A long-range advanced turboprop en route noise database was obtained with weather, tracking, and onboard measurements. In-flight noise directivity measurements were made. Data repeatability within a test day was excellent. Day-to-day variability existed and is not completely understood and therefore not predicted. Comparison of a two-dimensional ray-tracing propagation model with the ensemble average ground-measured data was good; however, as stated above, the day-to-day data variability was not completely predicted. Future research will include looking at alternative propagation models. Three-dimensional ray tracing, fast field program, and the parabolic equation are possibilities. The effect of turbulence needs to be accessed

Garber, Donald P.

Willshire, William L., Jr.

English

NASA Technical Reports Server

_-7/
N 9 0 --24 8 55:_3_
PTA EN ROUTE NOISE MEASUREMENTS
William L. Willshire, Jr.
NASA Langley Research Center
Hampton, VA
._v
Donald P. Garber
Planning Research Corporation
Hampton, VA
13
https://ntrs.nasa.gov/search.jsp?R=19900015539 2020-03-19T22:02:26+00:00Z
EN ROUTE NOISE TECHNICAL ISSUES
Development of the advanced turboprop has led to concerns about en route noise.
Advanced turboprops generate low frequency, periodic noise signatures of relatively
high levels. As demonstrated in a flight test of NASA LeRC's Propfan Test Assessment
(PTA) airplane in Alabama in October 1987, the noise of an advanced turboprop
operating at cruise altitudes can be audible on the ground. The assessment of the en
route noise issue is difficult due to the variability in received noise levels caused by
atmospheric propagation and the uncertainty in predicting community response to the
relatively low-level en route noise, as compared to noise associated with airport
operations.
The En Route Noise Test was designed to address the atmospheric propagation of
advanced turbroprop noise from cruise altitudes and consisted of measuring the noise
of an advanced turboprop at cruise in close proximity to the turboprop and on the
ground. The in-flight noise measurements were made by flying an instrumented
airplane in formation with the PTA airplane. The ground measurements were made
by flying the PTA airplane over a microphone array.
PTA EN ROUTE NOISE MEASUREMENTS
z
TECHNICAL ISSUES
• PROPAGATION INDUCED
• SUBJECTIVE RESPONSE
VARIABILITY
Figure 1
14
EN ROUTE NOISE TEST GOALS
The En Route Noise experiment had three goals. To acquire a long-range propeller
noise database designed to study propagation, to investigate propeller noise variability,
and to compare measured propagation data with ray-tracing propagation model
predictions.
• ACQUIRE LONG RANGE (VERTICAL) PROPELLER NOISE
DATA BASE DESIGNED TO STUDY PROPAGATION
• INVESTIGATE PROPELLER NOISE VARIABILITY
• COMPARE MEASURED AVERAGED PROPAGATION DATA WITH
RAY TRACING PROPAGATION MODEL
Figure 2
15
EN ROUTE NOISE TEST APPROACH
The approach taken to achieve these goals was to perform at White Sands Missile
Range a flight experiment using the Propfan Test Assessment airplane. The flight
experiment would use multiple-microphone array technology to measure on the
ground the noise levels of an advanced turbroprop operating at cruise conditions. The
in-flight noise directivity of the advanced turboprop blade passage harmonics would be
measured by flying an instrumented aircraft in formation with the test airplane. The
in-flight measured directivity of the turboprop would be used as input in propagation
models to predict the ground-measured average noise values. Participates in the En
Route Noise experiment were NASA Lewis Research Center, the FAA, and NASA
Langley Research Center. NASA LeRC was responsible for providing and operating
the PTA, and performing the in-flight noise measurements.
• CONDUCT PTA FLIGHT TEST AT WSMR WITH CONCURRENT
WEATHER PROFILES
• USE MULTIPLE-MICROPHONE ENSEMBLE-AVERAGING DATA
ANALYSIS
• MEASURE IN-FLIGHT SOURCE DIRECTIVITY
Figure 3
=
16
PROPFAN TEST ASSESSMENT AIRPLANE
The PTA airplane is shown in this photograph. The PTA airplane is a Gulfstream II
with an advance turboprop and engine mounted on its left wing. The advanced
turboprop is an eight bladed, 9 ft diameter, single propeller in a tractor configuration.
The advanced turboprop operated with supersonic helical tip Mach numbers. The PTA
airplane was instrumented with microphones mounted on the inboard boom on the
left wing and with surface-mounted microphones on the outside of the fuselage.
Engine and turboprop parameters, as well as other pertinent flight parameters, were
also measured on board the test airplane.
Figure 4
ORIG[NAE PAGE
BI.,ACK AND WHITE PHOTOGRAp_
17
EN ROUTE NOISE TEST MICROPHONE ARRAY
The microphone array used in the En Route Noise test was basically an eight-element
linear array with a 400-ft element spacing. The microphone array was located at
Gran Jean site in the North Range of WSMR.* Each of the eight array elements was
equipped with an analog and a digital microphone system mounted on ground boards.
Co-located at one array element were an analog and digital microphone pair mounted
1.2 m above the ground. The FAA had a ground-mounted and a 1.2 m mounted
microphone at another element of the microph6n6 array and at a site located
approximately5 miles north of the microphone array. The digital microphone systems
consisted of standard 1/2-in. condenser microphones with an analog-to-digital
converter located in the microphone power supply boxes. In the power supply boxes
the analog signal from the microphone was digitized at the rate of 2344 samples per
second. The data presented in this paper are from the digital microphone systems. The
test airplane flight path was parallel and over the microphone array.
GranJean Site
Balloon Site
1,1
FAA 5 Mile Site
Van Sitee
4,14
Scale
xlO0ft
40Oft r
1
North
2,12
3,13
•9,19
5,15 KEY
• Ground Board Mic. Pair
"6,16 • 1.2rn Mic. Pair
m7,17 • FAA grd and t.2rn Mlos.
=_8,18 " Ana_g and-Digital
microphone systems
t
Figure 5
ORTGINAE PAGE
BLACK AND WHLTE PHOTOGRAPh"
*White Sands Missile Range
18
EN ROUTE NOISE TEST WEATHER MEASUREMENTS
The various means used to measure weather information are illustrated in this
photograph. The primary weather information was obtained from free balloon
radiosonde releases. The radiosondes were released from the microphone array site
before and after each test session. A typical test session was an hour to an hour and
a half. The next important source of weather measurements was a tethered weather
balloon system which continuously made profiles up to 1500 m during a test session.
Six weather stations of various heights were located in a half-mile circle around the
microphone array. An acoustic sounder was located 4 miles northeast of the
microphone array.
|
!
Figure 6
ORIGrNAE PA(_E'
BLACK AND WHrTE PHOTOGRAPH
19
EN ROUTE NOISE TEST COMPLETED TEST MATRIX
The completed test matrix is illustrated in this table. Eighty-eight runs or passes over
the microphone array were recorded. The primary test parameters were aircraft Mach
number and altitude. The majority of the runs were the high-altitude cruise conditions
with a tip speed of 800 ft/s for the advanced turboprop. However, for a limited amount
of runs the advanced turboprop tip speed Was varied through the range of 620 to
840 ft/s. _ :
LL
PTA
SPEED, M
.5
.7
.77
ALTITUDE, 1000 FT. AGL
2 9 15 30
4 4 23
19 32
6
TOTAL RUNS: 88
Figure 7
:::q:
=_2
-:3 "._'_ZZ Z
_'.T _ .
2O
-_ ,¢t .....
-- Z Z
,'- .
ENSEMBLE AVERAGING
The data to be presented in this paper were obtained through ensemble averaging of the
eight ground-mounted digital microphone systems. The steps in the ensemble process
are the individual microphone time histories are high passed filtered at 80 Hz to
minimize the influence of wind noise; individual microphone 1/2-second mean
square pressure time histories are calculated; each microphone time history is shifted
in time based on measured ground speed of the test airplane along the microphone
array to give all microphone time histories a common time base; finally the eight
shifted time histories are averaged together to form an ensemble average 1/2-second
mean square pressure time history. Illustrated in the figure are noise level time
histories. However, the ensemble averaging is done on a linear pressure squared basis.
The ensemble result, the last plot in the figure, exhibits less variability than the
individual microphone time histories.
Mic
1
2
3
4
5
6
7
8
80
70
C_3
"0 60
_5o
U'I
40
30
I I [, , J , I , I I
0 60 120 180 240
Time, sec.
1
300
I
360
Figure 8
gl
EN ROUTE NOISE TEST ENSEMBLE AVERAGE RESULT
The previous ensemble average result is magnified in this figure for illustration. This
example is for a test condition of an airplane speed of Mach .7 and 30,000 ft AGL*
altitude. Plotted with the ensemble result are the 80-percent confidence intervals for
the average. The 80-percent confidence intervals bound an area in which there is an
80-percent probability that the true a_cerage ex_sts_ It should be noted that this _esult
and every result to be presented in this paper are from as measured ground level data.
The effect of pressure doubling due to the ground-mounted microphones remains in
the measured results. Ensemble average time histories like this were calcuiated for
each run. The maximum 1/2-second Overall Sound Pressure Level (SPL) from the
ensemble average time histories were determined. In this example the maximum
Overall SPL is 70 dB.
SPL,
dB
8O
6O
4O
A/C Speed: .7 Mach
Peak 70 dB_ 0 ft
'".... ' Confidence Interval: 80%
20 , l, v , 1 , ,..
0 100 200
| l I l ..... I r
Time, sec.
300
Figure 9
*Above ground level
22
EN ROUTE NOISE TEST AVERAGE MAXIMUM OVERALL SPL
Average ground level maximum 1/2-second Overall SPL's are given in this table
averaged over like-test conditions for the whole database. Average values and the
range of the values which went into the averages are given in the table. Approximately
20 runs were averaged for each of the 15 and 30-thousand-foot altitude results. Four
runs each were averaged in the 2 and 9-thousand-foot altitude averages. One thing
which stands out is the large range associated with the averages. Another is that
expected trends might be obscured in the averages by the wide data ranges. For
example, in the two 15,000-ft altitude test conditions, there is no change in the average
overall SPL for the two test speeds. The lower test speed would be expected to have a
lower noise level.
Alfitude, 1000 ff AGL
M(:::]c h #
.5
.7
2 9 15 50
91 81 73
(86-93) (80-81) (70-75)
73
(66-76)
Avg
Range
Avg
Range
Figure 10
23
EN ROUTE NOISE TEST DAILY AVERAGE MAXIMUM OVERALL SPL
In this table are presented daily averages for like-test conditions of maximum ground
level maximum 1/2 second Overall SPL. Standard deviations and number of runs in
the daily averages are also given in the table. In general there was good repeatability on
a daily basis for like-test condition. The standard deviations are often less than 1 dB.
On April the 8th, the standard deviation for 11 like-runs was -.8 to .7 dB. However,
there was considerable day-to-day variability. For the 32;000 ft, .7-M test_condit!on there
was a 12 dB range in average levels. The advanced turboprop source noise, measured
in flight, was very consistent within a test day and from test day to test day. The
observed average level day-to-day variability is propagation-induced.
TEST
CONDITION
30,000 FT.,
.7 M
15,000 FT.,
.7 M
15,000 FT.,
.5 M
KEY
AVG, dB
a, dB
No.
AVG, dB
a, dB
No.
AVG, dB
a, dB
No.
3
60.8
-1.6/1.2
2
75.0
-2.2/1.5
2
72.2
-.6/.6
2
illll
4
69.0
-.7/.6
4
III II
72.6
-.5/.5
2
TEST DATE
5 6 A.M.
60.7 65.1
-.2/.2 -1 ./.8
4 4
III II II
67.7 69.7
-1.3/1.0 -1./.8
4 4
70.7 70.2
-1.1/.9 -.2/.2
4 3
6 P.M. 8
=,1=|
75.0
-2./1.3
3
74.7
-.1/.1
2
74.4
-.8/.7
11
11 13
67.8 72.2
-3.5/1.9 -1 ./.8
3 4
I IFIII
74.3
-2.1/1.4
4
_=
Figure 11
24
EN ROUTE NOISE TEST AVERAGE SINGLE MICROPHONE DEVIATION
Another way to look at the variability of the ground measured PTA turboprop noise is
to look at the distribution of the eight microphones about the ensemble average for the
eight microphones. Plotted in this figure is the probability density function of the
deviation of the eight single microphones about the ensemble average of four 30,000-ft,
.7-M runs measured on the same day. Deviation in this figure is expressed as a
percentage and is defined as the difference between a 1/2 second time shifted mean
square pressure estimate for a single microphone and the corresponding 1/2-sec6n_
ensemble average estimate. The difference is then divided by the ensemble average.
Deviations were calculated for each microphone time history approximately 20 seconds
on either side of the time associated with the maximum overall Sound Pressure Level.
The average of the deviations is zero as it should be with a standard deviation of
64 percentage points. Once the actual probability density function is established,
probabilities of certain values of deviation can be ascertained. The general shape of the
probability density function is skewed to the left with the probability that the deviation
from the average is less than 0 being 62 percent. The shape of the PDF and the
associated probabilities are typical of the ones measured for other runs and other days.
30,000 FT., .7 M TEST CONDITION
1,
£2_
Avg 0.000 Sigma
.05 [--- Median -18.000/
I I I 1.
-I00.00
O.
63.941
Prob __ -50. is 0.23
Prob _ 0. is 0.62
Prob __ 50. is 0.85
Prob __ 100. is 0.93
Prob <: 200. is 0.98
1
o. loo.oo 2oo.oo 3oo.oo
Devic]fion, %
Figure 12
25
EN ROUTE NOISE TEST NOISE PREDICTION METHOD
In order to produce ray-tracing results to compare with the ground-measured PTA
advanced turboprop noise, the following procedure was used. The PTA advanced
turboprop source noise levels used as input to the ray-tracing propagation model were
predicted using Langley's Aircraft Noise Prediction Program (ANOPP). Measured
averaged flight parameters were used to generate a prediction for each_test_0ndLtion/_
Compared to the measured in-flight noise levels, the ANOPP predicted noise levels
were over predicted. In-flight measured noise levels from the chase airplane were Used
to empirically correct the amplitude of the predicted directivity patterns. The predicted
directivity patterns agreed well with the measured ones and were used i_he_ray -_-
tracing because the predicted directivities covered a larger angle range than the
measured directivity patterns. The ray-tracing model employed was a-2-dimensionai
model. Measured flight paths and atmospheric profiles were used in the fay-fracing
model. Atmospheric absorption was calculated by the ANSI standard method. A hard
ground assumption, 6 dB for pressure doubling for-the ground2m0unted microphones,
was used in the model. _
Source prediction performed with ANOPP
measured flight conditions
source level corrected using "in flight" measured data
predicted source directivity used
Propagation performed by 2-D ray tracing program
I= flight path from c-band radar
atmospheric profile from free flight balloon launch
atmospheric absorption by ANSI standard method
hard ground
Figure 13
Z
EN ROUTE NOISE TEST COMPARISON OF GROUND MEASURED DATA
WITH RAY-TRACE PREDICTION
Run 112
A comparison of a ray-tracing result with ensemble average PTA data is given in this
figure for a 30,000-ft, .7-M run. The 80% confidence bounds are included with the
ensemble average measured result. The ray-tracing result is the bold solid line. The
agreement between measurement and prediction for the flyover is good in amplitude
and in shape.
Comparison of Measured Data with
Predictions for PTA Flyover
Flight 112 of 6 April 1989
Scaled
8O
, , t t I i , i i I ' ' I J
- _ Mean estimate from -
eight microphone array
m ............. 80% confidence bounds
_ ._,..._" .,,::_.,,.. about mean estimate -_
d : :";""_. from microphone array60
I T
(J') ! ',.,. • ._,:.. :. :':_.:,;'_,:'" ._-, ,.
< j'_'
©
40I'i""'*:'/ I Prldicli°n__l I I I I I I '-_
0 5O 100 150
Time, sec
Data summary file:
Raytrace file:
DUA0:[GARBER.OPEN]WCF1 12.ANA; 1
DUA0:[GARB/R.TRAC]ADJ 112. OAS; 1
Figure 14
27
EN ROUTE NOISE TEST COMPARISON OF GROUND MEASURED DATA
WITH RAY-TRACE PREDICTION
Run 110
Lest anyone think that there is no problem, in this figure is another comparison
between measurement and prediction for another 30,000-ft, .7-M run. The agreement is
not as good as in the previous comparison. In this figure the ray-tracing result over-
predicted the measured result, and the predicted flyover shape is a little off. Ray tracing
does not predict all of the day-to-day variability seen in the measured results. For the
30,000-ft, .7-M test condition, the ray-tracing predictions showed approximately 7 dB
variation throughout the test, as compared to the 12 dB variation in the measured
average peak 1/2-second Overall SPL
Comparison of Meas ed Data With-Sca°ied
Predictions fOr-PTA Flyover
Flight 110 of 5 April 1989
8O
rn
-o
60
00
<
O
4O
Mean estimate from
eight microphone array
............80% confidence bounds
_ about mean estimate _
from microphone array
-- '.1_--' "" -_X , --
.f ' ..%, •
:, _; ,: :'.v':'"", :"v"''""/" ':"':'"'"'J '(_,.[i....""'".n"',-,".,.,/_.v_
0 15050 100
Time, sec
28.
Doto summary file:
Roytroce file:
DUAO:[GARBER.OPEN]WCF1 10.ANA; 1
DUAO:[GARBER.TRAC]ADJ 110.OAS; t
Figure 15
=
EN ROUTE NOISE TEST CONCLUDING REMARKS
In conclusion, a long-range advanced turboprop en route noise database was obtained
with weather, tracking, and onboard measurements. In-flight noise directivity
measurements were made. Data repeatability within a test day was excellent. Day-to-
day variability existed and is not completely understood and therefore not predicted.
Comparison of a two-dimensional ray-tracing propagation model with the ensemble
average ground-measured data was good; however, as stated above, the day-to-day data
variability was not completely predicted.
Future research will include looking at alternative propagation models. Three-
dimensional ray tracing, fast field program, and the parabolic equation are possibilities.
The effect of turbulence needs to be accessed.
• A LONG-RANGE PROPELLER DATA BASE WAS OBTAINED
• DATA REPEATABILITY WITHIN A TEST DAY WAS GOOD -
VARIABILITY BETWEEN DAYS IS NOT COMPLETELY UNDERSTOOD
COMPARISON OF RAY TRACING PROPAGATION MODEL TO
ENSEMBLE-AVERAGED GROUND MEASUREMENTS WAS GOOD
DAY TO DAY VARIABILITY NOT COMPLETELY PREDICTED
Figure 16
2O



PTA en route noise measurements

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