1 ALMA 2003 Winter Symposium World Cone Symposium World Cone Symposium A Simple Approach to Improve...

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ALMA 2003 Winter SymposiumALMA 2003 Winter Symposium

“ “World Cone SymposiumWorld Cone Symposium””

A Simple Approach to Improve Sensitivity ofA Simple Approach to Improve Sensitivity of

Quality Control Procedures of LoudspeakersQuality Control Procedures of Loudspeakers

Presented byPresented by

Mauro BigiMauro Bigi

Maurizio JacchiaMaurizio Jacchia

Audiomatica Srl - ItalyAudiomatica Srl - Italy

Imperial Palace, Las Vegas - January 8, 2003

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THE AUTHORS INVESTIGATE THE APPLICATIONS OF ONE CAPABILITY OF THE CLIO SYSTEM:

ADDING A DC COMPONENT TO THE EXCITATION SIGNAL.

WHILE THE IMPLICATIONS SPAN FROM R&D TO PERFORMANCE ASSESSMENT IN GENERAL,

THE AUTHORS FOCUS ON QUALITY CONTROL ENVIRONMENTS.

AN ENGINEERING REPORT COMPARES SEVERAL QUALITY CONTROL TEST METHODS

WITH AND WITHOUT THIS STRESSING TECHNIQUE.

ABSTRACTABSTRACT

www.cliowin.comwww.audiomatica.com

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INTRODUCTIONINTRODUCTION

• WE WOULD LIKE TO GIVE FIRST THE GUIDELINESWE FOLLOWED WHILE DEVELOPING

THE QUALITY CONTROL PART OF OUR MEASURING SYSTEM,

AS THESE ARE CONSISTENT WITH OUR PRESENTATION HERE


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• “HOW IS AN EFFECTIVE QC MEASUREMENT SYSTEM DEVELOPED?”

• “MERGING WELL ESTABLISHED AND DOCUMENTED MEASURING TECHNIQUES WITH REQUESTS COMING

FROM EXPERIENCED QC ENGINEERS”

• 10 YEARS OF EXPERIENCE TELL US THAT THE “QUALITY” OF A QUALITY CONTROL ENVIRONMENT

IS MORE DEPENDENT ON THE SKILL OFTHE QC ENGINEER(S) RATHER THAN ON

THE MEASUREMENT SYSTEM ITSELF


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• THE WORD “QUALITY” STAYS FOR THE FINAL RESULT, I.E. AVOIDING SHIPPING DEFECTIVE LOUDSPEAKERS

TO CUSTOMERS;THEY SHOULD ALSO BE AS CLOSE TO THE REFERENCE

AS POSSIBLE

www.cliowin.com


• THE MEASUREMENT SYSTEM MUST HOWEVERBE FLEXIBLE, ACCURATE AND RELIABLE;

IT SHOULD SPEED UP, AS MUCH AS POSSIBLE,ANY OPERATION INVOLVED

• WE ADDRESS PROBLEMSGIVING POWERFUL TOOLS TO ENGINEERS,NO WIZARDS OR UNIQUE MAGIC NUMBERS

WHAT WE PRESENT HERE IS JUST ONE MORE TOOL

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R&D ENGINEERS

PRODUCTION LINES

QC ENGINEERS

CRITICAL TIME CONSUMING ITEMS ARE:

A) USER’S LEARNING CURVEB) QC PROCEDURES DEVELOPMENTC) EASY FITTING IN EXISTING PRODUCTION LINESD) STATISTICS AND RESULT REPORTINGE) FEEDBACK TO TOLERANCE MASKS


YOUR COMPANY


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• THE STRESSING METHOD WE ARE PRESENTING APPLIES, AS AN ADD-ON,

TO VIRTUALLY ANY WELL ESTABLISHED MEASURING TECHNIQUE AUGMENTING

ITS “GOOD-TO-BAD” SPREAD

• OUR ENGINEERING REPORT WILL COVER TWO DIFFERENT CASES

ADDRESSING RUB&BUZZ DETECTION:


• SWEEP WITH RESPONSE ANDDISTORTION VS. FREQUENCY ANALYSIS

• SINGLE FREQUENCY THD


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• GENERIC TERM MEANING AN ACOUSTIC DEFECTWITH IMPULSIVE (LOW ENERGY) CHARACTERISTICS

MORE EASILY DETECTED BY THE END USERTHAN LINEAR AND NON LINEARCONVENTIONAL DISTORTIONS

• IT IS THEREFORE THE “BLACK BEAST” OF ANY QC ENGINEER, AS LACK OF ITS DETECTION

HAS GREAT IMPACT ON COMPANY’S IMAGE

RUB&BUZZRUB&BUZZ

• IT IS CAUSED, DURING PARTS ASSEMBLY,BY SEVERAL DIFFERENT REASONS RANGING

FROM DEFECTIVE PARTS TO PRODUCTION TOLERANCES

• IT CAN BE INTERMITTENT


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• STILL IN MANY ENVIRONMENTS AN HEARINGTEST PRECEDES AN AUTOMATED QC TEST

• ITS INTENSITY RANGES FROM MACROSCOPIC - DETECTABLE BY ANY MEASUREMENT, I.E. LEVEL -

TO VERY LITTLE - REQUIRING A QUITE ENVIRONMENTTO BE HEARD

RUB&BUZZRUB&BUZZ

• THE FIRST CASE OF STUDYADDRESSES THE LATTER SITUATION


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BLOCK DIAGRAMBLOCK DIAGRAM

ISense

From CLIOIn 1

D.U.T.

Ampli/SwitchBoxCLIOQC

OUTPUT ACLIO

INPUT A

OUTPUT B

INPUT B

To CLIO

Mic

Speaker

AC OUTPUT WITH DC CAPABILITY (± 2.5V)


CURRENT SENSING OUTPUT

DC COUPLED AMPLIFIER (WITH 10 dB GAIN)

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• 8” P.A. WOOFER

EXAMPLE “A”EXAMPLE “A”

• ACOUSTICALLY (BY EAR) THE BAD UNIT EMITSA VERY LOW LEVEL CLICKING NOISE

WHEN THE SWEEP REACHES THE RESONANCE REGION WHICH IS BETTER HEARD BELOW RESONANCE

Fs 80 Hz Re 5.2 OhmBl 12.44 TmCms 0.15 mm/N

Qes 0.45

Qms 3.51


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• BELOW RESONANCE THE PROPER SOUND EMISSION DECREASES WHILE DISPLACEMENT CONTINUES

TO INCREASE OR REMAINS THE SAME UNTIL DC (0 Hz)

• OBTAINING DISPLACEMENT IS THE FIRST REQUIREMENT TO UNDERLINE RUB & BUZZ

• IN RARE CASES DISPLACEMENT ALONEIS NOT SUFFICIENT AND A CONTEMPORANEOUS

HIGHER FREQUENCY TONE IS REQUIRED


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• THE LATTER OBSERVATION SUGGESTS THATWHEN THE CONE BEHAVES AS A RIGID PISTON

(LOW FREQUENCY - HIGH DISPLACEMENT)IS ABLE TO REDUCE SOME HOW THE RUB & BUZZ

• ADDING A HIGHER FREQUENCY TONE BREAKS CONE PISTON BEHAVIOUR MAKING RUB & BUZZ

MORE EVIDENT

• ALL THIS YIELDS TO THE POSSIBLE USEOF A TWO TONE SIGNAL FOR THE TEST LOOKING

FOR THD AND IMD COMPONENTS BY FFT ANALYSIS


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• UNFORTUNATELY THE CHOICE OFTHE FREQUENCY OF THE ADDED TONE IS NOT EASYTO BE DETERMINED ASIDE FROM EMPIRICAL BASIS

• WHILE THESE KIND OF TESTS ARE VERY FASTAND CAN THEREFORE BE REPEATED

FOR MANY SECOND TONE FREQUENCIES, THEY DO NOT ALLOW SIMULTANEOUS

FREQUENCY RESPONSE CHECK


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• THE BASIC IDEA FOR THIS EXAMPLEIS TO PERFORM A SWEEP TEST WITH FREQUENCY

RESPONSE AND HARMONIC CHECKAND OBTAINING DISPLACEMENT AT ANY FREQUENCY

DURING THE SWEEP BY ADDING A DC COMPONENTTO THE TEST SIGNAL.

•THE TEST SHOULD BE REPEATED TWICE,TO ADDRESS BOTH DISPLACEMENT DIRECTIONS


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CRITICAL CHOICES FOR THE TEST ARE:

•AC SIGNAL LEVEL•FREQUENCY RANGE

•DC SIGNAL LEVEL


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•TEST LEVELS, AC AND DC, AFFECT BOTHVOICE COIL DISSIPATION AND DISPLACEMENT

•FOCUS SHOULD BE ON DISPLACEMENTAS THIS IS THE LIMITING FACTOR;

DISSIPATION LIMIT IS NOT AN ISSUE HERE

•THE FREQUENCY RANGE SHOULD BE EXTENDED AROUND 1 OCTAVE BELOW RESONANCE


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0.1 1 10

0.5

1

1.5

2

• NORMALIZED EXCURSION VS. FREQUENCY FOR Q FROM 0.2 TO 1 IN 0.2 STEP FOR CONSTANT VOLTAGE DRIVE

0.2

0.40.6

0.8

1.0



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•THIS GRAPH, DERIVED FROM SMALL’S EQUATIONS [1],HOLDS FOR SMALL SIGNALS

•IT SHOWS THAT FOR VOLTAGE DRIVE, AS WELL ASFOR Q<0.7, WHICH IS THE CASE HERE AND IN MOST

REAL LIFE APPLICATION, MAXIMUM DISPLACEMENT IS REACHED WELL BELOW

RESONANCE AND APPROACHES THE DC ONE

•THIS STATEMENT SHOULD BE TAKEN WITH CARE


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•KLIPPEL [2], IN “ASSESSING VOICE COIL PEAK DISPLACEMENT”,

SHOWED INCREASED DISPLACEMENTAT RESONANCE DUE TO BL NON LINEARITYBEING DOMINANT OVER SUSPENSION LIMIT

•GANDER [3] GIVES THE FOLLOWING RELATIONSHIPFOR FORCE VS. VOLTAGE DRIVE AT RESONANCE

Bl

ZEF mt


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•THE BL IN THE DENOMINATOR SHOWS THATBL REDUCTION, DUE TO HIGH DISPLACEMENT, INCREASES FORCE AND GIVES REASON FOR

THE EXPANSION EFFECT AT RESONANCE

•HOWEVER DURING QC TESTS GREAT OVERLOAD,I.E. GOING IN STRONG NON LINEAR AREA,

SHOULD BE CONSIDERED WITH CAREFOR THE FOLLOWING REASONS:


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•QC HAPPENS, MOST OFTEN, JUST AT THE END OF THE ASSEMBLY PROCESS BEFORE PACKAGING;

IN THIS PHASE, GLUES AND CONE TREATMENTS SETTING MIGHT NOT BE COMPLETE

•QC DEFINES ITS PASS/FAIL LIMITS UPON A REFERENCEOR AN AVERAGE OF PRODUCTION.STRONG NON LINEARITY MAKES

LIMITS AMBIGUOUS AND THEREFORE DIFFICULT TO HANDLE

•NOISE POLLUTION IN GENERAL IS AN ISSUE BOTHFOR OPERATORS HEALTH AND INTERFERENCE BETWEEN LINES


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•THE FOUNDATION OF THIS APPROACHIS TO ACHIEVE COMPLETE KNOWLEDGE

OF CONE’S DISPLACEMENT

•WE CHOSE AN AC LEVELFOR A MAXIMUM DISPLACEMENT CORRESPONDING

TO 3÷5% OF 3RD HARMONIC DISTORTION [3]WHICH HAPPENS AT THE LOWEST SWEEP

FREQUENCY (45Hz)


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FROM THE FOLLOWING RELATIONSHIP [3],[4]

radiuspistona

C20atm

kg1.21air,ofdensityρ

2bypressurenearfieldp

where

afπ4ρ

px

0

peakN

220

peakNpeak

WE OBTAINED Xpeak=1.35mm=Xmax


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• WE GAVE A QUICK METHOD TO DETERMINE MAXIMUM EXCURSION;

SHOULD YOU KNOW IT AS A TECHNICAL SPECIFICATION YOU MAY EASILY REVERT THE FORMULA

TO OBTAIN THE DESIRED SPL

• IF THE TECHNICAL SPECIFICATIONCOMES FROM A COMMERCIAL BROCHURE,

WE WARMLY SUGGEST THE FIRST APPROACH!


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• WE CHOSE A DC VALUE CORRESPONDING TO 1/3*Xmax=0.45mm

BlC

R

3

xV

ms

emaxDC

• TO MAINTAIN THE MAXIMUM DISPLACEMENT WITH DC, ON THE DC SIDE, AROUND THE SAME,

THE AC LEVEL HAS BEEN REDUCED BY 2dB

• WHILE THE RELATIONSHIP BETWEEN DC CURRENT AND DISPLACEMENTIS FAR FROM BEING LINEAR [5],

WE CONSIDER THE ERROR ACCEPTABLEFOR OUR PURPOSES


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GRAPHICAL REPRESENTATION OF LEVELS AT VERY LOW FREQUENCIES

AC+VDC

AC ONLY

AC-VDC


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CONSIDERATIONS ON THE RESULTS:

• THE LEFT SIDE GRAPHS SHOW VERY LITTLE DIFFERENCE BETWEEN BAD AND GOOD UNITS IN THE LOWEST HARMONICS,

NOT USABLE TO ESTABLISH RELIABLE QC LIMITS

• HIGHER HARMONICS, BELOW RESONANCE,SHOW AN APPRECIABLE BUT STILL SMALL DIFFERENCE,

WHICH DOESN’T CORRELATE WITH THE HEARD PHENOMENON

• THE RIGHT SIDE GRAPHS SHOW A MUCH INCREASEDDIFFERENCE BETWEEN BAD AND GOOD,

EVEN IN LOW HARMONICS,FOR A WELL EXTENDED FREQUENCY RANGE


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2ND HARMONIC - NO DC 2ND HARMONIC - WITH DC


3RD HARMONIC - NO DC 3RD HARMONIC - WITH DC

FUNDAMENTAL

GOOD UNIT

BAD UNIT

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4TH HARMONIC - NO DC 4TH HARMONIC - WITH DC


5TH HARMONIC - NO DC 5TH HARMONIC - WITH DC

FUNDAMENTAL

GOOD UNIT

BAD UNIT

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• 8” P.A. WOOFER

EXAMPLE “B”EXAMPLE “B”

• THE SAME MODEL OF THE PREVIOUS EXAMPLE WITH A BAD SAMPLE SHOWING MORE EVIDENT

RUB&BUZZ DEPENDENT ON DISPLACEMENT ONLY

Fs 80 Hz Re 5.2 OhmBl 12.44 TmCms 0.15 mm/N

Qes 0.45

Qms 3.51


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• AS CLEARLY POINTED OUT BY TEMME [6],THD IS NOT ABLE TO RELIABLY DETECT RUB&BUZZ

• THIS MAINLY HAS TWO REASONS;RUB&BUZZ CREATES

IMPULSIVE LOW ENERGY DISTORTIONWHILE LOUDSPEAKERS SHOW RELATIVELY

HIGH ENERGY LOW ORDER HARMONICS

• NEVERTHELESS CLASSICAL THD MEASUREMENTIS VERY ATTRACTIVE DUE TO ITS ONE VALUE

RESULT AND, WHEN PERFORMED WITHACTUAL DSP SYSTEMS,

TAKES A FRACTION OF A SECOND TO EXECUTE


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• OUR APPROACH HERE IS TO REDUCE SUBSTANTIALLY THE AC TEST LEVEL

DEALING THEREFORE WITHA MUCH MORE LINEAR DEVICE

• AND, MOST IMPORTANT FOR THE QC ENGINEER, A SINGLE VALUE RESULT MEANS A SINGLE VALUE

LIMIT MASK

• THE AC LEVEL WAS INTENTIONALLY“OVER REDUCED”, IN THE DC CASE, BY 15dB

• THE DC LEVEL WAS SET TO OBTAIN HALF XMAX AS DETERMINED IN PREVIOUS EXAMPLE


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• THE RESULTS ARE SPECTACULAR IN TERMSOF BAD TO GOOD DETECTION IN THE DC CASE

DUE TO THE LOW AC LEVEL

• THE TEST FREQUENCY WAS SETAT THE DEVICE RESONANCE

• THE SPEED OF THE TEST PERMITSITS REPETITION AT DIFFERENT DC LEVELS

TO COVER FULL EXCURSION

• THE AC REDUCTION LEVEL TO THE NUMBEROF DC STEPS REQUIRED TO COVER

THE ENTIRE EXCURSION IS LEFT TO QC ENGINEER’S NEED


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GOOD UNIT - NO DC


BAD UNIT - NO DC

THD 5.13%

THD 3.65%

GOOD UNIT - WITH DC BAD UNIT - WITH DC

THD 23.7%

THD 0.63%

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CONCLUSIONSCONCLUSIONS


THE SIMPLE TECHNIQUE PROPOSEDHAS PROVEN TO BE EFFECTIVE

IN AUGMENTING GOOD-TO-BAD SPREADIN QUALITY CONTROL TESTS

IT CAN BE EASILY ADDED TO ANY MEASUREMENT SYSTEM AS AN EXTERNAL ADD-ON, TYPICALLY

INCLUDED IN THE POWER AMPLIFIER.

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• [3] M.R.GANDER “MOVING-COIL LOUDSPEAKER TOPOLOGY AS AN INDICATOR OF LINEAR EXCURSION CAPABILITY” - JAES V29, No1/2, JAN/FEB 1981

BIBLIOGRAPHYBIBLIOGRAPHY

• [4] D.B.KEELE, JR. “LOW FREQUENCY MEASUREMENT OF LOUDSPEAKER BY THE NEAR-FIELD SOUND PRESSURE TECHNIQUE” - PREPRINT No909, 45TH AES CONVENTION, 1973

• [6] S.F.TEMME “ARE YOU SHIPPING DEFECTIVE LOUDSPEAKERS TO YOUR CUSTOMERS?” - LISTEN, INC.

• [2] W.KLIPPEL “ASSESSING VOICE COIL PEAK DISPLACEMENT” - 112TH AES CONVENTION, 2002

• [5] J.SCOTT, J.KELLY, G.LEEMBRUGGEN “NEW METHOD OF CHARACTERIZING DRIVER LINEARITY ” - JAES V44, No4, APR 1996

• [1] R.H.SMALL “CLOSED BOX LOUDSPEAKERS SYSTEMS PART 1: ANALYSIS ” - AES LOUDSPEAKERS ANTHOLOGY V1-V25 (1953-1977)


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