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Author(s)
First Name
Middle Name
Surname
Role
Email
Hugh
W.
Fraser
ASABE
Member
hugh.fraser@om
afra.gov.on.ca
Affiliation
Organization
Address
Country
Ontario Ministry of Agriculture, Food
and Rural Affairs
4890 Victoria Avenue North,
Vineland Station, ON L0R 2E0
Canada
Author(s) – repeat Author and Affiliation boxes as needed-First Name
Middle Name
Surname
Vince
Gambino
Tony
Gambino
Role
Email
Affiliation
Organization
Address
Country
Aercoustics Engineering Ltd.
50 Ronson Drive, Suite 165,
Toronto, ON M9W 1B3
Canada
Publication Information
Pub ID
Pub Date
06-1146
2006 ASABE Annual Meeting Paper
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily
reflect the official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution
does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer
review process by ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this
work should state that it is from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2006. Title of Presentation.
ASABE Paper No. 06xxxx. St. Joseph, Mich.: ASABE. For information about securing permission to reprint or reproduce a technical
presentation, please contact ASABE at [email protected] or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
An ASABE Meeting Presentation
Paper Number: 06-1146
Field Study of the Movement of Sound Produced by
Wind Machines in Vineyards in Niagara, ON, Canada
Hugh W. Fraser, MSc., P.Eng.
Ontario Ministry of Agriculture, Food & Rural Affairs, Box 8000, Vineland, ON
L0R 2E0
Vince Gambino, B.A.Sc., P.Eng
Aercoustics Engineering Ltd., 50 Ronson Drive, Suite 165, Toronto, ON, Canada M9W 1B3
Tony Gambino, Engineering Technician
Aercoustics Engineering Ltd., 50 Ronson Drive, Suite 165, Toronto, ON, Canada M9W 1B3
Written for presentation at the
2006 ASABE Annual International Meeting
Sponsored by ASABE
Portland Convention Center
Portland, Oregon
9 - 12 July 2006
Abstract
Wind machines are tall, fixed-in-place, engine-driven fans that pull warm air down from high above
ground during a strong thermal inversion, raising air temperatures around cold-sensitive crops such
as grapes. They are used in Niagara, Ontario, Canada to protect against cold injury at three times;
winter’s extreme cold temperatures; spring’s late frosts; and autumn’s early frosts. Noise complaints
have multiplied as wind machine numbers have risen. A 3-year applied research project commenced
in 2005 to establish best environmental management practices so the use of wind machines would
be limited to those times when absolutely needed. One part of this project included a sound
characteristic study discussed in this paper. Results show that wind machines have several unique
acoustic properties and that some homes could be more susceptible to noise nuisance than others.
Keywords
Wind machine, cold-injury, grapes, normal farm practice, noise nuisance, low frequency sound
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the
official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not
constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by
ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is
from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2006. Title of Presentation. ASABE Paper No. 06xxxx. St. Joseph,
Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at
[email protected] or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
Introduction
Wind machines are tall, fixed-in-place, engine-driven fans that pull warm air down from high
above ground during a strong thermal inversion (Shaw, T.B., 2001) raising air temperatures
around cold-sensitive crops such as grapes and tender fruits (Figure 1). They help protect crops
from cold-injury that can affect the following year’s crop and long-term plant health. Wind
machines were first introduced in the United States in the 1920’s, and there are currently four
suppliers of wind machines used in Ontario. All have subtle differences in design, however all
are about 10.5 m high from the concrete pad they are anchored on to the axis or centre point of
their blades; all have long blades about 5.4 m to 6.0 m in length; and all have blade speeds in
the range of 375 to 525 rpm generating huge volumes of moving air that will help protect up to 4
ha of cropland. Most types of machines have two blades, but one type has four. Wind machines
are considered ‘upstream’ machines, as they operate with their blades ‘upstream’ of the tower.
Wind machines cost $30,000-$40,000 CDN to install and $30-50 CDN/h to fuel using propane,
diesel, gas, or natural gas. Regardless of make, all wind machines work under similar principles.
Wind machines are used in Niagara to protect against cold injury at three main times during;
winter’s extreme cold temperatures; spring’s late frosts; and autumn’s early frosts. The major
use in Ontario is against winter cold injury in grapes and tender fruit which is a serious problem
that is getting worse because less hardy cultivars are being grown. Normally, significant winter
damage is observed 1 year in 10 in the production area of Niagara, however the winters of
2003-2005 all resulted in a lot of vine and tree injury, with the last year (2004/2005) having
some of the coldest winter temperatures on record. Those growers with wind machines have
harvested larger crops and many growers say they ‘cannot justify not installing wind machines’
and consider it a long-term best management practice. The number of wind machines in Ontario
has about doubled annually from only a handful in the late 1990’s, to more than 425 in 2006.
The proliferation of wind machines has occurred mainly in Niagara-on-the-Lake’s grape growing
area bounded by Lake Ontario to the north, the Niagara River and New York State to the east,
the Welland Canal to the west and Niagara Falls to the south. Noise complaints have multiplied
as the number of wind machines (about 300 in NOTL) rises. Niagara-on-the-Lake has a
population of about 15000 rising over the past few years as a retirement destination. Complaints
about noise include, ‘it sounds like a helicopter’; ‘there’s a droning-sound’, ‘it’s a thumpingsound’, ‘my dishes are vibrating’, and ‘it is worse upstairs in the bedrooms’.
There are no siting controls for wind machines in Ontario. Farmers are protected from nuisance
complaints under Ontario’s Farming and Food Production Protection Act, 1998, providing the
nuisance is created as a result of a normal farm practice. A three-year applied research project
commenced in November 2005 to help establish best environmental management practices so
the use of wind machines would be limited to those times when absolutely needed to protect
crops from damage. One part of this project includes the sound study discussed in this paper.
2
Figure 1: Typical wind machine in a vineyard and schematic cross-section of how one works
Objectives
•
To characterize the sound produced by wind machines commonly used in Niagara
•
To learn why some neighbours are annoyed by wind machine noise more than others
•
To suggest ways to possibly reduce sound emissions
Methodology
Tests were conducted with a calibrated Larson Davis 2900 portable spectrum analyzer. Sound
was measured with a research grade microphone whose frequency response covered the range
4 Hz to 40 kHz. The LD 2900 processed and stored the microphone signal, while the signal was
recorded on a solid state device permitting further signal analysis later.
The test consisted of placing a microphone at 25 m, or about 4 fan blade diameters, away from
each of three wind machine types at a height corresponding to 20o above the centerline of the
airflow, which is in the fan’s slipstream. Measurement at this height minimizes the effects of
sound attenuation from ground influences, and allows better prediction of sound propagation in
the farfield. Because wind machines direct airflow downward 6o from horizontal (Figure 1), this
corresponds to a height of about 16.5 m above ground. A lift bucket with a 10.5 m lift height,
plus a 6 m extendable pole was used (Figure 2).
Three machine types were tested one evening under normal operating conditions, but with slight
northeast winds of between 4 to 8 km/h and temperatures of 4oC; Frost-Boss, 4-bladed;
Chinook, 2-bladed; and an Orchard Rite, 2-bladed. Background sound was also recorded.
Figure 2 shows a picture of one of the wind machines operating during the dark, which did
presented some measurement challenges.
3
Figure 2: Sound levels were measured high above ground in the slipstream of the wind
machines. Measurements were during the evening when wind levels were calmer.
Results and Discussion
Acoustic Terminology
There are two common descriptors used to rate equipment noise; sound power level and sound
pressure level. Sound power is a function of the source which radiates energy irrespective of its
surroundings the same way a 1 kW heater will radiate 1 kW of heat. Sound pressure level is the
magnitude of the pressure fluctuations perceived by the ear. Just as the temperature around a
heater depends on the thermal characteristics of the room and heater, sound pressure depends
on the room and source characteristics. Sound pressure is measured in decibels (dB).
Leq is the average, or equivalent continuous sound that has the same energy as the time
varying signal. However, the human ear’s response to sound is not linear over the audible
frequency range, so sound levels are often weighted in a manner analogous to the ear’s
characteristics. Leq (dBA) is the weighting method often used in noise impact assessment.
People hear in the range 20 Hz to 20 kHz. Low-frequency sounds from low infrasonic up to 100
Hz are difficult for people to hear, but can cause secondary acoustical and vibration problems.
Low frequency sounds are the ones you hear while sitting at a stop sign and a car rolls up
beside you with the bass of their radio shaking their car and yours. Low frequency sounds
decay at a slower rate with distance than higher frequency sounds, so they can travel very
efficiently for large distances. They can also penetrate homes, causing annoyances that are
often perceived to be worse inside than outside (Shepherd et al, 1990).
4
Five Key Areas of Noise Generation By Wind Machines
1. Blade noise is generated by the action of lift forces on the rotating blades. The relative
position of the blades and a listener changes in time as the blades rotate about their axis. This
motion is repetitive and periodic, so the result is a series of tones.
2. Blade slap noise is generated by vortex formation and shedding in the flow past the blade. It
is a distinctive low frequency throbbing sound which is dependent on the loading of the blades.
Blade slap consists of a broadband spectrum and is superimposed on the discrete frequency
noise at harmonics of the blade passage frequency.
3. Airflow noise is generated by the turbulent boundary layers that develop near the blade
surfaces, as well as the turbulent flow in their wake.
4. Aeolian tone noise is generated by the separation of airflow just ‘downstream’ of the tower
and by the shedding of a turbulent wake. This is the same process that causes musical notes
when wind blows through trees. For wind machines, these tones would occur at very low
frequencies of about 30 to 200 Hz. The turbulent boundary layer as well as the unsteady wake
will also generate additional middle frequency broad-band noise.
5. Engine noise is not considered critical to overall noise generated, but many do have mufflers.
Low Frequency Sounds
Figure 3 shows a narrow band spectrum (dB) vs. low frequency range (0.1 to 100 Hz) for one of
the wind machines. Each had its own characteristic blade passage frequency directly related to
the number of blades and its rpm, and each machine produced low frequency sound peaks. The
first peak occurred at the blade passage frequency then harmonics occurred at the first few
multiples of the blade frequency. For the wind machine demonstrated in Figure 3, the blade
passage frequency occurred at about 17 Hz, with harmonics occurring at about 34 Hz, 51 Hz,
68 Hz, etc. All three wind machines exhibited similar blade passage frequencies, sound level
peaks, and shape of curve as shown in Figure 3.
80
70
60
dB
50
40
Sound peaks at
multiples of blade
passage frequency
30
Low Frequency Level (Hz)
99.1
95.5
91.9
88.3
84.7
81.1
77.5
73.9
70.3
66.7
63.1
56
59.5
52.4
48.8
45.2
38
41.6
34.4
30.8
27.2
20
23.6
16.4
12.8
5.6
9.19
2
20
Figure 3: Narrow band spectrum (dB) vs. low frequency range (Hz) for a typical wind machine.
The first sound peak is at the blade passage frequency, with multiples thereafter.
5
As stated earlier, these low frequency sounds can travel a long distance, particularly if wind
assisted. In the case of wind machines, sounds may also travel in the ‘drift’ direction, following
the natural land drainage flow. In Niagara, this is from south to north towards Lake Ontario.
Low frequency sounds can penetrate homes and excite a variety of acoustic modes within
rooms (Shepherd et al, 1990). These excitations include acoustic modes within the room and its
building components, and the perceptible rattling of secondary structures in rooms such as wall
hangings, dishes or furniture. Windows, walls and floors have characteristic frequencies where
they begin to vibrate, depending on their density, materials, thickness and other variables.
Windows, especially of single pane thickness, are more excitable than walls or floors.
The frequency range of excitation is directly related to the geometry of a structure. Room sizes
from 1.5 m to 15 m would be affected by frequencies between 10 Hz to 110 Hz. However, the
movement of low frequency sounds is also influenced by the speed of sound in air. This varies
with air temperature, being about 331 m/s at 0oC, but only 319 m/s at -20oC. The use of wind
machines in Niagara would normally be between these two temperatures. Because temperature
can create subtle changes in low frequency sound movement, a home that ‘vibrates’ beside a
wind machine in -20oC weather might not vibrate at 0oC, and vice-versa. So, because of varying
house construction and varying sound movement under varying weather conditions, it is
possible that some homes could be more susceptible to noise nuisance than others.
Sound Pressure Levels Over Time
Figure 4 shows a graph of generalized sound pressure levels (dBA) versus time for a wind
machine as it cycles once around a vineyard. As noted earlier, wind machines are considered
‘upstream’ machines, as they operate with their blades ‘upstream’ of the tower.
Generalized Sound Pressure Levels (dBA) vs. Time for
One Cycle of a Wind Machine With Respect to Receiver
dBA
Blades
Tower
Airflow
direction
Rotation around
vineyard
Airflow away
from receiver
3-5 dBA variance
Airflow at
receiver
Airflow 90°
to receiver
8-11 dBA
variance
Airflow 90°
to receiver
Time (4.5 to 6.5 minutes, depends on machine)
Figure 4: Generalized sound pressure levels (dBA) vs. time over one cycle of a wind machine
6
Sound levels vary up and down over the cycle period of a few minutes in a sinusoidal fashion.
The greatest dBA levels occur when machines blow away from the receiver, which is when the
blades are between the tower and the receiver. This seems counter-intuitive because one
expects sounds to be greatest when high airflows hit the receiver. That is, when the blades are
on the opposite side of the tower to the receiver, pulling air down and blowing it at the receiver.
However, the sound levels are slightly lower at this point. The smallest dBA is when machines
are facing 90o to the receiver. In the tests, sound levels varied by up to 8 to 11 dBA from
greatest to least. This variance in sound levels over time is annoying to some neighbours.
Although the spectra of the three machines were slightly different, the overall A-weighted sound
pressure levels (dBA) were comparable and within the normal statistical spread that one might
expect with outdoor measurements.
Conclusions
Growers only operate wind machines when they have to protect their crops against cold injury,
and it is not intentional that anyone, including growers, would be bothered by their noise.
Unfortunately, wind machines are generally operated when:
•
People are sleeping at night
•
Rural areas are inactive and quiet
•
There is virtually no wind to make any noise outside
•
There is a lack of woodlots and vegetation to help muffle sounds, and
•
There is a denser, thermal inversion air level above for sounds to ‘bounce’ off
This study has examined and found several unique acoustic properties that are characteristic of
wind machines. A limited number of measurements have been conducted and it is clear that
there is still much to be studied about the various operating scenarios of this equipment.
However, the following conclusions can be drawn about wind machines:
•
Noise from wind machines is due to both aerodynamic and mechanical effects, but
aerodynamic sounds are deemed to be the most significant.
•
There is evidence of low frequency blade slapping or impulsive sounds, during little to no
winds (< 5 km/h), but more in the presence of even slightly greater winds.
•
Full range or broadband sounds manifest themselves as noise components that extend
throughout the audible frequency range from the blade passage frequency to upwards of
1000 Hz. The sound spectrum of a wind machine is full natural tones and impulses that
give it a readily identifiable acoustic character.
•
Atmospheric conditions including temperature, inversions/lapse rate, relative humidity,
mild winds, gradients and atmospheric turbulence all play a significant and sometimes
profound role in the long range outdoor propagation of sound from wind machines.
•
Factors such as hard, soft, ice or snow covered ground as well as barriers, buildings,
forested areas and topography (i.e. Escarpment) all affect outdoor sound propagation.
•
Low frequency energy from wind machines is capable of exciting lightweight
components that comprise a building structure such as a dwelling, thus causing
increased annoyance potential.
7
•
Multiple machines may interact and cause audible and/or adverse effects such as
cyclical variation, beating phenomena that can be detected at long range distances.
Further investigation on wind machines, to reduce noise, are worth pursuing such as:
•
Reducing the rpm of their blades, while still delivering acceptable airflow and distance
•
Improving blade design by addressing aerodynamic considerations
•
Changing tower design by incorporating vortex shedding mechanisms
•
Investigating the influence of existing tower ladders, which may affect sound production
•
Shielding of machines, where possible, behind outbuildings away from homes
•
Noise cancellation technologies
•
Review psychoacoustic and loudness properties of wind machines in order to set human
response and annoyance criteria
•
Review acoustic setbacks and noise buffer zones near residences and communities
Next Steps
Tests are already underway on measuring sound levels in some homes beside one, or more,
wind machines, and to measure sound levels at greater distances (100 to 400 m) from a wind
machine under extreme cold (-15oC, or colder), calm (5 km/h, or calmer) conditions.
Unfortunately, it is difficult to predict when and if these conditions will occur, since wind speeds
are often greater than this, except at night, and extreme cold temperatures only occur a few
days during winter, also usually at night.
Acknowledgements
The authors are very grateful for the field assistance of time, equipment and finances to:
•
•
Stephane Bosc, local distributor of Orchard Rite Wind Machines, St. David's, ON
Roger Vail, local distributor of Chinook Wind Machines, Campden, ON
The authors also want to acknowledge the field assistance of:
•
•
Doug Hernder, grape grower, Virgil, ON
Ernie Wiens, local distributor of Frost-Boss Wind Machines, Virgil, ON
References
Shaw, T. 2001. Final Report: Wind Machine Technology to Optimize Vineyard Conditions. Brock
University, ON.
Fraser, H.W., K. Slingerland, H. Fisher, K. Ker, 2006. Infosheet: Wind Machines for Protecting
Grapes and Tender Fruit from Cold Injury. Ontario Ministry of Agriculture, Food and Rural
Affairs, 2006.
Shepherd, K.P. and H. Hubbard, 1990. Physical Characteristics and Perception of Low
Frequency Noise From Wind Turbines. Noise Control Engineering Journal, Vol. 36, Number 1.
8