Aloft Stratospheric Testbed for Experimental Research on Infrasonic Activity
Courtney Ballard | Emily Daugherty | Connor Dullea | Kyle Garner | Martin Heaney | Ian Thom | Michael Von Hendy | Kerry Wahl | Emma Young
Topic
Presenter
Project Overview
•
Project Description
Kyle
•
Baseline Design
Kyle
Evidence of Baseline Feasibility
•
Key Project Elements
Kerry
•
Microphone Design and Data Collection
Kerry
•
Test Equipment and Infrasound Generation
Martin
•
Thermal System
Ian
•
Signal Detection In Relative Winds
Ian
Status Summary
Martin
Strategy for Conducting Remaining Studies
Martin
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
Develop a protoflight* high-altitude balloon payload
capable of measuring infrasonic events of frequencies
between 0.1 and 20 Hz with a minimum amplitude of
0.1 Pa.
Functional Requirements:
Requirement
Designation
Description
FR.1
ASTERIA shall measure and record simulated infrasonic
sources between 0.1 and 20 Hz, with a minimum wave
amplitude of 0.1 Pa.
FR.2
FR.3
ASTERIA shall be housed and operate on a balloon that
travels to 18 to 30.5 km.
ASTERIA shall operate autonomously for the duration of
the mission flight.
*Protoflight: Hardware that is designed to flight standards, but may not
Project
Baseline
Status
Remaining
incorporate
all the necessaryFeasibility
materials or testingSummary
required to be flight Studies
certified
Overview
3

Infrasound: Sound waves below
the threshold of human hearing
(0.1-20 Hz)

Generated by severe weather,
earthquakes, volcanoes, meteors
 Able to propagate for 1000’s of km

Current monitoring network
(CTBT-IMS*) is only capable of
detecting ~30% of 0.1-kiloton
events [Pinchon 2010]
Current CTBT-IMS Network
[Natural Resources Canada]
Microphone

Largest detection: Chelyabinsk meteor
 Issues with noise, mainly from wind

Desire to increase detection of
events

Important for design of re-entering
spacecraft, verification of atmospheric
models, testing CTBT-IMS network
Spatial Filter
CTBT-IMS station (UK) [Alden 2013]
Project
Baseline
Status
Remaining
*CTBT-IMS: Comprehensive
Nuclear
Test-Ban Treaty International
System
Overview
Feasibility
Summary Monitoring
Studies
4

>95% of bolides* produce a majority of their infrasound
at altitudes of 20-30km. [Pinchon, 2010]
 Infrasound propagates through stratospheric channels
 Waveguides efficiently propagate signals for 1,000’s of km
▪ Waveguides: thermal channels in the atmosphere

Only flight heritage: HASP 2014
STRATOSPHERE
PRESSURE WAVES (INFRASOUND)
ASTERIA
BOLIDE
GROUND STATIONS
(EXISTING)
*Bolides: Meteors that
explode in the Earth’s
Project
Baseline
Statusatmosphere Remaining
Overview
Feasibility
Summary
Studies
5
VOLTAGE
Current Project: Protoflight Ready Sensor & Support Systems
Bolides
ASTERIA
Patm , Tatm
TIME
Pressure
Data Out
P, T
Microphone
Acoustic
Filter
Infrasound
Source
Data Relay
Support Systems:
Power, Thermal,
Structural, Data
ASTERIA
Simulated Infrasound
Waves
Project
(0.1-20Hz)
Overview
Variable Simulated Flight Conditions
Baseline
Feasibility
Status
Summary
Remaining
Future
Application 6
Studies
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
7
0.5m
Microphones
(embedded in
spatial filter)
OVERVIEW
BALLOON
0.5m
GONDOLA
ASTERIA
ASTERIA
Spatial Filter
Microphone
Purpose:
1. Dictates the positioning of
microphones to achieve
omnidirectional coverage
2. House support systems
3. Thermal boundary between
internal hardware and
environment
Data Storage
Payload Structure/
Microphone Array
Thermal Control
Battery
Microprocessor
PAYLOAD STRUCTURE/
BASELINE
Baseline
CDD DESIGN: Project
MICROPHONE ARRAY
Overview
Feasibility
Overview:
5 microphones
• One on each planar face
• Three on curved face
evenly spaced at 120°
intervals
MICROPHONE
Status
Summary
SPATIAL
FILTER
Remaining
Studies
8
0.5m
Microphones
(embedded in
spatial filter)
OVERVIEW
BALLOON
0.5m
GONDOLA
ASTERIA
ASTERIA
Spatial Filter
Microphone
Low Pressure Differential Sensor:
• Two volumes, open to ambient
atmospheric pressures, separated
by a diaphragm.
• Ports oriented in opposite
directions. One port measures
infrasound while the other remains
at ambient pressure.
Data Storage
Payload Structure/
Microphone Array
Infrasound
Source
(0.1-20 Hz)
Thermal Control
Battery
Microprocessor
PAYLOAD STRUCTURE/
BASELINE
Baseline
CDD DESIGN: Project
MICROPHONE ARRAY
Overview
Feasibility
MICROPHONE
Status
Summary
SPATIAL
FILTER
Remaining
Studies
9
OVERVIEW
BALLOON
GONDOLA
ASTERIA
ASTERIA
*Spatial Filter: Porous material
attached to microphone to reduce
noise caused by atmospheric
turbulence.
Barrier Spatial Filter:
Spatial Filter
Microphone
Data Storage
Infrasound Source
(0.1-20 Hz)
Payload Structure/
Microphone Array
Thermal Control
Battery
Microprocessor
PAYLOAD STRUCTURE/
BASELINE
Baseline
CDD DESIGN: Project
MICROPHONE ARRAY
Overview
Feasibility
• Barrier acts as an acoustic low-pass
filter
• Microphone is embedded within
barrier spatial filter material
MICROPHONE
Status
Summary
SPATIAL
FILTER
Remaining
Studies
10
Key Project Elements
Microphone Design and Data Collection
Test Equipment & Infrasound Generation
Thermal System
Signal Detection in Relative Winds
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
11
Project Element
Description
Microphone Design & Data
Collection
0.01 Pa pressure changes correlate to voltages on
the order of μV, requiring extensive signal
processing and interfacing
Stratospheric Survivability of
Payload
Payload must operate in ambient temperatures
between -40 and 60 ° C
Infrasound Generation & Test
Equipment
Equipment that produces simulated infrasound is
critical to verifying microphone performance
Power Budget
The batteries on-board must be able to maintain active power draw for
24 continuous hours
Mass Budget
The payload mass cannot exceed the lifting capabilities of the balloon
(~20-50 kg) - The heaviest component would be the battery
Data Storage Survivability
Commercially available SD cards can withstand up to 500 Gs
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
12
Key Project Elements
Microphone Design and Data Collection
Test Equipment & Infrasound Generation
Thermal System
Signal Detection in Relative Winds
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
Key CPE addressed: Microphone Design and Data Collection
FR.1: Measure and record pressure data from simulated infrasonic sources between 0.1
and 20 Hz, with a minimum wave amplitude of 0.1 Pa
Designation
Description
DR.1
The ADC will sample the microphone at a minimum of 40Hz
DR.2
Spatial filtering system shall reduce wind noise by 15 dB.
DR.3
The microphone array configuration shall provide 360 coverage.
DR.4
The output voltage from the microphone shall be filtered with an analog low-pass filter to attenuate
frequencies above 20Hz.
DR.5
The microphone output voltage range shall be amplified to use the microprocessor’s full voltage range
DR.6
The amplified output voltage shall be converted from analog to digital signal
using 0.01 Pa (54 dB SPL) resolution
Feasibility Concerns:
Very low pressure resolution : 0.01 Pa (~54 dB), 0.004% of a dynamic range of 249 Pa
Low frequency range: 0.1-20 Hz (lowest human voice is 80 Hz)
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
14
Microphone components must have sensitivity of 0.01 Pa and rely on
support electronics – signal amplifier, digitizer, power source, etc. – that
are equally as sensitive.
Frequency Range
100000
Frequency Range (Hz)

10000
1000
100
10
1
0.1
Target
0.01
Studio Recording Condenser Pressure
Microphone
Field Microphone
Piezoelectric
Pressure Field
Microphone
Low Pressure
Differential Sensor
(Infra-NMT)*
*Low Pressure Differential Sensors do not have independent frequency responses
Project
Overview
Baseline
Feasibility
Status
Summary
Best option
Remaining
Studies
15
Metric
Requirement
Infra-NMT
Frequency Response
0.1 – 20 Hz
0.01 – 40 Hz
Linear Dynamic
Pressure Range
0.1 – 10 Pa
±124.5 Pa
Pressure Sensitivity
0.01 Pa
0.00063 Pa
Inherent Noise
0.006 Pa
0.00562 Pa rms
Analog Voltage
Data Connection
Has flight heritage on August 2014
NASA HASP (High Altitude Student
Payloads)
Differential Pressure Transducer
(Allsensors.com 0.5-INCH-D-MV)
To scale
Power
Regulator
[Marcillo & Johnson, 2012]
Project
Overview
Baseline
Feasibility
* Not Pictured: Mechanical Filter
Status
Summary
Remaining
Studies
16
Infrasound Signal Path
Infra-NMT Microphone
Power supply to sensor
Microphone Output
Pressure Waves
Acoustic Filter
Wind noise reduced
Mechanical Filter
Infrasound Signals
(0.1-20 Hz)
Port 1
Differential
Pressure Transducer
Stable supply
voltage
9V
Battery
Project
Overview
Baseline
Feasibility
ΔP measurement
Power
Regulator
Status
Summary
Voltage
Out
Port
2
Patm
Ambient
Atmospheric
Pressure
Remaining
Studies
17

Preliminary Results:
 Infra-NMT specifications satisfy
Resolution Calculation:
design requirements
 Resolution requirement met using 16bit ADC
 Amplification requirement met using
non-inverting operational amplifier
circuit
 Measuring waves between 0.1-20 Hz,
with an amplitude of 0.1 Pa, is feasible

Future Work:
𝑅𝑎𝑛𝑔𝑒
# 𝑜𝑓 𝐵𝑖𝑡𝑠 = log 2
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
249 𝑃𝑎
14.6 = log 2
→ 16 𝑏𝑖𝑡𝑠
0.01 𝑃𝑎
Amplifier Gain:
𝑉𝑜𝑢𝑡
𝑅1
= 1+
𝑉𝑖𝑛
𝑅2
3.3𝑉
330𝑘Ω
= (1 +
)
0.01𝑉
1.003𝑘Ω
 Test Infra-NMT to verify performance
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
18
Key Project Elements
Microphone Design and Data Collection
Test Equipment & Infrasound Generation
Thermal System
Signal Detection in Relative Winds
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies

Infrasound testing is part of verifying the sensor satisfies FR.1 and DR.1.
FR.1: Measure and record pressure data from simulated infrasonic sources between
0.1 and 20 Hz, with a minimum wave amplitude of 0.1 Pa
Designation
Description
DR.1
The microphone system shall measure pressure waves with a minimum amplitude of 0.1 Pa
(74 dB SPL) at a minimum sample rate of 40 Hz (twice the maximum frequency to avoid
aliasing).
DR.4
The output voltage from the microphone shall be filtered with an analog low-pass filter to reduce sound pressure noise
with a frequency above 20 Hz.
DR.5
The output voltage shall be amplified from the microphone output to the microprocessor’s voltage. Typical
microprocessor voltages range from 0 to 5 volts.
DR.6
The amplified output voltage shall be converted from analog to digital signal using 0.01 Pa resolution to accurately
represent the pressure changes.
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
20

Available Testing Facilities
 Sandia National Laboratories
▪ Pistonphone (0.1-14 Hz)
 NOAA
▪ Helmholtz Resonator (10-50 Hz)
▪ Piston Bellows (0.001-20 Hz)

Alternative Testing
 Ambient infrasound test
▪ Compare to historical archive
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
21
Bellows move air inside
sealed volume simulating
infrasound
 External data collection is
required
 No testing chamber

Metric
Requirement
Piston Bellows
Capability
Frequency Response
0.1 – 20 Hz
0.001 – 20 Hz
Pressure Amplitude
0.1 – 10 Pa
0.1 – 100 Pa
Piston Bellows Configuration
[Hubbard and Bedard, 1969]
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
22
Preliminary Testing Flow Diagram
JANA
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
23
Key Project Elements
Microphone Design and Data Collection
Test Equipment & Infrasound Generation
Thermal System
Signal Detection in Relative Winds
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
24
Key CPE addressed: Stratospheric Thermal Survivability
FR.2: ASTERIA shall operate between altitudes of 18 and 30.5 kilometers.
Designation
Description
DR.8
Mass and volume of the payload shall remain as small as possible – not to exceed 20 kg or 5 m3
respectively
DR.9
Capable of operating under stratospheric pressure conditions: 300 to 500 Pa
DR.10
Capable of operating under stratospheric temperature conditions: -60° to 40° C
DR.11
Maintain structural integrity under launch and landing loads: approximately 1400 to 1600 N9
FR.3: ASTERIA shall function autonomously for a minimum of 24 hours.
Designation
Description
DR.12
Measure the temperature of payload components to a minimum of 1° C resolution
DR.13
Maintain the temperature of payload components to a range that is 10 ± 1° C
below the maximum and above component minimum specifications.
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
25
Component
operating limit:
-20 to 55°C
 Required
operating range:
-10 to 45°C
▪ DR.13: Maintain
the temperature
10°C below
maximum
operation temp.
and above
minimum
operation temp.
Project
Overview
Mission Temperature Ranges
350
80
330
60
Temperature, (°C)

margin
55°C
45°C
margin
-10°C
-20°C
310
40
290
20
0
270
-20
250
-40
230
Ambient Temperature
-50°C
-60
210
0
5
10
15
20
25
Duration in Stratosphere, (hours)
Baseline
Feasibility
Status
Summary
Remaining
Studies
26

Payload Heat Transfer Rate:



Δ𝑄 = Δ𝑄𝑑𝑜𝑡 ∙ Δ𝑡
Qsolar radiation
𝐽
Temperature Change:


Heat Transfer Model:
Heat Transfer:


𝑄𝑖𝑛 = 𝑄𝑠𝑜𝑙𝑎𝑟 𝑟𝑎𝑑 +𝑄𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝑠𝑢𝑛 +𝑄𝑐𝑜𝑛𝑣. + 𝑄𝑖𝑛𝑡𝑒𝑟𝑛𝑎𝑙
𝑄𝑜𝑢𝑡 = 𝑄𝑟𝑎𝑑2𝑒𝑎𝑟𝑡ℎ + 𝑄𝑟𝑎𝑑2𝑠𝑝𝑎𝑐𝑒
Δ𝑇 =
𝑄
𝑚 𝑐𝑝
°𝐶
Qconvection
Assumptions:


Isothermal Sphere
3-Body System
▪

Payload Bus
Qinternal generation
0.5m
(Battery & Microcontroller)
Earth, Sun, Payload
Payload:
▪
▪
▪

Qrad2space
Mass = 20 kg
Diameter = 0.5 m
Specific Heat Capacity = 900 [J/(kg°𝐶)] (Aluminum)
Qreflected sunlight
Qrad2earth
Launch
▪
▪
Assume April launch, Ft. Worth, TX
Based on 05:30 am launch time, 07:00 am arrival at altitude
▪ Clearest weather conditions for launch in early morning
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
27


Absorptivity = 0.40
Emissivity = 0.87
 Values from white
Sunset
Sunrise
coating

9cm
Polyurethane
insulation used
Required Temperature Range
0.2cm
4cm
7cm
Operating Temperature Limit
 Thermal Conductivity
of 0.04 W/(m°C)
 Minimum of 7 cm thick
insulation required
Project
Overview
Baseline
Feasibility
External
Sphere Temp
Note: Numbers Indicate Insulation Thickness
Status
Summary
Remaining
Studies
28

Preliminary Analysis
 Feasible for payload to maintain temperature
within required bounds with use of 7 cm of
polyurethane insulation around the structure

Future Work
 Internal thermal model characterizing heat
transfer inside payload
 Material and coating analysis
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
29
Key Project Elements
Microphone Design and Data Collection
Test Equipment & Infrasound Generation
Thermal System
Signal Detection in Relative Winds
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
30

Determine if dynamic pressure remains
below microphone resolution of 0.01 Pa
FR.1: Measure and record pressure data from simulated infrasonic sources
between 0.1 and 20 Hz, with a minimum wave amplitude of 0.1 Pa.
Designation
Description
DR.1
The ADC shall sample the microphone at a minimum of 40 Hz.
DR.2
Spatial filtering system shall reduce wind noise by 15 dB with minimal artificially
generated false infrasound signatures
DR.3
The configuration shall reduce signal attenuation due to sensor array directionality.
DR.4
The output voltage from the microphone shall be filtered with an analog low-pass filter.
DR.5
The output voltage shall be amplified from the microphone output to the microprocessor’s voltage.
DR.6
The amplified output voltage shall be converted from analog to digital signal using 0.01 Pa resolution.
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
31

Need to reduce
dynamic pressure
noise to below 0.01 Pa

Compute dynamic
pressures on
microphone at altitude
 NASA Std. Atmosphere
 Relative wind speed of
0.5 m/s
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
32
 15dB Filter increases
allowable wind speed
 Operational altitudes
18-30.5 km tolerate up
to 1 m/s relative winds
with 15dB spatial filter
Maximum
Allowable
Wind Noise
▪ Expected 0.5 m/s relative
winds
Unfiltered Signal
15dB Filtered Signal
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
33

Requirements



Provide 15 dB of wind noise reduction
Provide pressure measurement SNR of 5 dB
Unfiltered
 Asignal 
0.1 
  20 log 10 
SNRdB,raw  20 log 10 
  4.73dB
 0.015 
 Anoise 
SNRdB,required  5dB

Requirement
Filtered
 Asignal 
  20 log 10  0.1   31.7dB Prediction
SNRdB,15dBfilter  20 log 10 
A

 0.0026 
 noise, filt 

Conditions for Calculation


Worst case wind speed: 0.5 m/s
Worst case altitude: 18 km
▪


Yields highest density and therefore largest dynamic pressure
Minimum signal amplitude: 0.1 Pa
Maximum dynamic pressure noise
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
34

Preliminary Analysis
 Spatial filter maintains 0.01 Pa resolution within expected
wind speeds
 Filter achieves minimum SNR of 31.7 dB
 Conclusion: Pressure measurement at operational altitude
18-30.5 km is feasible

Future Work
 Full 3-dimensional flow modeling
 Relative wind calculation from drag profile
▪ Predict relative wind generation
▪ Model forces on payload and resulting motion
 Testing: ITLL wind tunnel test, extrapolated to altitude
▪ Testing microphone with spatial filter inside wind tunnel
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
35
Budget
Schedule
Baseline Feasibility
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
36
General: $300
Funding:
• CU AES: $5000
• Time in NOAA Testing
Facilities: Donated
• Opportunity to apply
for EEF in Spring ‘15
Margin: $2,074
Electronics:
$1,406
Total : $2,926
Structures:
$1,120
*Conservative, order-ofmagnitude cost estimates
based on notional components
Testing: $100
Project
Overview
Margin: $2,074
Baseline
Feasibility
Status
Summary
Remaining
Studies
Budget
Schedule
Baseline Feasibility
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
38
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
39
Budget
Schedule
Baseline Feasibility
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
40
Microphone Sensor Design
• Feasible to measure infrasound waves between 0.1-20 Hz, with an amplitude of 0.1 Pa
• Infra-NMT sensor specifications meet requirements
Infrasound Generation & Test Facilities
• Feasible to test with NOAA piston bellows
• NOAA facilities can generate infrasound waves of frequency 0.001-20 Hz with
adjustable amplitudes from 0.1-100 Pa
Thermal System
• Feasible to maintain temperature within operational bounds
• 7 cm thick foam insulation fulfills temperature requirements
Signal Detection in Relative Winds
• Feasible to measure pressure at operational altitude range (18-30.5 km)
• Barrier spatial filter maintains 0.01 Pa resolution within expected relative wind
speeds
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
41
Power Budget
• Estimate power needs of all subsystems to function for 24 hours
• Determine power supply needs
Mass Budget
• Estimate subsystem masses based on notional components
• Determine which subsystem drives mass budget
• Total mass will not exceed lifting capabilities of balloon
Data Storage Survivability
• Assess impact of thermal and acceleration loads on data storage
system based on supplier specifications
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
42
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
43
44
Slide Number
Content
46
Amplified Low Pressure Sensors
47
Microphone Backup Specs
48
Microphone Sensor Comparison
49
Infra-NMT Sensor Package
50
Infra-NMT Test Data – Frequency Response
51
Infra-NMT Test Data – Self Noise Spectrum
52
Risk Mitigation
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
45
Metric
0.5-INCH-D-MV*
0.25-INCH-D4V**
Frequency
Response
N/A
N/A
Linear Dynamic
Pressure Range
±124.5 Pa
±62.25 Pa
Pressure
Resolution
N/A
N/A
Linearity Error
(0.05%FSS)
124.5 mPa
62.25 mPa
Sensitivity
0.040 mV/Pa
32.13 mV/Pa
Cost
$80-88
$162-180
These sensors form the core of the Infra-NMT
package, however the published datasheets on
them by themselves does not provide
confidence that they will work in our
application.
The primary concern is that the internal
linearity error is significant, already greater
than the target sensitivity. There is also no
published accuracy data to provide a
confidence level for the sensors.
This casts some doubt on any measurements
smaller than these values. Testing may reveal
that these sensors will still function for our
application, but without it, there is significant
risk to using them.
*http://www.mouser.com/ds/2/11/DS-0091%20Rev%20C1-262222.pdf
** http://www.mouser.com/ds/2/11/DS-0032%20Rev%20A-262032.pdf
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
46
Metric
PCB 377A14*
PCB 378B02**
Brüel & Kjær*
Frequency Response
3 – 100,000 Hz
3.75-20,000 Hz
0.1 – 20,000 Hz
Linear Dynamic Pressure
Range
1.1-10023 Pa
0.13 – 159 Pa
0.0002-2518 Pa
Pressure Resolution
1.1 mPa
0.00013 Pa
0.00056 Pa
Inherent Noise
0.0011 Pa
0.00013 Pa
0.00056 Pa
Sensitivity
1.0 mV/Pa
50 mV/Pa
12.5 mV/Pa
$839
+$3000
Cost
*http://www.pcb.com/TestMeasurement/Acoustics/1-4-inch-microphone_Model377A14
** http://www.pcb.com/Products.aspx?m=TLD378B02
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
47
Sensitivity Range
10000
Sensitivity (mPa)
1000
100
10
1
Target
0.1
Studio Recording
Microphone
Condenser
Pressure Field
Microphone
Piezoelectric
Pressure Field
Microphone
Low Pressure
Differential Sensor
(Infra-NMT)*
*Infra-NMT sensitivity based on 12 and 16-bit digitizers
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
48

The Infra-NMT has a linear response to
pressure input.
Digitizer input range
on the order of 0.1 V.
The signal will
require amplification.
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
49
Sensor
Capillary Tube
Length (m) Radius (µm)
Backing
Volume (m3)
Cutoff
Frequency
(Hz)
Infra-NMT 1
0.01
25
1 x10-6
0.01
Infra-NMT 2
0.01
25
1 x10-6
0.01
Infra-NMT 3
0.01
75
1 x10-6
1.22
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
50
Vacuum tested at
Sandia National
Laboratories
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
51
Utilize Infra-NMT sensor
package, with testing to
confirm previous data
Main Plan
Develop equivalent sensor, with
rigorous testing to ensure it will
meet the project requirements
Alternative #1
Low Risk: Adjust
pressure resolution
requirements based on
testing, which still
achieves customer’s
core goal
Off-Ramp #1
Project
Overview
High Risk: Redesign
ASTERIA for
microphones that
meet current
requirements
Moderate Risk: Redesign
ASTERIA for microphones
with minimal impact on
design and adjust
frequency requirements
Off-Ramp #2
Baseline
Feasibility
Off-Ramp #3
Status
Summary
Remaining
Studies
52
Slide Number
Content
54
Temperature Sensor
55
Accelerometer
56
Filtering
57
Amplifier
58
Analog-Digital Converter
59
Microcontroller
60
Storage
61
Main Supply
62
Maximum Load Power Budget
63
Nominal Load Power Budget
64
Regulator for Microphone
65
Converter for Off-Ramp #2
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
53



Minimum range of -15
to 75°C, based on
Infra-NMT family
operating range
Low cost, easy to
integrate sensors can
be incorporated into
microphone package
Example sensor:
BD1020HFV-TR from
Rohm Semiconductor
Project
Overview
Baseline
Feasibility
Purpose 1) Provide data to active
thermal controls to meet DR.10 and
DR.13.
Purpose 2) Collect data on
microphone temperature for postprocessing
Parameter
BD1020HFV-TR
Range
−30 𝑡𝑜 100°𝐶
Sensitivity
Voltage
Voltage Out
±2.5°𝐶 𝑚𝑎𝑥
2.4 𝑡𝑜 5𝑉, 3𝑉 𝑛𝑜𝑚𝑖𝑛𝑎𝑙
0 𝑡𝑜 𝑉𝑅𝑒𝑓 , 3𝑉 𝑛𝑜𝑚𝑖𝑛𝑎𝑙
Current
4 𝑡𝑜 7 𝜇𝐴
Cost
$0.78
Status
Summary
Remaining
Studies

3-axis accounts for
fluctuations for all
microphones

Minimum range of 0-5 g,
based on maximum
loading during balloon
decent

Example sensor:
ADXL325BCPZ from
Analog Devices
Project
Overview
Baseline
Feasibility
Motion of ASTERIA
Microphone diaphragm
moves due to acceleration
parallel to diaphragm normal,
creating false detection
ASTERIA
Accelerometer records
motion, post-processing
reveals false detection
Microphones
Parameter
ADXL325BCPZ
Range
± 5 𝑡𝑜 6𝑔, 6𝑔 𝑛𝑜𝑚𝑖𝑛𝑎𝑙
Sensitivity
Voltage
Voltage Out
Current
Operating
Temperature Range
Cost
174 𝑚𝑉/𝑔 𝑛𝑜𝑚𝑖𝑛𝑎𝑙
1.8 𝑡𝑜 3.6𝑉, 3𝑉 𝑛𝑜𝑚𝑖𝑛𝑎𝑙
0 𝑡𝑜 3𝑉 𝑛𝑜𝑚𝑖𝑛𝑎𝑙
350 𝜇𝐴 @ 3𝑉
Status
Summary
−40 𝑡𝑜 85°𝐶
$6.41
Remaining
Studies

Targeting narrow bandwidth
of infrasonic frequencies
 0.1-20 Hz

Variety of analog low pass
filters, all simple and low cost
First order low-pass filter:
1
𝑓𝑐 =
= 40𝐻𝑧
2𝜋𝑅1 𝐶1
𝑅1 ∗ 𝐶1 = 0.004𝑠
𝑅1 = 40𝑘Ω, 𝐶1 = 0.1𝜇𝐹
0.15W 40k Resistor: $1.19 each
Ceramic 0.1𝜇F Capacitor: $0.10 each
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies



Sensitivity of Infra-NMT:
45.13±0.23 μV/Pa
Full output range: 0-10 mV
Desired output range: 0-3.3V or
0-5V to interface with standard
ADC’s and microprocessors
Parameter
MCP6001
# of Circuits
Gain Bandwidth
Product
Supply Voltage
Voltage Out
1
1.8 𝑡𝑜 6 𝑉
0 𝑡𝑜 𝑉𝑆𝑢𝑝𝑝𝑙𝑦
Supply Current
0.1 𝑚𝐴
Cost
$0.29
Project
Overview
1 𝑀𝐻𝑧
Baseline
Feasibility
𝑉𝑜𝑢𝑡 = 1 +
𝑅1
∗ 𝑉𝑖𝑛
𝑅2
𝑉𝑜𝑢𝑡
𝑅1
−1=
𝑉𝑖𝑛
𝑅2
𝑉𝑖𝑛 = 10𝑚𝑉, 𝑉𝑜𝑢𝑡 = 3.3𝑉
𝑅1
→
= 329
𝑅2
1kΩ and 330kΩ both common values
+1000 1kΩ options, starting at $0.08
+300 330kΩ options, starting at $0.10
Status
Summary
Remaining
Studies
57



Current design uses 12 analog sensors
For microphone, 16-bits required. External chip
required (AD7171BCPZ)
Other sensors need less resolution, microcontroller
internal ADC sufficient
# 𝑜𝑓 𝐵𝑖𝑡𝑠 = log 2
Device
𝑅𝑎𝑛𝑔𝑒
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
10-Bit Resolution 16-Bit Resolution
(Arduino Uno)
(AD7171BCPZ)
Infra-NMT
0.243 𝑃𝑎
3.8 𝑚𝑃𝑎
BD1020HFV-TR
0.127 °𝐶
1.98𝑒 − 3 °𝐶
ADXL325BCPZ
5.86𝑒 − 3 𝑔
9.16𝑒 − 5 𝑔
Pressure sensitivity met by 16-bit ADC
Temperature sensor/Accelerometer limited by accuracy, 10-bit
adequate
Project
Overview
Baseline
Feasibility
Parameter
AD7171BCPZ
# of Bits
# of
Channels
Voltage
Power
Noise
Sampling
Rate
Cost
16
Status
Summary
1
3 𝑜𝑟 5𝑉 𝑛𝑜𝑚𝑖𝑛𝑎𝑙
6.9 − 11.5 𝜇𝑉𝑅𝑀𝑆
125 𝐻𝑧
$3.15
Remaining
Studies
58

Must have enough pins to accommodate all sensors
 14 analog, with minimum of 5 converted to digital externally

Must have data rate fast enough to sample all sensors
at target frequency
Example Microcontroller:
PIC18F87K22
- Available through University
with additional peripherals on
in-house breakout board
- Has SPI communication to
communicate with digital
devices, such as a MicroSD
card reader
Project
Overview
Baseline
Feasibility
Parameter
PIC18F87K22
Digital I/O
60
Analog I/O
24
Voltage
3.3𝑉 𝑜𝑟 5𝑉
Internal ADC
12 𝑏𝑖𝑡𝑠
Program
65,536 𝑏𝑖𝑡𝑠
Memory
Clock
30𝑘𝐻𝑧 𝑡𝑜 64𝑀𝐻𝑧
Frequencies
Cost
$4.85
Cost through ~$120 𝑖𝑛𝑐𝑙𝑢𝑑𝑖𝑛𝑔
University
𝑝𝑒𝑟𝑖𝑝ℎ𝑒𝑟𝑎𝑙𝑠
Status
Summary
Remaining
Studies
59

Assumptions:
𝑆𝑡𝑜𝑟𝑎𝑔𝑒 = (𝑆𝑖𝑧𝑒 ∗ 𝑛 + 8) ∗ 𝑇 ∗ 𝐹𝑆
 24 hour collection period
 24-bits per data point
▪ 16-bit ADC
▪ 8-bit delimiter character
Frequency
Storage Needed
40 𝐻𝑧
100 𝐻𝑧
1 𝑘𝐻𝑧
150 𝑀𝐵
355 𝑀𝐵
1 𝑀𝐻𝑧
3.55 𝑇𝐵
3.55 𝐺𝐵
 14 data points per cycle
▪ 5 microphones
▪ 3 axis accelerometer
▪ 6 temperature sensors
SanDisk 8GB MicroSD Card: $12
SanDisk 64GB MicroSD Card: $37
1.524 cm
 8-bit new line every cycle

Vary sampling frequency
to establish range
Project
Overview
Baseline
Feasibility
1.016 cm
Status
Summary
Remaining
Studies
60
Electronic components require power
for duration of flight, total current draw
on order of 24oomAh (estimated
100mA for 24 hours)
 Energy source must be stable

 No danger due to 5 g on descent
or -50 to 70°C environment

Lithium Polymer batteries:
 Variety of recommended
voltages to meet requirements
as parts are selected
 Require minimal padding to
protect from shocks
Project
Overview
Baseline
Feasibility
Parameter
SparkFun Cell
Capacity
Nominal
Voltage
Maximum
Current
Mass
2000𝑚𝐴ℎ
Cost
Status
Summary
3.7𝑉
1𝐴 (𝑖𝑛 𝑤𝑖𝑟𝑒𝑠)
37𝑔
$12.95
Remaining
Studies

11000mAh Electronics System Power Budget
Key Parts:
 PIC18F22K87
maximum
operating
current: 300mA
 MicroSD Card
write current:
100mA
microSD Card,
2400, 24%
Infra-NMT,
360, 4%
Accelerometer,
8.4, 0%
Temp. Sensors,
36, 0%
Op-amp,
12, 0%
PIC18F22K87,
7200, 72%
Project
Overview
16-bit ADC,
16.2, 0%
Baseline
Feasibility

Feasible Battery:
 3.7V, 11000 mAh
LiPo: $45
Status
Summary
Remaining
Studies
62
5200 mAh Electronics System Power Budget

 PIC18F22K87
microSD Card,
2400, 46%
maximum
operating
current: 100mA
 MicroSD Card
write current:
100mA
Infra-NMT,
360, 7%
Accelerometer,
8.4, 0%
Temp. Sensors,
36, 1%
Op-amp,
12, 0%
16-bit ADC,
16.2, 0%
PIC18F22K87,
2400, 46%
Project
Overview
Key Parts:
Baseline
Feasibility

Feasible Battery:
 3.7V, 6000 mAh
LiPo: $40
Status
Summary
Remaining
Studies
63
Amplified Low Pressure Sensors require extremely
stable supply voltages for stable radiometric output
 Regulator must resist voltage change due to:

 Temperature
 Drift over time

Infra-NMT package includes LT1021-7V regulator, no
further work required
 LT1021 family includes other voltages if redesign is
required, or for other areas of project
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies




Microphone selected for Off-Ramp #2 requires
200 V polarization voltage
3 mA current draw by sensor and amplifier
DC-DC converters allow for high voltages at low
current without massive battery packs
Example Converter: 5SMV200
Parameter
Converter
(5SMV200)
 Can be powered by same LiPo
Input Voltage
5𝑉
Input Current
333 𝑚𝐴
Output Voltage
Output Current
Output Power
200𝑉
6.25 𝑚𝐴 𝑚𝑎𝑥
1.25 𝑊 𝑚𝑎𝑥
Efficiency
75%
Cost
$92
batteries as other systems
 Surface mount component,
near-negligible mass and volume
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
Slide Number
Content
67
Piston Bellows Functional Diagram
68
NOAA – Helmholtz Resonator
69
Sandia – Pistonphone
70
Infrasound Generation Equipment
Comparison
71
Atmospheric Infrasound in Boulder, CO
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
66

Simulates infrasound by creating pressure
changes at infrasonic frequencies
Steel Wool
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
67






10-50 Hz
Differences in internal pressure creates
pressure waves
Frequency response decreases with time
Requires large volume and long neck to
produce required frequencies
100 gal, 2 m height
Governing Equation:
𝑓=
𝑎
2𝜋
𝐴
𝑉𝐿
𝑎 = speed of sound
𝐴 = neck area
𝑉 = Volume
𝐿 = neck length
NOAA’s Helmholtz Resonator
Bedard A. J. and Georges T. M.
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
68




0.1-14 Hz
Piston moves air in sealed chamber at low
frequencies to create infrasound
Isolated testing chamber to reduce noise
Used to calibrate infrasonic microphones
Sandia Testing Chambe. Hart, D. M.
Pistonphone Configuration
Marston, T. M.
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
69
Equipment
Pros
Pistonphone
Helmholtz
Piston Bellows
 Generates required
frequencies
 Includes test chamber to
decrease noise
Cons
 Travel: Located at in New
Mexico
 Reliably generates 10-50 Hz  Exposed to ambient
 Travel : NOAA access in Erie,
infrasound noise
CO
 External data collection
required
 Immobile device
 Generates lower than
required frequencies
 Pressure amplitude
variation
 Travel: NOAA facility in
Boulder, CO
Project
Overview
Baseline
Feasibility
 Exposed to external noise in
underground setting
 External data collection
required
 Immobile facility
Status
Summary
Remaining
Studies
70
Infrasound signatures could compare to for alternative infrasound testing
[Bedard and Georges, 2000]
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
71
Slide Number
Content
73
Assumptions & Equations
74
Barrier Properties
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
72

Assumptions
 Flow is incompressible.
▪ Relative wind speeds predicted below 1 m/s
 Body is point mass
 1-dimensional flow
 NASA Standard Atmosphere Model of Stratosphere


p
 kg 
0.2869  (T  273.15)  m 3 
p  22.65  e(1.730.000157alt) [kPa]
T  56.46[C]
Equations
 Bernoulli’s Equation
P0 +
1
rV 2 = Const.
2
 Filter Sound Pressure Level Reduction
-15dB = 20 log(
Project
Overview
Pfiltered
)
Pfiltered = P ×10-15/20
Baseline
Feasibility
Status
Summary
P
Remaining
Studies
73

Barrier Noise
Reduction
 Max: 25 dB
 Min: 15 dB
[Michael Hedlin]
Min. Noise Reduction
Max Noise Reduction
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
74
Slide Number Content
76
Convection Coefficient
77
Heat Rate Equations
78
Temperature vs. Altitude
79
Sphere Assumptions
80
Solar FOV Calculations
81
Reflected Solar Radiation
82
Operating Temperature Ranges
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
75

Assuming Relative Wind Speed, u∞, is 0.5m/s
kg
rmin = 0.0171 3
m
rmax = 0.1163
At 30.5km
kg
m3
Pr = 0.71
W
k = 0.028
mK
Project
Overview
Re =
At 18.3km
Nu = 0.332Re1/2 Pr1/3
h=
Assumed Constant for Air
Air Thermal Conductivity
Baseline
Feasibility
ru¥ x
m
Nu × k
L
hmin = 0.364
W
m2 K
hmax = 0.949
W
m2 K
Status
Summary
Remaining
Studies
76

Equations in Payload Power Balance
QDirectSolarRadiation    Fsun  Asurface  I sun
QRe flectedSunlight  a    FEarth  Asurface  I sun
QConvection = h × Asurface × (Tair -Tpayload )
4
QRaiatedToEarth = s × e × FEarth × Asurface × (Tpayload
-T 4Earth )
4
QRaiatedToSpace = s × e × Fspace × Asurface × (Tpayload
-T 4Space )
Project
Overview
Baseline
Feasibility
a = Earth albedo
α = material absorptivity
A = surface area
ε = material emissivity
F = view factor
I = solar constant
σ = Stefan-Boltzman constant
T = temperature
Status
Summary
Remaining
Studies
77

Ambient air
temperature based
upon average from
18-30.5km
 Tair= -50°C
Image From: http://eesc.columbia.edu/courses/v1003/images/atmprofile.gif
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
78
Sphere
Cylinder
r
r
h
Assuming r = 0.25m
h = 0.50m
Cylinder
Sphere
Relative Error
Surface Area
2πrh+2πr2
4πr2
33.3%
Top View Area
πr2
πr2
0%
Side View Area
2rh
πr2
21.5%
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
79


Assumed that half of
ASTERIA will be
exposed to direct
sunlight during day
ASTERIA in
Darkness
ASTERIA in
Light
Assumed parallel
incoming solar rays
Incoming Solar Rays
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
80



ASTERIA in
Darkness
Percent of Earth in
light varies during
day
Assumed all reflected
light is parallel to
ASTERIA
Earth albedo = 0.30
Project
Overview
Baseline
Feasibility
ASTERIA in
Light
Reflected Light
Half of Earth
visible to
ASTERIA
Status
Summary
Remaining
Studies
81
 Required temperature range is 10°C margin within
worst case component operating temperature range.
Component
Minimum Operating
Temperature (°C)
Maximum Operating
Temperature (°C)
Microphone:
Infra-NMT
-25
70
Microprocessor:
-20
85
Data Storage:
Sandisk 32GB SD
-25
85
Battery: Energizer 9V
-18
55
Accelerometer:
ADXL320
-20
70
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
82
Slide Number Content
84
Intro
85
Purpose, Trade Variables
86
Metric Definitions & Weights
87
Scoring Definitions & Trade Study Table
88
Results
89
FOV Calculations
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
83


The team carried forward two types of
configurations from CDD: radial and planar.
A follow-on trade study was performed to
determine the optimal microphone
configuration.
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
84

The purpose of the configuration:
 Determine the pointing of microphones
 House support systems
 Act as a thermal control boundary

Configuration trade study in CDD was
inconclusive
 Configuration is highly dependent on type
of microphones, spatial filter selection
 CDD determined differential pressure
sensor (microphone) and barrier (spatial
filter)

Tetrahedron
Cylinder
Sphere
New trade variables:
 Minimum number of microphones required
for omnidirectional sensing
 Surface area – Important for heat rejection
 Volume – Dictates space for microphones
and support hardware
Project
Overview
Baseline
Feasibility
Status
Summary
Cube
Remaining
Studies
85
Metric
Number of
microphones for
optimal signal
detection
Surface Area
Volume
Weight
Weight Rationale
Description of Metric
45%
Minimizing the number of
microphones needed while still
achieving omnidirectional detecting
capabilities reduces complexity and
cost. Fulfills FR.1.
Relates the field of view (FOV) of the
microphones to the geometry
needed to achieve full coverage
35%
Overheating of internal hardware is a
major concern. A larger surface area
allows more heat dissipation to the
surrounding environment. Fulfills
FR.2.
Surface area of the geometries were
computed using unit side
length/diameter
20%
A geometry with a large volume is
optimal for mounting microphones,
support hardware, and other payload
components.
Volume of the geometries were
computed using unit side
length/diameter
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
86
Trade Variable
Score 1
Score 2
Score 3
Number of
microphones
6 or more
5
4 or less
Surface area
(unit distance2)
Less than 2
2–4
Greater than 4
Volume (unit
distance3)
Less than 0.33
0.33 – 0.66
Greater than
0.66
Weight
Tetrahedron
Cube
Sphere
Cylinder
Number of
microphones
40%
3
1
3
2
Surface Area
35%
1
3
2
3
Volume
25%
1
3
2
3
Total
100%
1.8
2.2
2.4
2.6
Project
Overview
Baseline
Feasibility
Status
Summary
Trade study
scoring
definitions
Trade study
results
Remaining
Studies
87

Results:
 Cylinder was determined to be the
optimal configuration
 Geometry best fulfills all three
purposes of configuration: pointing
of microphones, housing of support
hardware, thermal boundary

Feasibility Concerns:
Cube
Sphere
Cylinder
Tetrahedron
 No concerns with manufacturability
 Structure will dominate mass
budget

Future Work:
 Conduct trade studies for
material/coating selection
 Complete SolidWorks design
Project
Overview
Baseline
Feasibility
Status
Summary
Remaining
Studies
88
• The angle of incidence in degrees of an incoming sound wave is a function of wavelength
in meters and the diameter in meters of the pressure sensor inlet:
• Wavelength is a function of the velocity of sound in meters per second at a given
altitude (which dictates ambient temperature) and the frequency of the sound wave in
Hz:
• Varying or unknown parameters:
• Altitude (18,288 – 30,480 m)
• Standard Atmosphere Tables were used to find the velocity of sound at
stratospheric altitudes
• Sound wave frequency (0.1 Hz – 20 Hz)
• Inlet diameter (0.001 – 0.002 m)
• Specifications from vendor are TBD
The range of angles of incidence that the sensor can detect, which determines the half angle
of the microphone field of view, is 80.89 – 84.33°.
This range will be finalized once the sensor’s inlet diameter is known.
89
• Total project cost: $2,926
• Margin: $2,074
• Preliminary budget based
on notional components
Item
Quantity
Total Cost
General
$300
Poster Printing
2
$50
$100
Misc.
-
-
$200
Electronics
$1406
Microphone Package
6
$200
$1200
Microcontroller
1
$120
$120
SD Card
1
$30
$30
Temperature Sensor
6
$1
$6
Accelerometer
1
$7
$7
Amplifier
6
$1
$6
A/D Converter
6
$4
$24
Microphone Power Supply
1
$13
$13
• Conservative, order-ofmagnitude estimates
• Funding sources:
• $5,000 AES
• Time in NOAA test
facilities donated at no
cost
• Opportunities to apply
for EEF in Spring
Item Cost
Structures
$1120
Stock Material (6061 Aluminum)
-
-
$500
Insulation (Foam)
0.1 m3
-
$20
3D printing
-
-
$500
Misc. Hardware
-
-
$100
Testing
$100
Misc.
TOTAL
-
-
$100
90
$2926
91
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from: http://can-ndc.nrcan.gc.ca/index-eng.php]
Alden, Andrew. “Infrasound Take A Bow. QUEST Northern California.21 Feb 2013.
[Retrieved from: http://science.kqed.org/quest/2013/02/21/infrasound-takes-a-bow/]
Pinchon, Alexis. Infrasound monitoring for atmospheric studies. Dordrecht New York:
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Marcillo, Omar, and Jeffrey Johnson. "Implementation, Characterization, and Evaluation of an
Inexpensive Low-Power Low-Noise Infrasound Sensor Based on a Micromachined
Differential Pressure Transducer and a Mechanical Filter." Atmospheric and Oceanic
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2014. <http://journals.ametsoc.org/doi/abs/10.1175/JTECH-D-11-00101.1>.
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<http://l2a.ucsd.edu/pub/Infrasonic_wind-noise_reduction_barriers_spatial_filters.pdf>.
92
Wolfe J. “Helmholtz Resonance,” University of New South Wales, Sydney, Australia
[http://newt.phys.unsw.edu.au/jw/Helmholtz.html. Accessed 10/12/14.]
Hart, D. M., “Evaluation of Inter-Mountain Labs Infrasound Sensors July 2007,” Sandia
National Laboratories, Albuquerque, NM, Oct. 2007
[http://prod.sandia.gov/techlib/access-control.cgi/2007/077020.pdf. Accessed
10/12/14.]
Marston, T. M., “Infrasonic Pistonphone Calibration," Ph. D. Dissertation, Graduate
Program for Acoustics, Pennsylvania State University, PA, 2009.
NOAA, 2014 [NOAA logo], [http://www.noaa.gov/ Accessed 10/12/14.]
Sandia National Laboratories, 2014 [Sandia logo], [http://www.sandia.gov/ Accessed
10/12/14.]
Bedard A. J. and Georges T. M., “Atmospheric Infrasound,” Physics Today, March 2000 pp
32-37.
[http://www.esrl.noaa.gov/psd/programs/infrasound/atmospheric_infrasound.pdf.
Accessed 10/12/14.]
Hubbard, E.A. and A.J.Bedard Jr
A Pressure transducer for use as a component of an infrasonic microphone
ESSA Research Laboratories Technical Memorandum ERL TM-WPL-4 38pp
93