Microflown Technologies develops particle velocity sensors called Microflowns that directly measure particle velocity rather than estimating it from pressure measurements, allowing for more accurate sound intensity measurements, especially in near fields and reverberant environments. The company was founded in 1998 and has grown to 20 employees, focusing on applications in automotive, aerospace, manufacturing, and military markets. Microflown sensors offer advantages over traditional pressure-pressure probes for sound intensity measurements.

1. Microflown Technologies
The Netherlands
www.microflown.com
info@microflown.com
Microflown: a new category of sensors
1

2. Agenda
• Introduction to Microflown Technologies [3-29]
• Sound Intensity [30-69]
• Measurement techniques – traditional [70-88]
systems
• Advanced scanning techniques :
Scan&Paint [89-126]
2

3. Company introduction
3

4. Company Introduction
1994: Invention Microflown by Hans-Elias de Bree at University Twente
1997: Ph.D. Hans-Elias de Bree
1998: Founding Microflown Technologies B.V. (de Bree, Koers)
2001: Industrializing product
2003: Introduction broad banded sensor element
2004: First applications scientifically proven / first arrays sold
2005: Rapid growth in (automotive + aerospace) industry
2005: De Bree appointed Professor ‘Vehicle Acoustics’ at the HAN University,
Arnhem School of Automotive Engineering
2008: Strategic decision to explore the defense & security market
2010: 20 FTE company, 1,3 MEURO turnover
2011: Microflown AVISA
4

5. Working principle
Microflown SEM picture: two heated wires
5

6. Working principle
Microphone measures sound pressure (result)
Microflown measures Particle Velocity (cause)
Acoustical <-> electrical <-> energy
Sound pressure <-> voltage <-> potential
Particle velocity <-> amperes <-> kinetic
6

7. Working principle
PRESSURE WAVE
7

8. Working principle
PRESSURE WAVE ≠ PARTICLE VELOCITY
8

9. Working principle
T[k] velocity
output
0
upstream downstream distance
9

10. Working principle
u u
u u u
u p
p p p p p C
u
u
sum sum p
p
A B
D
10

11. Working principle
Surface velocity measurement:
• No background noise and reflection problems
Figure of eight Low surface velocity High surface velocity
and high surface and low surface
pressure pressure
11

12. Working principle
Mems based sensor
Clean room technology is
used to create the small
elements on a waver
University of Twente
12

13. Working principle
Wirebonded elements
13

14. Microflown probes
14

15. Standard probes
Scanning Probes
• 1D Velocity
• For small object
• High temperatures
• Non contact vibration
15

16. Standard probes
PU probes
• Particle Velocity
• Sound Pressure
• 1D Sound Intensity
• 1D Sound Energy
• Impedance
16

17. Standard probes
PU Probes: Placement of p and u
17

18. Standard probes
Metal Mesh
• Wind shield, DC flow
up to 2 m/s
• Protecion of the
wires
• Calibration including
mesh
18

19. Standard probes
USP probes
• 3D Particle Velocity
• Sound Pressure
• 3D Sound Intensity
• 3D Sound Energy
• Impedance
• Acoustic Vector Sensor
19

20. Standard probes
High dB Scanning Probe
• Above 135dB acoustics becomes non linear
• Standard sensor overloads at 130dB
• Measurement at 170dB is possible with packaged sensor
20

21. Microflown applications
21

22. Standard probes
From product development till end of line control
22

23. Automotive
Scan & Paint Scan&Listen
Acoustic camera PNCAR
Insitu abosorption
23

24. Space and Aerospace
3D intensity stream lines
In-situ impedance
Reverberant room
characterization PNCAR
24

25. Environmental noise
3D sound source
location
Virtual Arrays
25

26. Manufacturing industries
Scan & Paint In situ impedance
Acoustic Camera Point by point
intensity
26

27. End of Line Control
Leak testing
Acoustic EOL
27

28. Room acoustics
Sound diffusion Impedance
measurement
3D intensity
visualization
28

29. Military applications
Air to ground
applications
Aircrafts location
Hostile fire location
Surveillance
29

30. What is intensity?

31. Theoretical approach

32. Intensity
Sound intensity is useful for measurement of sound power, identification and ranking of
sources, visualization of sound fields, measurement of transmission loss, identification of
transmission paths
Sound intensity is defined as the sound power per unit area
Intensity: Time averaged rate per unit area at which work is done by one element of fluid
on an adjacent element
Intensity and Particle velocity are vectors, therefor they have a direction related to their
magnitude.
Sound intensity units are W/m2
32

33. Intensity
In far field pressure and velocity have equal phase so I is a real quantity.
However, in the near field pressure and velocity are out of phase,
leading to have an active and reactive part of the intensity
Active intensity [ I ]
Active intensity is the real part of the time
Imaginary
averaged product between pressure and
velocity. This term is commonly called
‘acoustic intensity’ because is associated to
the acoustic energy that propagates away
J from the source
Reactive intensity [ J ]
I Real Active intensity is the imaginary part of the
time averaged product between pressure and
velocity. This term is associated with the
evanescent energy carried by the particle
velocity
33

34. Reactivity index
The reactivity index is the ratio between
reactive (J) and active intensity (I)
When reactivity takes high values lead to
low active intensity. This can be seem as
lack of radiation efficiency, i.e. there is a
vibrating surface which moves the air but
is not able to compress it.
The size of the near field is related to the
wavelength assessed, therefore the
reactivity index also depends on
frequency.
34

35. Pressure-Intensity index
35

36. Sound Intensity-PP probes

37. PP intensity
Traditionally the measurement of sound intensity is performed by P-P probes.
The measurement procedure makes use of two microphones. The sound pressure
is the average of the two corresponding pressure signals. The intensity is calculated
at the center of the space separating the two microphones.
The P-P intensity is then obtained by the following relation:
ˆ p1 t p2 t t p1 p2
I pp ˆˆ
pu t
d
2 r t
Where the first term is the promediated pressure value and the second term is the
estimated particle velocity.
37

38. PP intensity
Average pressure
between the two closely
spaced microphones
Estimation of the particle
velocity from the pressure
gradient valid for free field
plane waves
38

39. PP intensity - ERRORS
Phase mistmatch error
between pressure
microphones
Pressure-intensity index is directly
related with the measurement error
39

40. PP intensity - ERRORS
Phase mismatch error :
2 2
ˆ peprms prms / c
pe
I pp I pp I 1
k r c k r I
A small error in the microphones phase matching can lead to an uncorrect intensity
estimation.
This is the reason why the manufacturers need to pair the microphones, to try to
find in the production, the more similar sensors to form the probe.
40

41. PP intensity - ERRORS
Finite difference error (depends on the microphone separation):
ˆ sin k r
I pp / I
k r
The estimation of the velocity term is the pressure gradient between the two
pressure signals, this can lead to the following cases
Too low frequency: the pressure gradient is
too small to determine the velocity
Too high frequency: the wavelength is
too small compared to the microphone
spacing
41

42. PP intensity - Limitations
Reverberant sound fields:
The usable frequency region of these
sensors is drastically reduced when the
pressure-intensity index is HIGH,
because of the small ratio between
phase measurements at microphone
positions. This effect appears in
reverberant conditions where:
– high pressure level
– Intensity level tending to 0
Free field conditions:
The spacer needs to be changed for
each frequency range, in order to
adapt it to the interest wave legth.
42

43. PP intensity - Limitations
Near field measurements:
The probe can be used but the usable
frequency range is reduced drastically
because of the appearance of
evanescent waves.
The gradient of pressure to estimate
the particle velocity is on longer usable
NOTE: Evanescent waves: An
evancescent wave is a near field
standing wave with an exponential
amplitude decay from the boundary at
which the wave was formed
43

44. Properties of PP probes
Advantages
• Not sensitive to DC flow
• Flat frequency response
Disadvantages
• Only for plane waves
• Distributed sensor
• Exact microphone pairing needed
• Microphone spacing depends on frequency
• Accuracy is strongly dependent into the
pressure-intensity index
44

45. Sound intensity P-U probes

46. PU intensity
The working principle is based upon measuring the temperature
difference in the cross sections of two extremely sensitive heated
platinum wires that are placed in parallel. The incident sound flown
produces a difference in temperature, leading into a voltage
difference proportional to the flow.
P-U intensity :
Pressure and particle velocity are directly measured so no assumptions
about the sound field are required
Intensity is then described by the real part of the product of pressure and
particle velocity, both measured quantities.
46

47. PU probes - ERRORS
Reactivity index:
The reactivity is the ratio between active (Re) and
reactive ( Im) intensity of the sound field.
Reactive intensity : J pu 1 / 2 Im pu
If the reactivity takes a HIGH value there is not
intensity produced, the sound source is only pushing
air back and forward. In this case a small phase
mismatch between P and U sensor can produce an
error:
ˆ J
I pu I 1 e I1 e tan field
I
This is due to happen fat the vicinity of the sound
source at low frequencies.
This effect can be solved by the usage of the particle
velocity itself for sound localization purposes.
47

48. Calibration errors of P-U probes
Measurements show that a phase
matching of 1 degree is possible with
a calibration based on a short
standing wave tube method or the
piston on a sphere method. The
enhanced calibration based on the
sound power ratio technique a phase
matching error of 0.15 degrees can
be obtained
One can state that if the measured phase of the sound field is less than 80 degrees
(less than 7dB of reactivity index), a calibrated P-U probe has a measurement error
less than 0.5dB
48

49. Properties of P-U probes
Advantages
• Small size. Point measurement
• Usable for near field measurement
• Broadband solution
• Usable in reverberant conditions
• Pressure and Velocity measured
almost in same point ( non
distributed sensor).
Disadvantages
• Response decreases with frequency
• Sensitive to DC flow
• Accuracy is dependent into the
reactivity index
49

50. PU and PP performance comparisson

51. Experimental results
Sound intensity measurements of a broadband noise source using a P-U
probe (red) and a P-P probe (blue)
51

52. PP and PU intensity measurements
52

53. PP and PU intensity measurements
53

54. PP and PU intensity measurements
Difference
because of
area
assigned in
each method
54

55. Sound power measured at two surfaces
Expected 10,6 dB difference because of Dipole sound source
dimensions
1 dB deviation because of bad
location of PP probe while
measuring
55

56. Sound power measured at two surfaces
Very reactive
worst scenario for
PU probe
After correction of
phase mismatch of
PU, intensity
graphs coincide
56

57. PU probes calibration method

58. Piston on a sphere
As there is not a reference particle velocity sensor the principle is to insert the
pressure and velocity sensors into a known sound field in which P and U are
related by the known acoustic impedance ( Z).
Problem: this is not possible for
all frequencies, low frequencies:
• Lower loudspeaker radiation
• Spherical waves
Solution: 3 step method
• Step 1: High frequencies
• Step 2: Low frequencies
Step 3: Combination
Applying the 3 steps the calibration is usable for 20-20KHz
58

59. Step 1: High frequency calibration
• A known sound field is generated
• Known relation between U-P via Z ( Z= P/U)
• Usable for 100-20.000 Hz.
59

60. Step 2: Low frequencies
• Loudspeaker cannot radiate as much energy as in high frequencies
 Backgound noise too much influence
• Different method:
• Pref is inserted IN the sphere
• U is located next to the membrane
• Known noise field generated
• From the relation of the difference in pressure inside the sphee and
the movement of the membrane, is obtained the response.
• Usable until first mode of sphere
• The phase is obtaine dbut the results magnitud is not
determined. Need of Step 3
60

61. Step 3: Combination 1 and 2
• Not known magnitude of calibration at low frequencies because of lack
of:
• Vo: exact sphere volume
• Ao: piston area
• R: exact distance to membrane
Step 1 and 2 overlay
61

62. Result
Result: non flat response of the sensor. Needs to be equalized via Signal
conditioner
+
62

63. 3D intensity

64. 3D sound intensity probes
64

65. 3D sound intensity probes
100Hz
Sound intensity streamlines of loudspeakers vibrating in phase
(left) and vibrating in anti-phase (right).
65

66. 3D sound intensity probes
500Hz
Sound intensity streamlines of loudspeakers vibrating in phase
(left) and vibrating in anti-phase (right).
66

67. 3D sound intensity probes
Sound intensity streamlines of
a loudspeaker driven close to
a metal plate.
67

68. 3D sound intensity probes
Noise mapping
68

69. 3D sound intensity probes
Energy characterization and difussion
69

70. Measurement Techniques

71. Theoretical approach

72. Type of noises
Noise
Deterministic Non deterministic
Periodic Non periodic Random Transient
Complex Non-
Sinusoidal Stationary
periodic Stationary
Ergodic Non-Ergodic
72

73. Deterministic noise
• Deterministic: a signal whos values can be predicted from current or past
information
– Numerical: denoted by a number or colletion of numbers
– Analytic: denoted by an equation which defines the process.
73

74. Non deterministic noise
• Random / Stochastic process: a function usually of time, that takes on a definite
wave form each time a chance experiment is performed that cannot be
predicted in advance.
• DEF 2: a family of time dependent signals for which the value at a specific time
may be regarded as a random variable.
– Stationarity: invariance of stadistical properties with respect to the time origin.
• Narrow band process: stationary process in which significant samples
are limited to a slim band of frequencies in relation with a central
frequency of the band.
– Color noises: narrow band processes which energetic content and
statistical properties are distributed in a certain manner
• Wide band process: stationary process which significant values appear
in a range proportional of the magnitud of the central frequency of the
band.
74

75. Color noise
White noise is a signal/process with a flat spectrum. The
signal has equal power in any band of a given
bandwith.
Grey noise: is random white noise subjected to a
psychoacoustic equal loudness curve over a given
range of frequencies, giving the listener the
perception that it is equally loud at all frequencies
Pink noise: the frequency spectrum is linear in logarithmic
space, it has equal power in bands that are
proportionally wide.
Brown noise: stationary random signal who's power
spectrum falls of at a constant rate of 6 dB per octave
Violet noise: Violet noise's power density increases 6 dB
per octave with increasing frequency(density
proportional to f 2) over a finite frequency range
Blue noise: Blue noise's power density increases 3 dB per
octave with increasing frequency (density proportional
to f ) over a finite frequency range
75

76. Transient noise
Impulse: unwanted, almost instantaneous (thus impulse-like) sharp sounds
Burst noise : sudden step-like transitions between two or more discrete levels
Sweep noise: a signal, commonly of constant amplitude, that locally resembles a
sine wave but whose instantaneous frequency changes with time
Chirp noise: rapid frequency sweep signal
76

77. Measurement techniques

78. Conventional measurement techniques

79. Point by point measurements
Suitable for:
– Stationary noise
Measurement process:
– Definition of an imaginary measurement
plane.
– Definition of a grid on the plane
– In every grid position perform a
measurement for every noise component
to be characterized
Result:
– Vector per grid point.
79

80. Traditional scanning technique
Suitable for:
– Stationary noise
Measurement process:
– Definition of an imaginary measurement plane.
– Scanning of the whole interest area
Result:
– Single intensity value per area promediated value
80

81. Simultaneous measurement
• Suitable for:
– Stationary noise
– Transient noise
• Measurement process:
– Allocation of sensors/ array deployment
– Audio capture of several channels
– Direct measurement, no signal
processing
• Result:
– Color maps of noise distribution in time
81

82. Advanced measurement methods

83. New scanning techniques: Scan&Paint
Suitable for:
– Stationary noise
Measurement process:
– Definition of an imaginary measurement
plane.
– Scanning of the whole interest area
– Automatic post process assigning location
of probe- audio measurement
Result:
– Color map of various indexes
– Spectrograms of every located
measurement point
– Global index to characterize an area
83

84. Intensity based sound source localization
Suitable for:
– Any noise
Measurement process:
– Sensors allocation
– Simple signal processing
Result:
– DOA: direction of arrival of noise
– Spectrograms of each 3D directions
– Global and narrow band levels
Limitations:
₋ Free field assumptions for simple
algorithm
₋ Increase number of sensors to detect
coherent noise sources
84

85. Conventional beamforming
Suitable for:
– Stationary noise
– Transient noise
Measurement process:
– Definition: Signal processing techniqued ised in arrays for directional signal
transmission . This directional information is obtained by combining elements in
the array
– Allocation of sensors/ array deployment
– Audio capture of several channels
– Beam forming signal processing
Result:
– Color maps of noise distribution
Limitations
― Frequency limitations by spacing and array size
― High cost
85

86. Holography
Suitable for:
– Stationary noise
Measurement process:
– Definition: Method to estimate the sound field near a source by measuring
acoustic parameters away from the source via an array of pressure and/or
particle velocity transducers.
– Processing after acquiring information from array
Result:
– Color map of the interest area
Limitations:
— Frequency limitations because of spacing and array dimension
— Assumes free field
— Regular grid
— Heavy calculations
— High cost
86

87. Airborne transfer path measurements
Suitable for:
– Stationary noise
Measurement process:
– Combination of the characterization of a noise source
with the propagation path to the listener in order to
𝑦
obtain information about the contribution of a specific
noise in the whole perceive sound pressure level S 𝑥
– Measurements divided in two steps: source and
transfer path characterization
Result:
– Noise source listener ranking
Limitations:
— High cost
— Typically measured in reverberant environments
— Surface noise source detected not structural problems
87

88. Virtual arrays beamforming
Suitable for:
– Stationary noise
Measurement process:
– Deffinition of an imaginary measurement plane.
– Scanning of the whole interest area
– Measurement of two reference positions
– Automatic post process assigning location of probe-
audio measurement
Result:
– Color map of various indexes
– Spectrograms of every located measurement point
– Global index to characterize an area and noise source
location
Limitations:
– Size and distance
– Heavy calculations
88

89. Advanced measurement techniques:
Scan & Paint

90. Theoretical approach

91. Scan&Paint principle
The PU probe is moved along
the virtual plane while the
movement is recorded by
the video camera.
The location of each measured
position is extracted from
the video and synchronized
with the 2 audio channels.
91

92. Scan&Paint principle
Pressure Velocity
92

93. Scan&Paint principle: post-processing
Two methods to cover the full
frequency range:
- Velocity method (for low
frequencies)
- Intensity method (for high
frequencies)
93

94. Low frequencies
In the near field of the surface the particle velocity is equal to the surface
velocity. The influence to background noise is low.
At higher frequencies the velocity method fails because:
• The area of consistent velocity is too small. There are many modes
in the material which require many measurement points
• The sensor is not in the near field any more
High frequencies
At high frequencies the sensor is not in the (very) near field any more and
the intensity is used. There are no P-I index problems like with P-P intensity
probes
At low frequencies the intensity method fails because the sound source is
too reactive
94

95. Measurement procedure

96. Measurement procedure
96

97. Measurement examples

98. Scan & Paint
Example 1: Large gas turbine enclosure
There are big stationary engines ( used for Heat & Power )
The goal was to measure the performance of the special designed enclosures.
Specially regarding acoustic leakages. With Scan & Paint we could perform the
measurement on a large surface in short time period in highly reverberant
conditions.
98

99. Scan & Paint
Example 1: Large gas turbine enclosure
Selection of measurement points on the backside of the housing
99

100. Scan & Paint
Example 1: Large gas turbine enclosure
Velocity map at 65Hz
100

101. Scan & Paint
Example 1: Large gas turbine enclosure
Low frequency High frequency
101

102. Scan & Paint
Example 2: Building acoustics
102

103. Scan & Paint
Example 3: Leak detection in buildings
Studying the spectra of
different areas allows to
produce narrow band maps
focused on detecting
weaknesses
This technique is suitable for
localizing acoustic leakage
with a very high spatial
resolution in a clear and easy
way
103

104. Scan & Paint
Example 4: Automotive | Comparison test of compo-
nents in a windtunnel
See the effect of the noise due to windflow
related to the interior noise when using
different type of components or make
adjustment to the components used on the
outside of a car like a mirror or window wiper.
The test are performed with the car in a
windtunnel when using Scan & Paint to map
the effect to the noise in the interior inside
the car.
104

105. Scan & Paint
Example 4: Automotive | Comparison test of compo-
nents in a windtunnel
Left the velocity map of the standard wiper and right the velocity map of
the wiper with adjustments made.
105

106. Scan & Paint
Example 4: Automotive | Comparison test of compo-
nents in a windtunnel
Left the velocity map of the car without rearview-window and right the
velocity map of the car with the rearview-window.
106

107. Scan & Paint
Example 5: Automotive | Optimization material package
To see where to place absorbing
materials effectively and measure the
effect after installing the materials. First
the door was measured without
materials and secondly with materials
placed based on the first measurement.
A sound source is positioned in the
interior and with pink noise as
excitation.
107

108. Scan & Paint
Example 5: Automotive | Optimization material package
Door | no damping Door | with damping
108

109. Scan & Paint
Example 6: Automotive NVH| Component optimization
The opening of the ventilation
system of the dashboard show
important acoustic leakages
The shell radiation of an intake system
is measured on the test bench exciting
the plastic filter with white noise from
a loudspeaker
109

110. Scan & Paint
Example 6: Automotive NVH| Component optimization
A volume super-charger show the crank
frequency emission from the aluminum
case.
The intake system radiation at lower
frequency in engine running condition
can be optimized
110

111. Scan & Paint
Example 7: Automotive NVH| Sound source localization
Exterior noise of a car. The colormap
show the engine radiation through the
weak areas.
The front part of the engine without
cover show high velocity emission.
111

112. Scan & Paint
Example 8: Electronic / white consumer goods
Optimize the noise performance of a washing
machine. Localize the hotspots suggest and
adapt changes and compare result.
Overall result of this case: 4dB lower Sound
Power ( measured by the standard sound
pressure method )
112

113. Scan & Paint
Example 8: Electronic / white consumer goods
Dominant source
200Hz
113

114. Scan & Paint
0.5Kg damping
72dB PVL 68dB PVL
114

115. Scan & Paint
Example 9: Electronic goods | Commercial printer
Optimize the noise performance of a printers
developed for offices. Localize the origin of the
noise problem.
A mode is created in the
backplate. This mode made the
printer very noisy but the origin
causing the mode was needed
to be localized.
115

116. Scan & Paint
Example 9: Electronic goods | Commercial printer
With Scan & Paint the gear
wheel that was causing
structure borne noise ( the
created mode in the
backplate).
The amount of teeth, the
material of the gear wheel
or the connection with the
backplate could be options
to reduce this structure
borne noise.
116

117. Scan & Paint
Example 9: Electronic goods | Commercial printer
117

118. Scan & Paint
Example 10: Electronic goods | Clima and Microwave
With Scan & Paint low frequency noise
from the cooling system (airflow)can be
separated by the noise coming from the
body frame of the clima.
The button panel on the right
side show higher sound emission
above 2000Hz.
118

119. Scan & Paint
Example 11: Ground Vehicles | High speed train | In situ
transparency measurements
In situ transparency measurements
using Scan & Paint were performed
as alternative to the traditional
transmission loss measurements.
Mainly to identify positions of
leakages.
119

120. Scan & Paint
Example 11: Ground Vehicles | High speed train | In situ
transparency measurements
The pressure distribution (and the velocity distribution) is
measured both out and inside the train, to correct the non-
uniformity of the sound field as the excitation pattern
(emitter side).
The average velocity over the surface is calculated for both
sides, and a simple formula is applied to estimate the
transmission loss from the velocity or so called the
transparency:
120

121. Scan & Paint
Example 11: Ground Vehicles | High speed train | In situ
transparency measurements
Outside TGV – velocity distribution Inside TGV – velocity distribution
121

122. Scan & Paint
Example 11: Ground Vehicles | High speed train | In situ
transparency measurements
122

123. Scan & Paint
Example 12: Industrial machinery | Sound source localization
Industrial machinery can be tested in
non-anechoic conditions.
123

124. Scan & Paint
Example 13: Airplane | Leakage detection
Acoustic leakage on a plane section. The
sound is generated out from the plane to
simulate the engine noise level.
124

125. Scan & Paint
Example 14: Airplane | In situ absorption
The Scan&Paint
absorption show
the effect of the
flame cover on a
plane seat.
The colormap of
absorption can
be calculated
measuring with
the impedance
gun.
125

126. Scan & Paint
Example 15: Absorption & Reflection coefficients
126