© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009 UK: +44 (0)1353 648 888 US: +1 973 983 9588 Same

© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009

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Same Speaker, Different Data?

Loudspeaker design and analysis tools

Presented at ALMA Asia Symposium 2010


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Introductions

Simon WoollardPrism Sound

• Prism Sound:– Established 1987 (23 years)– Founders Ian Dennis and Graham Boswell designed

world’s first digital audio console at AMS Neve– Ian Dennis co-wrote AES-ABU digital interface standard– Professional audio logging / recording / editing solutions – Audio test and measurement equipment

• Simon Woollard:– BSc (Hons) Audio Technology, University of Salford, UK– Worked as loudspeaker design engineer (NXT, Mission)– Now Applications Engineer for Prism Sound’s Test and

Measurement Division


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Agenda

• Introduction

• Typical Measurement Setup

• Discussion: Sources of Inconsistency

• Recap

• Conclusion


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Part 1

• Introduction• Typical Measurement Setup


• Recap

• Conclusion


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Introduction: Measurement Inconsistency

• THE PROBLEM - Same Loudspeaker:

• Different data!


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• We ship samples & employees across continents to investigate



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+• We ship samples & employees across continents to investigate


$

• Measurement inconsistency costs time & money!


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Part 2

• Introduction

• Typical Measurement Setup• Discussion: Sources of Inconsistency

• Recap

• Conclusion


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Typical Measurement Setup

• Typical Loudspeaker Measurements:

– On-axis frequency response– Off-axis frequency response (polar plot)– Power response– Distortion / non-linearities – Impedance– Thiele-Small parameters

• Today we will discuss on-axis frequency response;

Perhaps the most common loudspeaker measurement


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• Typical Loudspeaker Measurements:

– On-axis frequency response– Off-axis frequency response (polar plot)– Power response– Distortion / non-linearities – Impedance response– Thiele-Small parameters

• Today we will discuss on-axis frequency response;

Perhaps the most common loudspeaker measurement


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• Typical frequency response measurement setup:



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1) Test System


1



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1) Test System

2) Amplifier


1 2



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1) Test System

2) Amplifier

3) EUT


1 2

3



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1) Test System

2) Amplifier

3) EUT

4) Baffle


1 2

3

4



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1) Test System

2) Amplifier

3) EUT

4) Baffle

5) Microphone


1 2

3

45



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1) Test System

2) Amplifier

3) EUT

4) Baffle

5) Microphone

6) Room / Chamber


1 2

3

45

6



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1) Test System

2) Amplifier

3) EUT

4) Baffle

5) Microphone

6) Room / Chamber

POTENTIAL SOURCESOF MEASUREMENT

INCONSISTENCY

1 2

3

45

6



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Part 3

• Introduction



• Recap

• Conclusion


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1) Test System

2) Amplifier

3) EUT

4) Baffle

5) Microphone

6) Room / Chamber

1 2

3

45

6

Sources of Inconsistency:Test and Measurement System


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• Calibration of test system (2 main types):

1) Dedicated Hardware:– Usually calibrated by manufacturer

– Typical accuracy ±0.05dB (absolute), ±0.05dB flatness

– Should include amplitude ranging to ensure maximum noise / distortion performance at low signal amplitude

2) Generic Soundcard:– Requires calibration by operator, or errors could be ~dBs!

– Noise and distortion of soundcard may affect accuracy at low amplitude (frequency response affected less)



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Test and Measurement SystemFor Today’s Examples

dScope Series III HardwaredScope Software

USB

Cyrus III

G.R.A.S. 46AE / 26CA

IEC Baffle


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• Possible stimuli for frequency response measurement:

Stepped sine (sweep)

Broad band noise

Maximum length sequence (MLS)

Continuous sweep (chirp)

Multi-tone



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• Impulse response (using broad band stimulus) is widely used:



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• Impulse response methods require a broad band stimulus

• Stepped sine sweep cannot remove room reflections

Stepped sine sweep

Log swept sine +

Impulse response



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Window Function:

Prism 7

Prism 5

Gaussian

Blackman-Harris

Hamming

Hann

Blackman

Triangular

Rectangular


• Choice of window function affects frequency response:

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

0 2 4 6 8 10 12ms

Gain


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• Choice of window function affects frequency response:

Window Function:

Prism 7

Prism 5

Gaussian

Blackman-Harris

Hamming

Hann

Blackman

Triangular

Rectangular



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• Length of window function affects LF cut-off:


t


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• Length of window function affects LF cut-off:

• HF also affected by reflections in non-anechoic measurements

Window Length:

t = 5ms

t = 10ms

t = 15ms

t = 20ms

t = 25ms

t = 30ms


Shorter window


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• Low frequency cut-off is given by:

f = 1 / t

f = lowest valid frequency in measurement (Hz)

t = length of time domain window (seconds)



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• Stimulus Amplitude:

• Voice coil driving force is non-linear WRT transducer excursion

• At high excursion, efficiency will be lower (e.g. low frequency)

6.0

4.5

1.5

0.0

3.0

Excursion @2V, mm


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• At high excursion, efficiency will be lower (e.g. low frequency):

Stimulus Amplitude:

15V

7.5V

3.75V

500mV

250mV

100mV

50mV



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• Start position of window can affect low frequency response:

Correct Window Position



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Correct Window Position

Window Starts too early

• Start position of window can affect low frequency response:

• LF attenuation due to early start of window function



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1) Test System

2) Amplifier

3) EUT

4) Baffle

5) Microphone

6) Room / Chamber

1 2

3

45

6

Sources of Inconsistency:Power Amplifier


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• Output / cable impedance:

• Output impedance and cable impedance should be minimised, and ideally calibrated in the test and measurement system

Vload

Zout Zcable

Zload



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• Assume a loudspeaker impedance as follows:

• Assume amplifier output impedance + cable impedance is 0.5Ω;

• We see error if we measure without calibration

-2

-1.5

-1

-0.5

0

10 100 1000 10000Hz

dBLoudspeaker impedance

Frequency Response Measurement Error



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1) Test System

2) Amplifier

3) EUT

4) Baffle

5) Microphone

6) Room / Chamber

1 2

3

45

6

Sources of Inconsistency:Baffle


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+ +

- -

Why do We need a Baffle?

Positive voltage > outward cone motionAir compresses in front of diaphragm (positive pressure)Air expands behind diaphragm (negative pressure)Acoustic ‘short circuit’ at low frequencyNo significant acoustic radiationUsing a baffle restores air pressure in front of diaphragm


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• If the wavelength is long compared to the baffle dimensions, radiation is omnidirectional (4π)

• If the wavelength is short compared to the baffle dimensions, radiation is in the forward direction only (2π) : SPL is 6dB greater


Low Frequency

High Frequency


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• IEC / AES standards for baffle size; varies with transducer size

1350mm

1650mm

150mm

225mm

IEC / AES standard baffle dimensions for transducers up to 8” diameter



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• Baffle should be large compared to wavelengths of interest

• If baffle is too small, we see low SPL at low frequencies:


Baffle Size:

1650 x 1450mm (IEC)

650 x 550mm


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• Ensure a good air seal between transducer and baffle

• If air seal is poor, we see errors at low / mid frequencies:


Air Seal:

Airtight

Poor Seal


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1) Test System

2) Amplifier

3) EUT

4) Baffle

5) Microphone

6) Room / Chamber

1 2

3

45

6

Sources of Inconsistency:Microphone


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• Use a dedicated test and measurement microphone

• Calibrate the microphone using manufacturer’s data

Calibration data for G.R.A.S. 46AF mic S/N 57302

Title: Microphone, integrated preamplifierUTID: 769, nodeID: E00000019B430914Manufacturer: G.R.A.S. Sound & VibrationModel: 46AFSerial no.: 57302Calibration date: juli 15, 2005Sensitivity @ 250Hz: 49.225m V/PaPrepolarized (0/1): 0Meas. position ID: 0User data (ascii): -8

-7

-6

-5

-4

-3

-2

-1

0

1

2

10 100 1000 10000 100000Hz

dB



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• For highest precision, use a pistonphone or calibrator

G.R.A.S. pistonphone type 42AA

G.R.A.S. calibrator type 42AB



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• Correct for actual atmospheric pressure

Atmospheric Pressure Corrections for G.R.A.S. Pistonphone

Type 42AA

-5

-4

-3

-2

-1

0

1

600 700 800 900 1000 1100

Atmospeheric Pressure (hPa)

Co

rrec

tio

n F

acto

r (d

B)



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• Use the correct microphone capsule! Typically a choice of 2:

Pressure microphone:

For Measuring SPL at a boundary

Mic does not modify pressure field

Free-field microphone:

For measuring SPL in free space

Mic modifies pressure field, so is acoustically equalised to correct error


+

+

+

+

+

+

+

+

+

+

+

+

+

+


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• Free-field response of a ½” pressure mic capsule:

Image from Cirrus Research



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• Using wrong capsule type gives error at high frequency:


Free-field measurement:

Free-field mic capsule

Pressure mic capsule


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• Use of non-instrument grade mic, and / or generic calibration data will be less accurate:


Mic:

G.R.A.S. Type 46CA (calibrated)

‘Brand x’ mic, no calibration data

supplied


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• Be careful if you are not measuring at a distance of 1m

• Response is affected at close mic distances:


We can measure at any distance and normalise to the equivalent

1m sensitivity…

x


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• Be careful if you are not measuring at a distance of 1m

• Response is affected at close mic distances:


Microphone Distance:

x = 0mm

x = 50mm

x = 100mm

x = 200mm

x = 400mm

x = 800mm


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• ‘Comb filtering’ caused by path differences from cone to speaker:


x

y

• If y – x = n*wavelength:

CONSTRUCTIVE interference

• If y – x = (n - 0.5)*wavelength:

DESTRUCTIVE interference

(where n is integer)


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• ‘Comb filtering’ caused by path differences from cone to speaker:

• x and y approximately equal in far field; comb filtering is minimised


x

y


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1) Test System

2) Amplifier

3) EUT

4) Baffle

5) Microphone

6) Room / Chamber

1 2

3

45

6

Sources of Inconsistency:Room / Chamber


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• Reflections from room boundaries cause ‘comb filtering’


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x


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y y

x


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x

• If 2y – x = n*wavelength:

CONSTRUCTIVE interference

• If 2y – x = (n – 0.5)*wavelength:

DESTRUCTIVE interference

(where n is integer)

Path difference = 2y - x

y y


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Early Reflection

No Early Reflection


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Early Reflection

No Early Reflection



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• Reflections can be removed from an impulse response

• But we lose low frequency information from our measurement



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• Reflections can be removed from an impulse response

• But we lose low frequency information from our measurement


Window Length:

5ms

10ms

15ms

20ms

25ms

30ms


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• Use a large room



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• Move the microphone & speaker away from room boundaries



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• Move the microphone & speaker away from room boundaries



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• Use absorption if possible (anechoic chamber)



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• Small chambers are very sensitive to mic position and reflections



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• Small chambers are very sensitive to mic position and reflections

– Make the microphone mounting secure and accurate

– Use non-simple ratios between chamber x, y, and z dimensions

– If possible, use non-parallel chamber walls

– Use high specification acoustic absorption

– Use high stiffness, high damping, high mass chamber walls to prevent panel resonances and noise transmission

– Ensure that multiple chambers are built to identical specifications



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Part 4

• Introduction



• Recap• Conclusion


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• Earlier we saw 2 measurements of the same loudspeaker:

• We have now identified the major causes of the discrepancies

Recap


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• Causes of frequency response measurement inconsistency:

– Test System:• Calibration• Stimulus type / measurement method• Stimulus amplitude• Window function (for impulse response)• Window length• Window start time

– Power amplifier:• Output impedance / cable impedance• Calibration for output impedance

– Baffle:• Baffle size - IEC / AES recommend standard baffle sizes• Air seal between transducer and baffle

Recap


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• Causes of frequency response measurement inconsistency:

– Microphone:• Use a dedicated T&M mic (omni-directional)• Capsule type: pressure or free-field• Calibration for sensitivity, frequency response and mic pre gain• Correction for atmospheric pressure• Correction for any gain in ear couplers• Distance from mic to loudspeaker

– Room / Chamber:• Nearby boundaries• Room dimensions• Windowing (if used)• Position of mic

Recap


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Part 5

• Introduction



• Recap

• Conclusion


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– Sources of ERRORS should be MINIMISED / calibrated, e.g.:• Cable and amplifier impedance • Microphone frequency response• Environmental noise

– Measurement VARIABLES should be STATED with measurement data, e.g.:• Stimulus type• Stimulus amplitude• Microphone distance• Type of window function• Baffle dimensions• Room dimensions

Conclusion


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On-axis frequency response re 2.83V, 1m

Conclusion

• Suggested datasheet format:

Brand XYZ, model ADCD, S/N 0123456


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Conclusion

• By minimising ERRORS and documenting VARIABLES, we can all save time and money!


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Further Considerations

• We could also talk about measurement inconsistencies in:

– EUT; atmospheric temperature, humidity, voice coil temperature due to test cycle duration etc

– Impedance, TSPs, distortion measurements etc– Electrical measurements – power amplifiers, crossovers etc– Headphones / ear couplers / HATSs

• But we are out of time! Perhaps a topic for ALMA 2011…

Documents

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