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© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Same Speaker, Different Data?
Loudspeaker design and analysis tools
Presented at ALMA Asia Symposium 2010
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Agenda
• Introduction
• Typical Measurement Setup
• Discussion: Sources of Inconsistency
• Recap
• Conclusion
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Part 1
• Introduction• Typical Measurement Setup
• Discussion: Sources of Inconsistency
• Recap
• Conclusion
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Introduction: Measurement Inconsistency
• THE PROBLEM - Same Loudspeaker:
• Different data!
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• We ship samples & employees across continents to investigate
Introduction: Measurement Inconsistency
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
+• We ship samples & employees across continents to investigate
Introduction: Measurement Inconsistency
$
• Measurement inconsistency costs time & money!
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Part 2
• Introduction
• Typical Measurement Setup• Discussion: Sources of Inconsistency
• Recap
• Conclusion
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Typical Measurement Setup
• 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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Typical frequency response measurement setup:
Typical Measurement Setup
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
• Typical frequency response measurement setup:
1
Typical Measurement Setup
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
2) Amplifier
• Typical frequency response measurement setup:
1 2
Typical Measurement Setup
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
2) Amplifier
3) EUT
• Typical frequency response measurement setup:
1 2
3
Typical Measurement Setup
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
2) Amplifier
3) EUT
4) Baffle
• Typical frequency response measurement setup:
1 2
3
4
Typical Measurement Setup
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
2) Amplifier
3) EUT
4) Baffle
5) Microphone
• Typical frequency response measurement setup:
1 2
3
45
Typical Measurement Setup
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
2) Amplifier
3) EUT
4) Baffle
5) Microphone
6) Room / Chamber
• Typical frequency response measurement setup:
1 2
3
45
6
Typical Measurement Setup
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Typical frequency response measurement setup:
1) Test System
2) Amplifier
3) EUT
4) Baffle
5) Microphone
6) Room / Chamber
POTENTIAL SOURCESOF MEASUREMENT
INCONSISTENCY
1 2
3
45
6
Typical Measurement Setup
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Part 3
• Introduction
• Typical Measurement Setup
• Discussion: Sources of Inconsistency
• Recap
• Conclusion
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• 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)
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Test and Measurement SystemFor Today’s Examples
dScope Series III HardwaredScope Software
USB
Cyrus III
G.R.A.S. 46AE / 26CA
IEC Baffle
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Possible stimuli for frequency response measurement:
Stepped sine (sweep)
Broad band noise
Maximum length sequence (MLS)
Continuous sweep (chirp)
Multi-tone
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Impulse response (using broad band stimulus) is widely used:
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Impulse response (using broad band stimulus) is widely used:
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Impulse response (using broad band stimulus) is widely used:
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Impulse response methods require a broad band stimulus
• Stepped sine sweep cannot remove room reflections
Stepped sine sweep
Log swept sine +
Impulse response
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Window Function:
Prism 7
Prism 5
Gaussian
Blackman-Harris
Hamming
Hann
Blackman
Triangular
Rectangular
Sources of Inconsistency:Test and Measurement System
• 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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Choice of window function affects frequency response:
Window Function:
Prism 7
Prism 5
Gaussian
Blackman-Harris
Hamming
Hann
Blackman
Triangular
Rectangular
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Length of window function affects LF cut-off:
Sources of Inconsistency:Test and Measurement System
t
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• 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
Sources of Inconsistency:Test and Measurement System
Shorter window
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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US: +1 973 983 9588
www.prismsound.com
• Low frequency cut-off is given by:
f = 1 / t
f = lowest valid frequency in measurement (Hz)
t = length of time domain window (seconds)
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Sources of Inconsistency:Test and Measurement System
• 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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
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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
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Start position of window can affect low frequency response:
Correct Window Position
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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US: +1 973 983 9588
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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
Sources of Inconsistency:Test and Measurement System
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
2) Amplifier
3) EUT
4) Baffle
5) Microphone
6) Room / Chamber
1 2
3
45
6
Sources of Inconsistency:Power Amplifier
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Output / cable impedance:
• Output impedance and cable impedance should be minimised, and ideally calibrated in the test and measurement system
Vload
Zout Zcable
Zload
Sources of Inconsistency:Power Amplifier
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• 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
Sources of Inconsistency:Power Amplifier
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
2) Amplifier
3) EUT
4) Baffle
5) Microphone
6) Room / Chamber
1 2
3
45
6
Sources of Inconsistency:Baffle
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
+ +
- -
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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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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
Sources of Inconsistency:Baffle
Low Frequency
High Frequency
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• 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
Sources of Inconsistency:Baffle
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Baffle should be large compared to wavelengths of interest
• If baffle is too small, we see low SPL at low frequencies:
Sources of Inconsistency:Baffle
Baffle Size:
1650 x 1450mm (IEC)
650 x 550mm
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Ensure a good air seal between transducer and baffle
• If air seal is poor, we see errors at low / mid frequencies:
Sources of Inconsistency:Baffle
Air Seal:
Airtight
Poor Seal
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
1) Test System
2) Amplifier
3) EUT
4) Baffle
5) Microphone
6) Room / Chamber
1 2
3
45
6
Sources of Inconsistency:Microphone
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• 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
Sources of Inconsistency:Microphone
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• For highest precision, use a pistonphone or calibrator
G.R.A.S. pistonphone type 42AA
G.R.A.S. calibrator type 42AB
Sources of Inconsistency:Microphone
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
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www.prismsound.com
• 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)
Sources of Inconsistency:Microphone
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• 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
Sources of Inconsistency:Microphone
+
+
+
+
+
+
+
+
+
+
+
+
+
+
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
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• Free-field response of a ½” pressure mic capsule:
Image from Cirrus Research
Sources of Inconsistency:Microphone
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Using wrong capsule type gives error at high frequency:
Sources of Inconsistency:Microphone
Free-field measurement:
Free-field mic capsule
Pressure mic capsule
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Use of non-instrument grade mic, and / or generic calibration data will be less accurate:
Sources of Inconsistency:Microphone
Mic:
G.R.A.S. Type 46CA (calibrated)
‘Brand x’ mic, no calibration data
supplied
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
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www.prismsound.com
• Be careful if you are not measuring at a distance of 1m
• Response is affected at close mic distances:
Sources of Inconsistency:Microphone
We can measure at any distance and normalise to the equivalent
1m sensitivity…
x
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Be careful if you are not measuring at a distance of 1m
• Response is affected at close mic distances:
Sources of Inconsistency:Microphone
Microphone Distance:
x = 0mm
x = 50mm
x = 100mm
x = 200mm
x = 400mm
x = 800mm
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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www.prismsound.com
• ‘Comb filtering’ caused by path differences from cone to speaker:
Sources of Inconsistency:Microphone
x
y
• If y – x = n*wavelength:
CONSTRUCTIVE interference
• If y – x = (n - 0.5)*wavelength:
DESTRUCTIVE interference
(where n is integer)
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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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
Sources of Inconsistency:Microphone
x
y
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
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Sources of Inconsistency:Room / Chamber
• Reflections from room boundaries cause ‘comb filtering’
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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Sources of Inconsistency:Room / Chamber
• Reflections from room boundaries cause ‘comb filtering’
x
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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Sources of Inconsistency:Room / Chamber
• Reflections from room boundaries cause ‘comb filtering’
y y
x
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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Sources of Inconsistency:Room / Chamber
• Reflections from room boundaries cause ‘comb filtering’
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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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Sources of Inconsistency:Room / Chamber
• Reflections from room boundaries cause ‘comb filtering’
Early Reflection
No Early Reflection
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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Sources of Inconsistency:Room / Chamber
Early Reflection
No Early Reflection
• Reflections from room boundaries cause ‘comb filtering’
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Reflections can be removed from an impulse response
• But we lose low frequency information from our measurement
Sources of Inconsistency:Room / Chamber
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• Reflections can be removed from an impulse response
• But we lose low frequency information from our measurement
Sources of Inconsistency:Room / Chamber
Window Length:
5ms
10ms
15ms
20ms
25ms
30ms
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Use a large room
Sources of Inconsistency:Room / Chamber
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Move the microphone & speaker away from room boundaries
Sources of Inconsistency:Room / Chamber
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Move the microphone & speaker away from room boundaries
Sources of Inconsistency:Room / Chamber
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Use absorption if possible (anechoic chamber)
Sources of Inconsistency:Room / Chamber
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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• Small chambers are very sensitive to mic position and reflections
Sources of Inconsistency:Room / Chamber
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
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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
Sources of Inconsistency:Room / Chamber
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Part 4
• Introduction
• Typical Measurement Setup
• Discussion: Sources of Inconsistency
• Recap• Conclusion
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
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• Earlier we saw 2 measurements of the same loudspeaker:
• We have now identified the major causes of the discrepancies
Recap
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• 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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
• 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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Part 5
• Introduction
• Typical Measurement Setup
• Discussion: Sources of Inconsistency
• Recap
• Conclusion
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
– 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
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
On-axis frequency response re 2.83V, 1m
Conclusion
• Suggested datasheet format:
Brand XYZ, model ADCD, S/N 0123456
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
Conclusion
• By minimising ERRORS and documenting VARIABLES, we can all save time and money!
© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009
UK: +44 (0)1353 648 888
US: +1 973 983 9588
www.prismsound.com
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…