A program is being conducted to develop noise standards for wind turbines which minimize annoyance and which can be used to design specifications. The approach consists of presenting wind turbine noise stimuli to test subjects in a laboratory listening chamber. The responses of the subjects are recorded for a range of stimuli which encompass the designs, operating conditions, and ambient noise levels of current and future installations. Results to date have established the threshold of detectability for a range of impulsive stimuli of the type associated with blade/tower wake interactions. The status of the ongoing psychoacoustic tests, the subjective data, and the approach to the development of acoustic criteria/standards are described

Grosveld, F.

Shepherd, K. P.

Stephens, D. G.

English

NASA Technical Reports Server

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WINED TURBINE ACOUSTIC STANDARDS k
David G. Stephens, Kevln P. Shepherd*, and Ferdinand Grosve!d*
National Aeronautics and Space Administration
The Bionetics Corporation*
Langley Research Center
Hampton, Virginia 23665
ABSTRACT
A program is being conducted to develop noise
standards for wind turbines which minimize annoy-
ance and-which can be used in design specifica-
{i_sV=Th_a_proach consists of presenting wind
[urblne noise stimuli to test subjects in a
labora£oryiistening chamber. The responses of
the subjects are recordedfor a range of stimuli
which encompass the designs, operating conditions,
and ambient noise levels of current and future
installa_ions. Results to date have established
the threshold of detectability for a range of
impuisivestimuli of the type associated with
blade/tower-wake interactions. The status of
the ongoing psychoacoustic tests, the subjective
data, and the approach to the development of
acoustic criteria/standards are described.
INTRODUCTION
The development of wind turbines which are
acoustically acceptable to the community requires
an understanding of the human perception of, and
response to, wind turbine noise and any noise
induced building vibrations resulting from their
operation. Subjective data on wind turbine
operations, however, are virtually nonexistent
with the exception of a few cases of reported
annoyance with the MOD-I, a 2000 kw downwind
machine located at Boone, North Carolina. Further-
more the noise associated with wind turbines, and
in particular the low frequency blade/tower-
interaction noise, have temporal and spectral
characteristics which are in a range where very
little subjective data and/or experience are
available (ref. I). Thus, a laboratory study has
been initiated to better understand wind turbine
type noise to provide guidance in the design and
siting of future machines.
The factors which are believed to be important to
the development and application of wind turbine
noise criteria are shown schematically in figure i.
As indicated, the turbine may produce noise
through several source mechanisms which may
result in both impulsive characteristics due to
blade/tower-wake interactions ("thumping") and
nonlmpulsive ("swishing") noise due to unsteady
_low over the blades. These source characteris-
tics are modified before reaching the receiver
due _o atmospheric propagation and terrain effects.
Finally, the effects of wind turbine noise on the
receiver may be modified by many factors such as
the background or ambient noise level, the time
of day, the activity and location of the receiver
(indoors outdoors), and the presence of any per-
ceptible house vibrations induced by the noise.
To assess the impact of the noise, criteria must
be developed which consider the receiver's per-
ception of, and response to, the acoustical
factors (noise level and frequency, for example)
and nonacoustlcal factors (e.g. time of day)
associated with the operation of the wind turbine.
These criteria must include the various source
and path effects both separately and in combina-
tion in order to be useful for the design and
siting of future machines.
431
IMPULSIVE I_ PATH
BROADBANO • Di STANCE
_04 \ -WINDITEMP GRADIENTS
/ -TERRAINk/J
/ _. / • BACKGROUND NOISE
i "_k / /, INDOOR/OUTDOOR
",_ /. TIME-OF-DAY
k /. ACTIVITY
k%k _ ..__RATION
_IANNOYI NG
i '
.... i .......
CRITERION
-'.-PERCEPTIBLE
Figure i - Wind turbine noise factors.
This paper is a status report of an ongoing
program to develop noise criteria to be applied
at the receiver location. Emphasis to date has
been on the determination of the perception
threshold of wind turbine spectra covering the
range of anticipated designs and operating con-
ditions. It should be noted that building vibra-
tion, despite the fact that it has received con-
siderable attention in the case of the MOD-I
operation will not be considered in this report
for _wo reasons. First, the levels recorded to
date suggest that the building response is below
the human (whole-body) threshold of detectability
and, secondly, it is believed that any noise
standards resulting from these studies will yield
noise levels low enough to preclude perceptible
building vibration (refs. 2 and 3).
PROCEDURE
Tests were conducted to determine the threshold
of detectability for the impuisive _ithumping"
sounds which result from blade/tower-wake inter-
actions. This stimulus is believed to be the
dominant source of annoyance in large downwind
machines such as the MOD-I configuration.
Although the thump resulting from the blade
passing the wake of the tower is uniquely defined
by the time history of the pressure pulse, it is
more common to define the noise by a frequency
spectrum which, with information on the phase
relationship between harmonic components,
completely describes the noise signature. Since
phase information is not always available from
https://ntrs.nasa.gov/search.jsp?R=19820015857 2020-03-21T17:02:29+00:00Z
measurements or calculations, a preliminary study
was conducted to examine the importance of phase
to the subjective detectability of the noise.
Four phase conditions were examined; three having
coherent phase relationships and one being random.
For the first three (nonrandom) conditions, the
threshold of detectability was found to be
independent of phase and lower in level than that
found for the random phase condition. For this
reason, the sounds used in this study had a
coherent phase relationship between harmonic
components.
Test stimuli were computer generated and con-
sisted of a fundamental frequency (blade passage)
and up to 250 harmonics for which amplitude and
phase were defined. Since the sound amplifica-
tion/reproduction system introduces phase and
amplitude distortion, the transfer function
between the output from the computer and a micro-
phone placed at the location of the test subject's
ear was calculated. This transfer function was
incorporated in the noise generation software,
enabling the desired spectra and time histories
to be produced in the anechoic test facility,
figure 2. This facility has dimensions of
4m x 2.5m x 2.5m (cutoff frequency of 150 Hz) and
is equipped with two loudspeakers having a fre-
quency response of 5 Hz to 20 kHz.
Figure 2 - Anechoic test facility.
Each Subject was seated in front of the loud-
speakers and instructed to press a hand-held
switch when they heard the sound (fig. 3). This
switch activated a light which indicated to the
test conductor that a given sound was heard.
The sound pressure level of the sound was slowly
reduced until no longer detectable and then
slowly raised until detectable again. This
process was repeated until consistent ascending
and descending thresholds were achieved. The
mean of these two values was considered to be
the threshold of detectability.
J
LIGHT SIGNAL
!
ANECHOIC ROOM
(_M ICROPHONE
0
HANDSWITCI-
UB'WOOEERJl
Figure 3 - Stimuli presentation and
subjective response system.
RESULTS AND DISCUSSION
The primary objective of the study to date has
been the determination of the threshold of
detectability for impulsive turbine noise having
a variety of spectra in the frequency range from
20 to II0 Hz. Frequencies below 20 Hz, were
considered to be unimportant due to the extreme
insensitivity of the ear to wind turbine noise
levels in this low frequency region, These
spectra were synthesized based on measured
data from the MOD-I site as well as calculations
of the spectra resulting from blade/tower-wake
interactions (refs. 4 and 5). No broadband
(_wishing") noise has been included nor have
ambient noise effects been studied.
Ten spectra, having a fundamental frequency of
either 1.0 Hz or 0.5 Hz, were designed to be
dominated, subjectively, by harmonics at dif-
ferent frequencies. Those having a 1.0 Hz funds-
mental were dominated by components at 20, 40,
60, 80 and i00 Hz and those having a 0.5 Hz
fundamental were dominated by components at 30,
50, 70, 90 and ii0 Hz. Detectability thresholds
were determined for each of the i0 sounds using
nine test subjects, none of whom had significant
hearing loss. The standard deviations of the
threshold measurements were found to be typically
2.5 dB, with a tendency for the spectra having
0.5 Hz fundamental to have the higher standard
deviations.
The narrowband spectra at the mean threshold
level are presented in figure 4. Tangential
curves were fitted to these spectra and are
presented in figure 5 for comparison. Due to
the higher harmonic density, the curve for the
spectra having 0.5 Hz fundamental is lower than
the 1.0 Hz case. Also shown in the ISO pure tone
or minimum audible field (MAF) threshold (ref. 6)
which has the same general shape. The difference
in level between the turbine curves and the MAF
curve may be attributed to the critical bandwidth
of the human ear (ref. 7) which is far greater
than the bandwidth used in the spectral analysis.
432
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SOUND
PRESSURE
_VEL+ 30
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20
30Hz DOMINANT
..... 50 Hz DOMINANT
--C,---- 70 Hz DOMINANT
....... go Hz DOM INANT
_i .... I]O Hz DOMINANT
,.. +' _"_,
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ZO
__ ___--L_ _.L__ I I I
IO 20 30 40 50 60 70 80 90 lO0 llO
- -- - FREQUENCY, Hz
a) 0.5 Hz fundamental
70
60
50
40
SOU_
FRESSURE
LEVEL 30
dB
20
+ ::
]0
0
-I0
20 Hz DOMINANT
A ...... 40 Nz DOMINANT
/ \ --O---- 60 Hz DOMINANT
/ \ ....... 80 Hz DOMINANT
/ _ ..... I00 Hz DOMINANT
/;N /"
_ ," '+l
_I 3._ I I I L __L__ __._J
1O 20 30 40 50 60 lO 80 90 I00 II0
FREQU._NCY, Hz
b) l.O Hz fundamental
rlgure 4 - Narrowhand spectra at the mean
threshold level
The data (fig. 5) suggest that the frequency of
the fundamental is a significant variable. It
is recommended that the i Hz fundamental curve
be used and then an adjustment made on a
logarithmic (energy) basis for the actual funda-
mental blade passage frequency. Thus, the curve
for 0.5 Hz fundamental would be 3 dB lower
and the curve for 2 HZ would be 3 dB higher
than the I Hz fundamental curve. Furthermore,
the frequency analysis bandwidth should be
less than the fundamental frequency. The use
of 1/3 octave or octave band analysis is not
recommended due to the steep slope of the
threshold curves.
433
40
SOUND
PRESSURE 30
LEVEL
dB 20
0
-IO
x_ MAF
_'. _ ..... I Nz FUNDAMENTAL
_,\ _ _.-- 0,5 HZFUNDAMENTAL
lO _0 30 40 50 60 70 BO 90 I00 llO
FREQUENCY, Hz
Figure 5 - Thresholds of detectability
Certain limitations of the preceding results need
to be considered. Mean threshold data have been
presented and consequently many people will be
able to detect sounds at lower l_vels, Also
these tests were conducted in an anechoic environ-
ment wi_h extremely low background noise (masking
by higher levels of background noise will be
considered in the near future). The spectra
used to generate the threshold curves were
specifically designed such that detectability was
achieved over a narrow frequency range. The
threshold level of sounds which have components
at or near the threshold curve over a wider fre-
quency range is unknown at this point in time,
but will be studied in future testing.
CONCLUDING REMARKS
The thrust of the program is to develop the
psychophysical functions relating human response
to each of the wind turbine noise components such
as "thumping" and swishing." Furthermore, the
variation or dependence of these functions on
other acoustical factors such as ambient noise
will be examined, This will enable assessment
of the acceptability of a predicted or measured
noise condition at a receiver location. For
example_ if "thumping" is the dominant noise
source in a low ambient noise situation, the
ideal solution would be to design for noise levels
below the threshold of detectability for impulsive
noise (fig. 5). However, if the measured or pre-
dicted spectrum exceeds the detection threshold,
annoyance may result. The degree of annoyance
cannot be predicted at the present time due to
absence of subjective data under actual operating
conditions but an indication of the growth of
annoyance may be found in classical loudness
studies which show that the growth of loudness
is frequency dependent as shown in Table i.
Frequency dB/doubling of loudness
20 4.5
50 6.1
i00 7.2
500 8.6
i000 i0.0
Table I - Growth of loudness
Exceedlng the threshold of detectability at
20 Hz has more serious consequences than at
higher frequencies. In general, the major use
of the psychophysical relationships will be to
guide the designer/operator In pinpointing the
exact frequency components andhence source
mechanisms which may cause problems with wind
turbine operations.
REFERENCES
i. Tempest, W.: Infrasound in Transportation.
Infrasound and Low Frequency Vibration,
W. Tempest, ed., Academic Press, 1976,
pp. 19-36.
2. Vibration and Shock Limits for Occupants in
Buildings. ISO Standard 2631, 1974.
3° Stephens, D. G. and Mayes, W. H.: Aircraft
Noise-lnduced Building Vibrations. Community
Noise, Peppln and Rodman, eds., Am. Society
for Testing and Materlals, 1979, pp. 183-194.
4, Wells, R. J.:
Noise Studies.
1980.
MOD-I Wind Turbine Generator
General Electric Company,
5, Greene, George C. and Hubbard Harvey H.:
Some Calculated Effects of Non-Unlform
Inflow on the Radiated Noise of a Large
Wind Turbine. NASA TM 81813, 1980.
6. Normal Equal-Loudness Contours for Pure Tones
and Normal Threshold of Hearing Under Free
Field Listening Conditions. ISO Recommenda-
tion R 226, 1961.
7. Kryter, Karl D.: The Effects of Noise on
Man. Lee, Hewson, and Okun, eds., Academic
Press, 1970.
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QUESTIONS AND ANSWERS
D.G. stephens
From: Anonymous
Q: Have you varied the background ambient noise level in your test room to simulate
that at a residence?
A: No, but we intend to.
From: W. Beans
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Q: From test subjects did you obtain the standard deviation due to subject age?
A: No. AIZ subject8 used in the test had good hearing and consequently were relatively
the same young age.
From: D.W. Thomson
Q: Have you considered setting the individual subjects on a shaker table driven from
-_ your synthesizer system so as to provide "body" input? Another possibility would
: _be_o include a loaded china closet in the chamber which might pick up the vibrating
stemware source.
A: No answer was prouided,
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Wind turbine acoustic standards

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