PENNSTATE
Project: PS 4.1
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Modeling of Rotorcraft Noise in
Maneuvering Flight
PI: Kenneth S. Brentner (814)865-6433,
ksbrentner@psu.edu
Graduate Students:
Hsuan-Nien Chen (started Dec 2002 – PhD)
2005 RCOE Program Review
May 3, 2005
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
1
PENNSTATE
Overview of Work

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Project Overview (Ken Brentner)
Acoustic Analysis (Sam Chen)
Summary (Ken Brentner)
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
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PENNSTATE
Background/Problem Statement:



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Current rotor aerodynamics and noise
prediction primarily for steady flight
conditions
Noise of maneuvering rotorcraft can be
significantly higher than for a similar steady
flight condition
A tool is needed that is able to predict noise
generated by rotorcraft in maneuver —
including the transient aircraft motion and
blade loading.
Technical Barriers or Physical Mechanisms to Solve:


Acoustics
 Very complex source motion and time dependence
 Complicated time-dependent noise directivity
 Transient blade loading and motion are an “additional” noise source
Aeromechanics
 Nonperiodic blade loading and motion is unique to each blade
 Rotor-wake interaction extremely challenging problem
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
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PENNSTATE
Task Objectives:
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
Develop a noise prediction capability for rotors in steady
AND transient maneuvers (including multiple rotors)

Gain better understanding of noise directivity in
maneuvering flight—especially the components (thickness,
loading, transients, etc.) of maneuver noise

Quantify the importance of transients

Assess the requirements for wake fidelity and airloads
accuracy in the context of maneuver noise-prediction

Improve maneuver noise prediction through the utilization
and/or development of maneuvering wake
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
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PENNSTATE
Approach:
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Develop acoustics code with full rotor-blade motion and
complete aircraft motion
 Utilize best available comprehensive analysis tools for
initial developmental work, accepting known weaknesses
 Incorporate advanced maneuver airloads/wake modeling as
it becomes available
Emphasis is on approaching the problem from the acoustics
point of view, then working to provide required input data

Expected Research Results or Products:



A new rotorcraft noise prediction code—much more useful
and general purpose than the current generation of codes
Understanding of the extra noise generated in maneuvers
Guidance for the development of maneuver aerodynamics
and flight dynamics (acoustic requirements)
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
5
PENNSTATE
Overview of Work



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Project Overview
Acoustic Analysis
Summary
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
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PENNSTATE
Maneuver Noise Analyzed

Several maneuvers were analyzed:

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Arrested descent
Left turn entry (with three different roll rates)
Right turn entry (with three different roll rates)
Left-right-left roll reversal maneuver
Right-left-right roll reversal maneuver
Quick stop maneuver
Level acceleration maneuver
Climb maneuver
Focus of this presentation on maneuvers with roll motion
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
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PENNSTATE
Code Validation – BVI Condition
Mic 7
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Mic 9
Predicted levels
lowered by 20
Pa for clarity

Compared predictions with DNW acoustic measurement
 Contemporary design 4-bladed rotor for utility helicopter
 μ = 0.2 and CT=0.0056 and zero shaft tilt angle (wind tunnel conditions
not fully reported)
 Two mic positions, Mic 9: Ψ=150º and 25º below; Mic 7: Ψ=150º in-plane.

Aerodynamic calculation was performed by RCAS free vortex-wake
model
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
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PENNSTATE
Transient Maneuver Noise Identified
Moving Observer Location
70
60
0.7
70
65
85
90
Thickness Noise
80
1.1
75
75
70
0.9
65
0
20
40
Time (sec)
95
80
90
Total Noise
80
65
85
80
95
0
85
75
95
0
Thickness Noise
Loading Noise
85
90
70
75
0
60
80
60
95
80
95
90
75
Loading Noise
90
75
OASPL (dB)
Rotor Normal Force Ratio
1.3
0.7
80
50
85
95
Thickness Noise
OASPL (dB)
1.1
85
OASPL (dB)
80
OASPL - Thickness (dB)
1.3
OASPL - Loading (dB)
Rotor Normal Force Ratio
Fixed Observer Location
0.9
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85
80
75
0
20
40
Time (sec)
20 Noise
40
Total
60
80
60
80
60
80
LoadingTime
Noise
(sec)
Observer
Location: 30R
form rotor hub,
45º below rotor
and 120º
20
40
60
80
Time (sec)
azimuth
angle
20
40
60
80
Time (sec)
Total Noise
Observer Location : (800, - 400, 0) m
90
90
Rotor Normal
Force85Ratio*
85
OASPL (dB)
80
0
20
80
0
40
Time (sec)
* Rotor Normal Force / Gross Weight
Kenneth S. Brentner, Dept. of Aerospace Engineering
60
80
20
40
Time (sec)
RCOE Review, May 3, 2005
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PENNSTATE
Turn-Entry Maneuvering Flight
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Right Turn

Both right and left turn-entry
maneuvering flights.

Three different turn transient
duration settings: 0.5, 1 and 5
seconds.

Focus on the helicopter roll
maneuver.
Left Turn
0.5 sec duration
1 sec duration
5 sec duration
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
10
PENNSTATE
Acoustic Signature with
Different Roll Rates
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OASPL - Thickness (dB, ref 20 Pa)
OASPL - Thickness (dB, ref 20 Pa)
Thickness noise
85
80
75
70
OASPL - Loading (dB, ref 20 Pa)
OASPL - Loading (dB, ref 20 Pa)
95
90
85

0
2
4
6
Time (sec)
0.5 sec duration
1 sec duration
5 sec duration
75
70
65
Loading noise
Retreating Side
80
80
8
10
Advancing Side
95
90
Observer locations:

45º below rotor tip path plane

30 R from rotor hub

Upstream ±60º from centerline
85
80
75
0
2
6
4
Time (sec)
8
10
OASPL “spike” amplitude is a strong function of transient duration
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
11
Disk Loading in Right Turn-Entry PENNSTATE
Maneuver
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0.5s duration
Kenneth S. Brentner, Dept. of Aerospace Engineering
1.0s duration
5.0s duration
RCOE Review, May 3, 2005
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PENNSTATE
Rotor Wake Geometry for Right Turn

Wake bundling effect starts from Rev 27

Interaction of wake bundle and blade result in a “Super BVI”
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 occurs in both Revs 28 and 29

Helicopter roll overshoot during maneuver is partially responsible
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
13
BVISPL Prediction in Right
Turn-Entry Maneuver
0.5s duration
Kenneth S. Brentner, Dept. of Aerospace Engineering
1.0s duration
PENNSTATE
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5.0s duration
RCOE Review, May 3, 2005
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Disk Loading in Left Turn-Entry
Maneuver
0.5s duration
Kenneth S. Brentner, Dept. of Aerospace Engineering
1.0s duration
PENNSTATE
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5.0s duration
RCOE Review, May 3, 2005
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PENNSTATE
Rotor Wake Geometry for Left Turn
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The wake bundling effect observed in the retreating side.

The wake bundling effect also occurred in the advancing side but
less interaction with rotor blades.

The strength of the “super BVI” is less than what we observe in
the right turn.
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
16
BVISPL Prediction in Left
Turn-Entry Maneuver
0.5s duration
Kenneth S. Brentner, Dept. of Aerospace Engineering
1.0s duration
PENNSTATE
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5.0s duration
RCOE Review, May 3, 2005
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PENNSTATE
Summary for Turn Maneuvers
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
Both right and left turns experienced vortex bundling in the
transient maneuver condition. Right turn maneuver has
stronger bundling and interaction in the aggressive turn.

The overshoot in roll attitude results in strong BVI during
the right turn maneuver. It is like a mini roll-reversal
maneuver.

A more aggressive maneuver triggers a stronger wake
bundling condition. As this bundled tip vortices encounter
the rotor during the maneuver has the potential to generate
very high level of impulsive loading and BVI noise.

Right turn maneuver generated higher noise level than the
left turn.
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
18
A More Complex Example:
LRL Roll Reversal Maneuver
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PENNSTATE
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The LRL roll reversal maneuver
consists of three components
within 6 sec:
 A -50º left roll over approximately
2 sec.
 A 100º right roll over
approximately 2 sec.
 A second left to zero roll angle
over approximately 2 sec.

The advance ratio for maneuver
was relatively low, μ=0.093
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
19
Disk Loading in the LRL Roll
Reversal Maneuver
Kenneth S. Brentner, Dept. of Aerospace Engineering
PENNSTATE
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RCOE Review, May 3, 2005
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PENNSTATE
LRL Roll Reversal Maneuver

The high level BVISPL
concentrated in the forward area
at beginning of the right roll
(t =7.24 s)

The very large BVISPL levels
ahead of the rotor at t = 8.25 s
and t = 8.65 s are primarily
caused by BVI loading during
Revs 36 and 37

As helicopter returns to level
flight, both advancing and
retreating side BVI are present
(t = 10.66 s)
Kenneth S. Brentner, Dept. of Aerospace Engineering
1 8 5 5
RCOE Review, May 3, 2005
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PENNSTATE
Summary for Roll Reversal Maneuver
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
In these maneuvers, BVI noise dominates

BVI noise during a transient maneuver is different than in
steady flight
 Vortex bundling
 Dynamic state of vortex system (not steady after start of
maneuver)

The formation of the vortex bundle and its subsequent
interaction with the rotor blades was strongly influenced by
the pilot overshoots in the turn-entry maneuver

Due to the short duration of maneuver duration, the
helicopter is constantly in the transient maneuver state and
the noise generated in this condition can be considered as
transient maneuver noise
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
22
PENNSTATE
Accomplishments

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2004 Accomplishments
 Limited noise prediction system validation against wind
tunnel measurement for both thickness and loading noise
 Systematically unraveling the source of maneuver noise
 Transient maneuver noise for climb, acceleration
maneuver flights.
 Compute maneuver noise with BVI using UMD maneuver
wake
 Rotor wake interaction analyzed for elemental maneuvers
 Roll maneuver, quick stop, roll reversal maneuvers

2005 Planned Accomplishments
 Investigate issues of signal processing for aperiodic
conditions
 RCAS maneuver model with free vortex-wake model
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
23
PENNSTATE
Schedule and Milestones
Milestones
CODE DEVELOPMENT:
•Initial aircraft motions and complete
rotor motions
•Validate with WOPWOP
•Self-scheduling parallel
implementation
• Coordinate transformation
enhancements
•Acoustic analysis of non-periodic
time history data
•“Flight-test” modeling (GENHEL
coupling)
•Efficiency enhancements (real-time?)
2001
2002
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2003
Complete
In Progress / Near Term
Long Term
Moved from last year’s
schedule
2004
2005
ANALYSIS
•Determine spatial regions where
noise depends strongly on wake.
•Simple maneuvers analysis
•Simple flight path and attitude
determination
•Validation (with data – flight or
wind tunnel)
•Advanced wake modeling (RCAS
or UMD maneuver wake)
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
24
PENNSTATE
Technology Transfer Activities

Papers:
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AHS Specialists’ Meeting, San Francisco, Jan 2004
AIAA Aerospace Science Meeting and Exhibit, Jan 2004
AIAA/CEAS Aeroacoustics Conference, May 2005
AHS Annual Forum, Grapevine, TX, June 2005
Other Interactions:
 Collaboration with Gordon Leishman, University of
Maryland
 Work with Professor Horn: GENHEL coupling and work
toward acoustic prediction capability in new flight
simulator
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
25
PENNSTATE
Other Impact
Leveraging
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or Attracting Other Resources or Programs
DURIP equipment funding for RCOE
124 processor RCOE parallel cluster
Rotorcraft flight simulator with acoustic simulation capability
NASA LaRC contract for high-speed maneuver noise prediction
modifications to PSU-WOPWOP (Burley/Boyd)
Teamed with Georgia Tech for DARPA “Helicopter Quieting” Project
Phase I SBIR with Continuum Dynamics for real-time rotor noise
prediction (NASA LaRC)

Recommendations at the last
review (2004)
 It is recommended to pick
concrete physical problems and a
firm plan is needed to solve
physics or physical mechanisms,
such as effects of roll or Lock
number on noise. And also
validation of analysis is needed
for steady flight first, before
deeply involved with maneuvering
flight conditions.
Kenneth S. Brentner, Dept. of Aerospace Engineering

Actions Taken (2004)
 Some validation for steady flight
performed
 Focus on physical mechanisms of
associated with aircraft roll, including
BVI noise in maneuver
 Gaining understanding of role of BVI
and nonimpulsive noise in maneuver
RCOE Review, May 3, 2005
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Any Questions … ?
Kenneth S. Brentner, Dept. of Aerospace Engineering
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RCOE Review, May 3, 2005
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PENNSTATE
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Auxiliary Presentation Material
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
28
Validation of GENHEL/PSU-WOPWOP:
Comparison with Wind Tunnel Data
measured
(Visintainer et al., 1993)
10
Predicted
0
(GENHEL/PSU-WOPWOP)
-10
-20
0
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20
Microphone 7
Microphone 7, MAT=0.690
0.25
0.5
0.75
1
Acoustic Pressure (Pa)
Acoustic Pressure (Pa)
20
PENNSTATE
Top View
0
-20
-40
0
Microphone 7, MAT=0.796
0.25
Normalized Time
0.5
0.75
1
Normalized Time
1.5 D
30 Deg.
1.5 D
Microphone 1
20
10
Side View
0
1. 5 D
-10
In-plane Microphones
Microphone 1, MAT=0.690
-20
0
0.25
0.5
0.75
1
Normalized Time
Kenneth S. Brentner, Dept. of Aerospace Engineering
Acoustic Pressure (Pa)
Acoustic Pressure (Pa)
20
0
-20
Microphone 1, MAT=0.796
-40
0
0.25
0.5
0.75
1
Normalized Time
RCOE Review, May 3, 2005
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PENNSTATE
80-Second Maneuver Flight Simulation
Level
t = 0 sec
t = 1 sec
Climb
t = 1 sec
t = 14 sec
Acceleration
t = 1 sec
t = 18 sec
Level
t = 14 sec
t = 22 sec
Coordinated Turn
t = 22 sec
t = 56 sec
Level
t = 56 sec
t = 80 sec
35
15
-5
210
Pilot controls
90
55
20
60
45
30
70
100
50
-10
30
0.3
70
0.2
0.1
0
0
Kenneth S. Brentner, Dept. of Aerospace Engineering
Lateral Cyclic (%)
Pitch Angle (deg)
-15
Long. Cyclic (%)
End Time
-5
Collective (%)
Start Time
5
Pedal (%)
Maneuver
Roll Angle (deg)

Aircraft response
Yaw Angle (deg)

Helicopter gross weight: 74800N
4-bladed articulated main rotor and tail
rotor
Main rotor radius: 8.18 m
Adv. Ratio

1 8 5 5
20
40
60
Time (sec)
80
55
40
0
20
40
60
Time (sec)
RCOE Review, May 3, 2005
80
30
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Arrested Descent Wake (From UMD,
Ananthan & Leishman)
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Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
31
PSU-WOPWOP Validation
Comparison with WOPWOP
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WOPWOP - Thickness
PSU-WOPWOP - Isom thickness
PSU-WOPWOP - Thickness
40
20
Acoustic pressure (Pa)
PENNSTATE
Operating conditions:
• UH-1H model scale
0
untwisted rotor
• MH=0.88
• Observer at 3.09 R in
plane
• Rotation only (hover)
-20
-40
-60
-80
 Thickness and loading noise
predictions validated
-100
-120
-140
0.0255
0.026
0.0265
0.027
time (s)
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
32
PENNSTATE
PSU-WOPWOP Features

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Permeable surface
formulation
 Coupling with CFD
for high-speedimpulsive noise

Upper surface
Object oriented
approach
 Modularity and
flexibility for
complex rotor
configuration
Lower surface
Tip
Main rotor blade description
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
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PENNSTATE
Arrested Descent Maneuver

Starts from 6º flight path angle
and μ=0.186.

A half-doublet collective pitch
input applied between t = 5 and
t = 6 s.

At the end of the maneuver,
the helicopter is pitched up by
over 20º.
Kenneth S. Brentner, Dept. of Aerospace Engineering
1 8 5 5
RCOE Review, May 3, 2005
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PENNSTATE
Acoustic Pressure Prediction
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Free Vortex-Wake Model
– ● – Pitt-Peters Inflow Model
Observer location Ψ=135º, 22º below
the helicopter and 7R away.
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
35
PENNSTATE
Summary For Arrested Descent
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
This arrested descent maneuver is a simple maneuver by
applying collective pitch input.

In the steady descent condition, BVI is not dominant source of
noise due to steep flight path angle.

In this maneuver, the primary effect of the maneuver is that the
rotor wake goes through the rotor disk resulting in several BVIs
in the rear of the disk that are nearly parallel to the rotor blade
during the interactions.

Less BVIs were observed after the maneuver due to helicopter
attitude.
Kenneth S. Brentner, Dept. of Aerospace Engineering
RCOE Review, May 3, 2005
36
PENNSTATE
Arrested Descent
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collective pitch (deg)
8
-0.5
6
-1
4
-1.5
2
-2
0
2
Z (m)
– Initial condition: 3 degree steady
descent
– Total time: 2 sec
– Flight speed: 40 m/s
8
Deceleration (m/s )
• Case Description
height
deceleration
0
-2.5
6
-3
4
10
-2
-4
0
20
40
X (m)
60
80
Roll
Pitch
Yaw
8
2
Aircraft 6
Attitude, 4
deg
0
2
-2
0
0.5
1
1.5
2
time (s)
Descent arrested by collective pulse
Kenneth S. Brentner, Dept. of Aerospace Engineering
0
-2
0
100
X, ft
200
300
RCOE Review, May 3, 2005
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PENNSTATE
Sound Pressure Level Computation
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• Frequency analysis issues
– Non-periodic signal
– Noise widely fluctuating in amplitude and frequency
Extract slice of data
Discrete Fourier Transform
Compute sound pressure level
Move
slice of
data
Acoustic pressure (Pa)
Apply Hanning Window
6
4
2
0
-2
0
Kenneth S. Brentner, Dept. of Aerospace Engineering
0.5
1
time (s)
1.5
RCOE Review, May 3, 2005
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38