leipzig, 1992-1993 Scholarship at the University Waterloo (Canada) 1993-1995 Harman International, USA 1995-1997 Consultancy 1997 Managing the KLIPPEL GmbH 2007 Professor for Electro-acoustics, TU Dresden

My interests and experiences: • electro-acustics, loudspeakers • digital signal processing applied to audio Wolfgang Klippel • psycho-acoustics and measurement techniques Klippel GmbH Agenda

Left Right Audio Audio Channel Channel

Audio-System Transducer Perception (Transducer, DSP, (woofer, tweeter) Final Audio Application Amplifier) (Room, Speaker, Listening Position, Stimulus)

1. Perceptual and physical evaluation at the listening point  perceptive modeling & sound quality assessment  auralization techniques & systematic listening tests 2. Output-based evaluation of (active) audio systems  holografic near field measurement of 3D sound output  prediction of far field and room interaction  nonlinear distortion at max. SPL 3. Comprehensive description of the passive transducer  parameters (H(f), T/S, nonlinear, thermal)  symptoms (THD, IMD, rub&buzz, power handling)

3 Objectives

Left Right Audio Audio Channel Channel

Audio-System Transducer Perception (Transducer, DSP, (woofer, tweeter) Final Audio Application Amplifier) (Room, Speaker, Listening Position, Stimulus)

• clear definition of sound quality in target application • filling the gap between measurement and listening • numerical evaluation of design choices • meaningful transducer data for DSP and system design • selection of optimal components • maximal performance-cost ratio • smooth communication between customer and supplier

4 Objective Methods for Assessing Loudspeakers

Room Loudspeaker Parameter-Based Parameters Parameters Method e.g. T/S parameter, amplitude and phase response, nonlinear and thermal parameters

Loudspeaker- Psychoacoustical Stimulus Room Model Model Sensations nonlinear nonlinear

e.g. music, test signals Distortion Perceptual Measurement Quality Method e.g. THD, IMD, rub&buzz e.g. predicted distortion preference

5 Perceptual Evaluation of Signal Distortion

distortion Ideal reference Basic Conceptions signal monaural processing Basic Auditory test signal Sensations

Loudness Perceived Binaural Fluctuations Defects Processing Sharpness S DS Overall stimulus + Coloration V DV Quality Loss Spaciousness R DR test signal Localization Basic reference Perceived defects consider the signal monaural ideal conceptions and the impact processing on quality distortion The basic auditory sensations are the dimensions of the perceptional space and describe the audibility of the distortion

7 Auralization of Signal Distortion

Input Output Signal Signal

u (t ) es 0 p(t)

Linear dlin (t) OBJECTIVES: Model Linear 1. Virtual enhancement or Distortion attenuation of distortion n(t) components Nonlinear dnlin (t) Model Nonlinear Distortion 2. Systematic Listening Tests Noise 3. Defining a value S in d (t) DIS Unpredictable irr dB describing the Dynamics Irregular Distortion distance to the audibilty

Model threshold

10 Finding Audibility Thresholds

histogram of the audibility thresholds of 55000 participants of a listening test at www.klippel.de

weighted up and down method

low distortion audibility threshold SDIS enhancement attenuation SDIS=-15 dB

11 Subjective and Objective Evaluation

Objective Subjective

Engineering Evaluation Evaluation Marketing Management Listening Test + Auralization

Perceptive Modeling SDIS Physical Data • Distortion, Maximal Output Audibility of distortion • Displacement, Temperature Perference,

• Evaluation of Design Choices • Defining target specification • Clues for Improvements • Tuning to the market

Performance/cost ratio

13 Agenda

Left Right Audio Audio Channel Channel

Audio-System Transducer Perception (Transducer, DSP, (woofer, tweeter) Final Audio Application Amplifier) (Room, Speaker, Listening Position, Stimulus)

14 Evaluation of the Audio Product

Measurement in Target Application Measurement under Standard Condition

(Standard) living room Anechoic room Suppressing the considering room, influence of distance, ambient acoustical noise and other transfer of the environment conditions audio system

P P U U S S H H

I I P 0 C C P P W P P U U R 1 2 S S O O H H U U MIC1 LINE2 T T LINE1 MIC2 1 2

I I P 0 C C W P P R 1 2 O O U U MIC1 LINE2 T T LINE1 MIC2 1 2

Definition of target performance as Physical characteristics perceived by final (comprehensive, simple user to interpret, comparable, reproducible)

Auralization/Listening Test Loudspeaker Development Perceptual Evaluation

15 Characteristics defined by IEC 60268-5

1. Impedance (rated value, Z(f)-curve, Qts, Vas) 2. Input voltage (rated noise, short + long term maximal) 3. Input power (rated noise, short + long term maximal) 4. Frequency characteristics (rated range, fs, fvent) 5. SPL in stated band, sensitivity for 1 W 6. SPL response for voltage, H(f), effec. freq. range 7. Output (acoustic) power, efficiency 8. Directivity (pattern, rad. angle, index, coverage) 9. Amplitude nonlinearity (THD, IMD)

The scope of this standard is limited to passive loudspeaker systems !

16 Active Loudspeaker Systems

Properties of the black box depend on control parameters and stimulus Sound Field digital audio Black box Near Field Far Field stream No access to internal states Evaluation is based on evaluation of drivers acoustical output

control parameters (e.g. attenuation)

17 IEC 60268-5 applicable to Active Systems ?

can be applied, need modification, not applicable

1. Impedance (rated value, Z(f)-curve, Qts, Vas) 2. Input voltage (rated noise, short + long term maximal) 3. Input power (rated noise, short + long term maximal) 4. Frequency characteristics (fs, fvent) 5. SPL in stated band, Sensitivity for 1 W 6. SPL response for voltage input, H(f), effec. freq. range, 7. Output (acoustic) power, efficiency 8. Directivity (pattern, rad. angle, index, coverage) 9. Amplitude nonlinearity (THD, IMD)

18 Modern Audio Systems

New Requirements:

• Audio systems become active  no access to the electrical terminals of the transducer  digital signal processing dedicated to the transducer  amplifiers with more capabilities • Audio systems become portable  main axis of radiation, sweet point and position of the listener are not defined  battery powered • Audio systems become personal (hand-hold devices)  listener is in the near field of the source • Audio systems become smaller, lighter  using green transducer technologies (efficient, nonlinear)

19 Integration of DSP, power amplification and electro-acoustical conversion

amplifiers Nonlinear components DSP protection

Tweeter Digital protection Equalizer audio Limiter X-over linearization input Midrange Gain protection Control linearization

Woofer drivers

Control input • Smart technologies (DSP) saves hardware resources and energy • more acoustical output at reduced weight, size and cost

 Green Speaker Technology

20 New Standards required for Evaluating Active and Passive Loudspeaker Systems

• Applicable to active and passive systems (prototypes, final and competitive products) • Describing the radiated direct sound at any point within the listening area (including near field) • Consideration of room-loudspeaker interaction • Assessment of maximal acoustical output • Irregular loudspeaker defects (rub, buzz, leakage, particles, loose connections) • Comprehensive set of data (low redundancy, easy interpretation) • Bridging QC and R&D • Bridging perceptive and physical evaluation

 Currently discussed in standard committees

21 Small Signal Performance

Specifications for Active and Passive Loudspeaker Systems

• On-Axis Sound Pressure at reference distance rref=1m  SPL(f) response  Phase response (group delay (f) response)

• Directivity a) single-value characteristics

 sound power response Pa(f)  directivity index Di(f) b) 2D far-field data  pressure distribution p(θ, ) on a spherical surface at large distance from the source p(θ, ) (balloon plot, beam pattern) c) 3D near/far-field data new  sound pressure p(θ,  , r ) at any point r in the space beyond the sound source (spherical wave expansion)

22 2D far-field data

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

23 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

24 Holografic Measurement of the radiated direct sound in the complete 3D space

1. Scanning the sound pressure in the near field of the source

2. Expansion in spherical waves Loudspeaker microphone monopol

z dipols

r quadropols

Hankel function N n (2) m φ pout (r,,,)   cn,m ()hn (kr)Yn (,) n0 mn Coefficients Spherical Harmonics

3. Results:

- frequency dependent set of coefficients cn,m () - point r0 of expansion - radius rs of validity (scanning surface) - order N of expansion

25 Practical Realization near field scanning by using robotics

scanning in various Example of a Near-Field Scanner coordinates (cylindrical, spherical, cartesian)

Objectives of the robotics: 1. Acoustical properties • transparent, • low noises 2. Flexible scanning grid • scanning close to the source • accurate positioning on multiple layers • 2 π half-space (driver in baffle) • 4π full-space (compact sources) 3. High-Speed measurement • simultanous positioning in 3 coordinates • multiple channel aquisition (mic array) 4. Wide range of application • from smart phone to line array • heavy systems (> 500 kg) • slim system (> 4 m) • cost effective, portable

26 Evaluation of a Notebook Application of Near-field Acoustical Holography

far3. fieldExtrapolation of the sound pressure at any 1. Measurement of the sound point2. Expansion outside the into scanning spherical surface waves pressure distribution

near field r

 r0



r s

scanning surface close to the source

27 Line Arrays in Professional Audio Application of Near-field Acoustical Holography

1) Near-field Measurement a  large dimension a of box  anechoic room is too small for far field condition: distance r >> dimension a distance r >> wave length λ

2) Comprehensive 3D data  coefficients of spherical wave expansion  direct sound in near and far field (any distance)  no redundancy (angular resolution at minimal data size)  more information provided by GLL files

3) Input for Numerical Simulation Tools  superposition of wave expansions  design and evaluation of line arrays  room interaction

Large Signal Performance

Specifications for Active and Passive Loudspeaker Systems

• Maximal SPLmax at reference point (1 m, on-axis), in rated frequency range

• Effective frequency range (Upper and lower limits flower,l < f < fupper,l ) • Compression of fundamental component (thermal and nonlinear effect) • Harmonic distortion (Equivalent input distortion) • Intermodulation distortion (IMD, MTD) • Impulsive distortion (PHD, CHD) indicating rub&buzz, loose particles • Modulated noise (MOD) indicating air leakage • Durability verified in accelerated life test

29 Condition for Large Signal Measurements new IEC TC100 standard project “Output-based Evaluation of audio systems”

gain and u max maximal input sound field depend on value u ? measurement max setup ! Output*

SPLMAX gain ? optical active audio analogue system re wavefile evaluation ~ u(t) wireless (no access to point* internal states) test signal* drivers (chirp in rated frequency range)

*conditions defined by control manufacturer parameters*

30 Interpretation and Benefits of SPLMAX

Example as specified by a manufacturer:

SPLMAX, =108 dB for 60 Hz < f < 3kHz

(default test chirp, 1m on-axis)

SPLMAX • is a single-valued characteristic describing the limit of the acoustical output (but not sound quality of the system) • is rated by the manufacturer considering target application • depends on rated conditions (working range, reference point, stimulus, ...) • can be generated by the audio system without damage • provides a fast way for adjusting the input level of any stimulus during measurements

31 Definition of SPLMAX

SPLMAX is the mean short-term SPL in the rated frequency range generated by a sinusoidal chirp at the reference point.

Fundamental 120 Fund. mean (0.06 to 3 kHz) SPLMAX 100 THD

[V] (rms)

- 40

dB rated frequency range

-20 20 50 100 200 500 1k 2k 5k 10k 20k Frequency [Hz] fl fu

32 Short-Term Compression reveals mechanical nonlinearities only (no voice coil heating )

system excited by a chirp (T=1 s) generating SPLmax at the evalution point 125 KLIPPEL 120 115 linear prediction 110 105

100 ) short-term fundamental (1 s)

95 rms

[V] ( [V]

- 85

dB dB 80 75 70 65 60 20 50 200 500 2k 20k Frequency [Hz]

33 Long-Term Compression reveals effects of mechanical nonlinearities and voice coil heating

system excited by a chirp (T=1 min) generating SPLmax at the evalution point 125 KLIPPEL 120 115 linear prediction 110 105

100 long-term fundamental (1 min) )

95 rms

[V] ( [V]

- 85

dB dB 80 75 70 65 60 20 50 200 500 2k 20k Frequency [Hz]

34 Harmonic Distortion

system excited by a chirp (T=1s) generating SPLmax at the evalution point

Fundamental 120 Fund. mean (0.07 to 20 kHz)

100 THD 3rd Harmonic 80 2nd Harmonic

[V] (rms)

- 40 dB

-20 20 50 100 200 500 1k 2k 5k 10k 20k Frequency [Hz]

35 Higher-Order Distortion for assessing rub&buzz and other irregular loudspeaker defects

system excited by a chirp (T=1s) generating SPLmax at the evalution point

Fundamental 120 Fund. mean (0.07 to 20 kHz)

100

PHD limit (-40dB)

[V] (rms)

- 40 dB

20 Absolute PHD peak value of higher-order 0 distortion

-20 20 50 100 200 500 1k 2k 5k 10k 20k Frequency [Hz]

36 Agenda

Left Right Audio Audio Channel Channel

Audio-System Transducer Perception (Transducer, DSP, (woofer, tweeter) Final Audio Application Amplifier) (Room, Speaker, Listening Position, Stimulus)

38 Interfaces between Signal Processing, Electronics, Transducer, Acoustical Environment

Example: Active Loudspeaker System enclosure

software transducer horn HP x x sound field

BP x x x acoustical Digital measurement input LP DSP x x mechanical amplifiers measurement electrical measurement

39 How to Specify the Optimal Transducer ?

Parameters give a comprehensive 1. Parameters (independent of stimuli) set of data !! • Acoustical transfer functions (from near-field holography) • Mechanical transfer functions (from laser scanning) • Small signal parameter T/S • Large signal parameters (thermal, nonlinear)

2. Stimulus-based Characteristics • Maximal SPL Should be transformed into • Nonlinear distortion (THD, IMD, XDC) parameters • Symptoms of irregular defects (rub, buzz, leakage,...) • Coil temperature, compression, Pmax

40 Important Transducer Parameters

1. Linear parameters of Motor and Suspension

 T/S parameters (Re, Mms, Rms, ...), lambda (), electr. impedance 2. Nonlinear lumped parameters of motor and suspension

 Bl(x), Le(x), Kms(x), Le(i) 3. Thermal parameters

 thermal resistances Rtv, Rtm and capacities Ctv, Ctm 4. Linear distributed mechanical parameters

 mechanical transfer function Hx(rc,j), cone geometry z(r), AAL response 5. Sound pressure responses (transducer in infinite baffle)  spherical wave expansion, on-axis response, directivity 6. Mechanical or acoustical load  Mechanical Admittance Y(j) of the coil

41 Conclusions

Left Right Audio Audio Channel Channel

Audio-System Transducer Perception (Transducer, DSP, (woofer, tweeter) Final Audio Application Amplifier) (Room, Speaker, Listening Position, Stimulus)

1. Development of modern audio system requires different kinds of models, characteristics and measurement techniques 2. Perceived sound quality depends on the final audio application, perceptual processing, training and expectations of the customer 3. Output-based evaluation of active audio-systems is under discussion (join the IEC or AES standard groups) 4. Parameters (independent on the stimulus) play an important role in tranducer design and system integration

49 Thank you !