A 11 ID 3

PUBLICATIONS

RRbMMb

NBSIR 79-1599

Microphone Windscreen

Performance

Robert N. Hosier

and

Paul R. Donavan

Acoustical Engineering Division

Center for Mechanical Engineering

and Process Technology

National Bureau of Standards

Washington, DC 20234 „

January 1979

Final Report

Prepared for Office of Noise Abatement and Control

U.S. Environmental Protection Agency

Washington, DC 20460

NaHenaP Bj,rean of Standards MAY 1 5 1979

NBSIR 79-1599

MICROPHONE WINDSCREEN

PERFORMANCE

Robert N. Hosier

and

Paul R. Donavan

Acoustical Engineering Division

Center for Mechanical Engineering

and Process Technology

National Bureau of Standards

Washington, DC 20234

January 1979

Final Report

Prepared for Office of Noise Abatement and Control

U S. Environmental Protection Agency

Washington, DC 20460

U S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
J
TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. WINDSCREENS AND WINDSCREEN PERFORMANCE 1

2.1 Types of Windscreens 1

2.2 Commercially Available Windscreens 2

2.3 Performance of Windscreens 3

3. TEST APPARATUS 4

4. TEST PROCEDURE 6

5. TEST RESULTS 6

5.1 Wind-Induced Noise 6

5.2 Acoustic Insertion Loss 9

6. CONCLUDING REMARKS 9

REFERENCES 11

TABLES 13

FIGURES 16

APPENDIX A: TABULATION OF WIND-INDUCED NOISE LEVELS FOR

GRAZING FLOW INCIDENCE 52

APPENDIX B: TABULATION OF WIND-INDUCED NOISE LEVELS FOR

NORMAL FLOW INCIDENCE 64

APPENDIX C: TABULATION OF ACOUSTIC INSERTION LOSS MEASUREMENTS

FOR GRAZING FLOW AND NORMAL ACOUSTIC INCIDENCE.. 76

1. INTRODUCTION

Microphone windscreens are used in conjunction with almost every

outdoor noise measurement. As the name implies, windscreens screen or shield microphones from wind in order to improve the signal-to-noise

ratio of an acoustic measurement. Windscreens also serve to protect the

microphone diaphragm from foreign materials carried in the wind (e.g.

dust, insects, or moisture).

In order to empirically characterize a number of microphone

windscreens of the type currently in use and to provide some indication

of the mechanisms of wind-induced noise, measurements of wind induced

noise and windscreen insertion loss were made under laboratory

conditions. Ten microphone windscreens were included in the study.

Eight of these were reticulated polyester or polyether spheres of varying

porosity. The remaining two windscreens were of the

cloth-over-metal-cage type often used in long term outdoor noise

monitoring. The measurements were made at two flow orientations and at seven wind speeds.

Section 2 of this report presents a discussion of previous work

dealing with assessments of windscreen performance along with a description of the type of windscreens currently in use. Details of the

apparatus and measurement techniques used in the current study are discussed in Sections 3 and 4. The results of the measurements are contained in Section 5 and concluding remarks are presented in Section 6.

2. WINDSCREENS AND WINDSCREEN PERFORMANCE

2.1 Types of Windscreens

With a few exceptions, microphone windscreeen design has evolved very little

since the prototype of Thorne [1] 1 and Spotts [2] which consisted of either

spherical or spheroidal wire cages covered with a silk or nylon gauze.

In an early work Phelps [3] developed an expression for the wind pressure

around a sphere [microphone] placed in incompressible, non-turbulent

flow. He then designed a perforated windscreen to take advantage of the

pressure and phase differences over the surface of the sphere in order to

reduce the wind pressure at the microphone diaphragm. More than twenty

years later Bleazy [4] extended the work of Phelps to relate the

attenuation of wind noise to the enclosed volume of the windscreen. His

experiments were conducted in an anechoic chamber with a paddle-wheel as the wind source. The wind generated by this device was turbulent, but

controlled in frequency and velocity so as to simulate outdoor wind

conditions. Bleazy found the following relationship to hold over the

wind speed range from 2 m/sec to 13 m/sec:

A = 6.77 log V + 10.4

1 Numbers in brackets indicate the literature references at the end of

this report.

1
Where A is the unweighted attenuation in dB and V is the windscreen

volume in units of in 3

.

1

The windscreens tested ranged in volume from 2 in 3 to 800 in 3

. Most recently, the U. S. Army [5,6] has used concentric

hemispherical and cylindrical cage windscreens of very large volume (e.g.

0.3 m

3

) to reduce the wind- induced noise for low-frequency microphone

systems. Ballard and Izquierdo [5] found that this approach enhanced the

viscous dissipation of turbulent energy in the air stream by creating

large velocity gradients (small eddies). Gradually, the cage windscreens

have been replaced with reticulated foam spheres for many outdoor

measurements

The only major change in windscreen design came in response to the

need to measure sound levels in air ducts containing turbulent flow.

Nakamura, et. al. [7] and Neise [8] performed extensive theoretical and

experimental studies of long (24 cm) slit-tube windscreen probes. Their

results indicate that turbulence- induced noise levels due to air flow

parallel to the axis can be significantly reduced by using these special

purpose windscreens. Shorter (3 cm), less sophisticated, nose cones are also frequently used in very directional flow environments. Because

omni-directional response is needed for most outdoor measurements, use of

the slit tube and nose cone windscreens is generally restricted to

acoustic measurements in wind tunnels or air ducts.

2.2 Commercially Available Windscreens

For most outdoor applications today, either spherically or cylindrically shaped windscreens are used. These windscreens are commercially available in a number of models (see Figure 1). The

simplest spherical model is made of reticulated polyester foam. It is

available in sizes from 6 cm to 9 cm in diameter and has a hole reaching

nearly to the center for insertion of the microphone (e.g. windscreens A

and C in Figure 1).

A second commercially available spherical model is the wire cage

type similar to the one used by Thome and identified as windscreen B in

Figure 1. It is about 11 cm in diameter with a nylon gauze cover and

contains a conically shaped adaptor Inside the cage to receive the

microphone. A multi-spiked assembly is often fitted to the sphere in

order to prevent birds from roosting on the windscreen.

A third, hybrid windscreen is made by one manufacturer (see

windscreen J in Figure 1). This windscreen consists of a number of

components, beginning with a 6 cm diameter polyester sphere which is

placed over the microphone. Both are then inserted about two-thirds of

the way into a cylindrical, stainless steel, perforated cage and held in

place by a base plate attached to the microphone preamplifier. The cage

has a hemispherical cap at one end and a microphone adaptor plate at the

1 NBS recommends the use of metric units; however, to maintain the

integrity of the cited references, non-metric units were used here.

2
other. A reticulated polyester cover is fitted over the cage. The

entire assembly is about 22 cm long and 7 cm in diameter. It also comes equipped with a 13 cm "bird spike" which is mounted on the hemispherical

cap. The stainless steel cage serves to prevent rain from impinging on the internal windscreen by causing it to run off between the cover and

the cage.

2.3 Performance of Windscreens

When assessing the performance of windscreens, it is common practice

to measure two fundamental descriptors: wind- induced noise and acoustic

insertion loss. Wind- induced noise is the noise produced by air flow

over the microphone and windscreen assembly. As is shown in Figure 2,

wind-induced noise is a function of five parameters: 1) mean flow; 2)

inflow (free-stream) turbulence due to thermal and wind speed gradients

in the atmosphere; 3) self-generated turbulence caused by boundary layer

separation and vortex shedding; 4) flow through the windscreen; and 5)

flow incidence angle. The flow through the windscreen is dependent upon

windscreen characteristics such as porosity and diameter. In a 1976

paper, Oswald [9] showed that inflow turbulence and self-generated

turbulence are major factors in determining the low and high-frequency

levels, respectively, of the wind-induced noise spectrum. This effect

was also substantiated by the work of Kldtzsch [10] which contained an excellent investigation of the wind-induced noise levels measured by

microphones in flows of varying turbulence levels and with different grid

cap designs. In a subsequent paper [11], Kl'dtzsch measured the

wind- induced noise for cage windscreens of different diameters and with

various covering materials. He found that the wind-induced noise depends

on the mean flow velocity, the degree of turbulence, the direction of the

approach flow, the windscreen diameter and covering material, and on the

arrangement of the microphone within the windscreen. His data show that

the wind- induced noise increases with increasing flow velocity and

increasing turbulence levels in the inflow. Kldtzsch also found

cylindrical and spherical windscreens to perform equally well in reducing

wind- induced noise.

Acoustic insertion loss (IL) is the difference between the sound

pressure levels measured without (L . ) and with (L ) the windscreen In

w/o w

place. In this test, it was assumed that the flow field around the

windscreen may interact with the incident acoustic field and thereby

affect the insertion loss. For this reason the IL is assumed to depend

on the approach velocity according to the following definition:

IL = L . (0) “ L (v) , dB,

w/o W

where v = the mean flow velocity and where L is a sound pressure level

descriptor such as one- third octave band level or A-weighted level. Note

that the level without the windscreen in place is measured at v=o.

3
Most windscreen manufacturers provide only a modest amount of data

on wind- induced noise and insertion loss for their type of windscreen.

Experience has shown these data to be difficult to convert to one- third

octave, A-weighted, and overall levels over the wind speed range

generally considered acceptable for outdoor testing (0 m/sec to 6 m/sec).

Furthermore, little information is available on the effects of porosity

and manufacturing variability on the wind- induced noise and insertion

loss. Thus, correcting acoustic data for these effects is often not

possible.

With the exception of the studies by Nakamura [7] and Neise [8],

little attention has been given to relating the fluid dynamics of the air

flow field to the sound levels measured when a microphone windscreen is

used. This is particularly true for the porous polyester windscreens

frequently used in outdoor noise measurements today. (Blomquist [12] has

published some data on the effects of porosity and diameter on insertion

loss and A-weighted sound level for a wind speed of 40 km/hr.) The

experiment described in the remainder of this report was designed to

provide a preliminary indication of the physical mechanisms contributing

to the wind- induced noise and insertion loss of windscreens.

3. TEST APPARATUS

The ten windscreens tested are shown in Figure 1. The windscreens

are shown in greater detail in Figures 3 through 6. Windscreens A, C, D,

E, and F are commercially available polyester windscreens (see Figures 3

and 4) . They differ in that A and C were selected at random from the

manufacturers’ stock, while D, E, and F were selected to cover the range

of porosity judged acceptable by one manufacturer. Windscreens G, I, and

H, shown in Figure 5, were specially made to cover the porosity range

from 400 ppm to 2200 ppm. 1 B and J (see Figure 6) are commercially

available cage windscreens with bird spikes. B is covered with a nylon

gauze and J with a polyester foam. When J is assembled, windscreen C is

placed over the microphone and the two are placed inside the perforated

cage. An adaptor (shown next to windscreen J in Figure 1) is used to

hold the microphone in place. Table I summarizes the physical

characteristics of these windscreens.

1 Foam porosity is commonly measured in units of pores per unit length,

in this case pores per meter (ppm). Thus, a high porosity foam

would have many pores per unit length, but the open area of each pore

would be small. Conversely, a low porosity foam would have fewer pores

per unit length, but their open area would be larger. In terms of flow

resistance, low porosity would imply low flow resistance and high porosity

large flow resistance.

4
The measurements of wind- induced noise and insertion loss were made

in the 450-m3 NBS anechoic chamber. For the wind-induced noise

measurements, each windscreen was placed on a "half-inch” 1 diameter

microphone which was mounted at the end of a 1.57 m rotating arm. The

rotating arm was constructed out of symmetric airfoil tubing and oriented

for zero lift. Thus, air recirculation in the test facility was minimized. The microphone could be swiveled in the mounting bracket to

allow normal and grazing flow incidence to be simulated. This assembly

was then placed on a precision rate table (see Figure 7) capable of

providing a specified angular velocity to within one-half percent. To

measure insertion loss, a loudspeaker of known directional

characteristics was mounted at the center of the rotating assembly. The

directivity pattern of the speaker was calculated to be distorted by less

than two degrees in the horizontal direction, at the microphone position,

for the maximum rotational speed' used in these tests.

Figure 8 shows a block diagram of the data acquisition system. Two

microphones were used in these tests. The primary microphone was mounted

on the rotating arm and the stationary microphone was supported in the

anechoic chamber 2.4 m from the closest approach of the arm. This second

microphone was used to monitor the test and proved useful in verifying , that no spurious signal, such as that due to vortex shedding from the

rotating arm, was present. The signal from the rotating microphone was recorded on two channels of the FM recorder. The first channel consisted

of the pressure signal as obtained directly from the rotating microphone

after amplification, while the second channel consisted of a 150 Hz

high-pass-filtered version of the Channel 1 signal. The Channel 2 signal

was amplified relative to the Channel 1 signal in order to increase the

high-frequency dynamic range of the wind- induced noise measurements since

most of the acoustic energy fell below 150 Hz, All data were recorded

for a minimum of 60 seconds in order to determine the sound level within

+ 1 dB with 95 percent confidence for the lowest one-third octave band of

interest (6.3 Hz). Both signals were monitored by peak level detectors

to assure the maximum use of the tape recorder dynamic range. The

microphone signals were also monitored during recording with a one-third

octave band real time analyzer and an oscilloscope. A digital data coder

was used to log test parameters such as run number, windscreen number,

microphone orientation, and speaker activation.

For the insertion loss data, the loudspeaker mounted at the center

of the rate table was driven by a pink noise signal which had been passed

through a 150 Hz high pass filter. The high pass filter was necessary to

make the input signal comparable with the loud speaker frequency

characteristics and thereby avoid speaker signal distortion. The

speaker excitation voltage was set as high as possible to assure that the

wind- induced noise would be negligible over as large a frequency range as possible. Using this technique, insertion loss measurements could be

made up to 4000 Hz. Above this frequency the difference between the loud

speaker signal level and the wind-induced noise level was less than 10 dB

and the data were not used. To assure a constant electronic signal level

to the loudspeaker, the voltage of the input signal was constantly

monitored.

1 Commercial designation of microphone type.

5
4. TEST PROCEDURE

The apparatus described in Section 2 were used to measure the

wind-induced noise and insertion loss for each of the windscreens shown

in Figure 1. The wind induced-noise was measured at normal and grazing

flow incidence. After mounting each windscreen on the rotating

microphone, the levels were recorded at zero air flow speed. The

microphone and windscreen were then rotated at tangential speeds up to 14

m/sec in 2 m/sec increments and recordings were made of the wind-induced

noise. The recorded data were then reduced to obtain overall (flat

frequency response 20 Hz to 20 kHz), A-weighted, and one-third octave

band levels.

As shown in Figure 9, insertion loss measurements were made for

three combinations of acoustic and flow incidence angles. To make these

measurements, the noise from the loudspeaker , as excited with the

high-pass- filtered, pink noise signal, was recorded. Insertion loss was calculated for each windscreen by computing the difference between the

levels measured with and without the windscreen in place. In each case,

the level without the windscreen in place was a static (v=0) measurement

while the levels with the windscreen in place were measured over the

speed range from 0 to 14 m/sec in 2 m/sec increments.

5. TEST RESULTS

5.1 Wind- Induced Noise

Plots of the overall and A-weighted wind-induced noise levels as a function of wind speed and windscreen type for grazing (parallel to

microphone diaphragm) and normal (perpendicular to microphone diaphragm)

flow incidence are presented in Figures 10 through 13. The curves shown

in these figures were determined by using a third order regression

analysis, the equations for which are presented in Table II. The

one-third octave band wind-induced noise data are presented in Figures 14

and 15 for each windscreen and wind speed. It will be noted in these

plots that some data points for the lower speeds have been omitted. This

was done because narrow band frequency analysis revealed that structural

resonances in the arm assembly were sufficiently excited by the rate

table governing system to contaminate some of the small amplitude

acoustic signals through excessive vibration of the microphone. This

phenomenon was examined thoroughly, and only those data where such

contamination did not occur are presented.

In considering the results of the wind-induced noise measurements,

it must be remembered that the windscreen was mounted one meter above the

rotating arm so that the incident free-stream turbulence was only due to

the windscreen intersecting its own wake on each revolution (a distance

of more than 100 windscreen diameters) . Therefore, the laboratory

environment, compared to outdoor conditions, was relatively

turbulence-free. For this reason, the results cannot be directly applied

6
to the outdoor wind environment as characterized solely by a mean wind

speed. To apply the results of the laboratory wind- induced noise

measurements to estimating outdoor wind- induced noise, the turbulence

intensity and length scale present in the outdoor environment must be

considered in addition to mean wind speed. Realizing this limitation,

the data of Figures 10 through 13 can still be useful when considering

outdoor noise measurements. These data indicate that commercially

available windscreens can provide on the order of 15 to 25 dB reduction

of wind- induced noise, depending on the wind speed and flow incidence

angle. The graphs can also be used by an experimenter as an aid in the

selection of the most appropriate windscreen for a given measurement if

there is some knowledge of the wind environment to be encountered and

the frequency content of the noise to be measured. Furthermore, the data

of these figures can be used to .roughly estimate the wind-induced-noise

content in existing data if wind speed information is available.

For both grazing and normal incidence, the curves for each

windscreen are more tightly grouped for the overall levels than for the

A-weighted levels. This suggests that the wind-induced noise is

dominated by low-frequency components. These components can be related

to vortex shedding in the subcritical Reynolds number range encountered

in these tests (8,000 <_ Re 90,000) through the Strouhal number S = fd/V

for spheres of varying surface roughness as measured by Achenbach [13].

His data show that, S>0.18 with S becoming smaller as surface roughness

is increased. Using S = 0.18 for a sphere of diameter d = 0.095 m gives

a Strouhal shedding frequency f = 27 Hz at a wind speed V = 14 m/sec. An

examination of the spectral data obtained during these tests (Figures

14(a) through 15 (i)) reveals that the maximum wind- induced noise levels

occur in the 16 Hz band at v = 14 m/sec for most of the windscreens

tested. This lower peak frequency seems to suggest that a Strouhal

number lower than S = 0.18 is characteristic of porous spheres. Also,

the smooth distribution of levels about the spectrum peak indicates that

complex interactions are occurring between the flow over and through the

porous windscreens and that the wind-induced noise cannot be totally

attributed to vortex shedding from the windscreen.

The spectra from a number of the windscreens tested (e.g. D, E, F,

G, H, and I) show a secondary velocity-dependent peak in the frequency

range between 1 kHz and 10 kHz. This peak is believed to be related to

the flow through the windscreen pores, with the magnitude and frequency

of the spectrum peak being inversely proportional to pore size. Because

a one- third octave bandwidth analysis was used in Figures 14 and 15, it was not possible to determine the exact dependence of peak frequency on porosity; however, the magnitude of the spectrum peak seems to decrease

about 8 dB to 10 dB per doubling of porosity.

Although it did not prove possible to relate the secondary peak

frequency to porosity, the overall and A-weighted levels do show a porosity dependence. For example, the data from Figures 10 through 13 at

v=12 m/sec for the spherical foam windscreens have been replotted in

Figure 16. It can be seen that the overall levels increase as the

7
porosity increases. A possible explanation for this dependence is that

as the windscreen porosity increases the windscreen flow characteristics

approach those of a smooth, solid sphere and the acoustic signal is

dominated by low-frequency, discrete, vortex-shedding noise. In this

case turbulence fluctuations are restricted to the boundary layer because

they are of insufficient energy to propagate through the windscreen to

the microphone. As the windscreen porosity is decreased the boundary

layer becomes more intense and the wake becomes increasingly turbulent.

Flow through the windscreen is also Increased. The spectra in Figures

14(g) and (i) for grazing flow and 15(g) and (i) for normal flow show

predominating low-frequency energy for the high-porosity windscreens and

increased high-frequency energy for the low-porosity windscreens. It is

not possible from these tests to absolutely determine whether the

high-frequency noise is attributable to increased flow through the

windscreen, increased turbulence intensity in the boundary layer and

wake, or a combination of these effects; however, it seems that the

high-frequency peaks may scale to a reduced frequency based on an effective velocity through the pores and an effective pore size.

In contrast to the overall levels, the A-weighted levels (Figure 16)

decrease with increasing porosity. This dependence is not related to a new flow phenomenon but rather to the A-weighting filter characteristics.

The spectra in Figures 14 (k) and 15 (k) are from the microphone with

no windscreen at grazing and normal flow incidence. Both spectra show

intense low frequency content; however, the spectrum shapes are dissimilar. With the windscreen removed, the flow impinges directly on the microphone diaphragm-grid cap assembly. When the microphone is

oriented for grazing flow incidence, boundary layer fluctuations are sensed directly by the microphone. This effect is mitigated when the

microphone is oriented for normal flow incidence because the flow

stagnates at the microphone diaphragm. For grazing flow incidence,

however, the vortex shedding frequency for a 1.27 cm diameter cylinder is

about 160 Hz at a flow speed of 10 m/sec and the spectra of Figure 14 (k)

show a velocity- dependent peaking in that region. Oswald's data [9] seem

to indicate that the low frequency portion, below the 100 Hz region, may

be attributed to inflow turbulence. However, no measurements of

turbulence were made in these tests to verify that hypothesis. It does

seem reasonable that such an effect would be much more noticeable in the

measurements without the windscreen since the windscreen would shield the

microphone from low intensity turbulence.

In general, the performance of the cage windscreens was not as good

as most of the foam windscreens. This was particularly true for the

spherical cage windscreen (see Figure 6). For grazing flow incidence,

the cylindrical windscreen also did not provide as much wind induced

noise reduction as most of the foam windscreens; however, for normal

flow, the cylindrical windscreen performed better than any of the foam

windscreens over most of the wind speed range.

The data in Figures 10 through 15 (k) have been tabulated in

Appendices A and B. Tables A1 through All contain the data for grazing

flow incidence and Tables B1 through Bll for normal flow incidence.

8
5.2 Acoustic Insertion Loss

As part of this study, measurements were made of the acoustic

insertion loss for each windscreen as a function of wind speed. The test

was designed to acquire insertion loss data at each of the combinations

of flow and acoustic incidence angles shown in Figure 9 in order to

identify not only the magnitude of the insertion loss, but also its

dependence, if any, on air flow. Pink noise emitted by the loudspeaker

at the center of the rate table was measured by the rotating microphone

over the speed range 0 <v< 12 m/sec with the windscreen in place and at

v=0 without the windscreen. The insertion loss was computed for

one-third octave bands between 250 Hz and 4 kHz (the range of useful

signal-to-noise ratio) as well as for the overall and A-weighted levels

by subtracting the levels measured without the windscreen in place at v=0

from the levels with the windscreen in place over the speed range of 0

m/sec <v< 12 m/sec.

The results of the insertion loss measurements for Orientation B in

Figure 9 (grazing flow and normal acoustic incidence) are tabulated in

Appendix C. It can be seen from these data that the insertion loss is

small (0.1 dB to 0.6 dB, generally). Also, in most cases the insertion

loss for each one- third octave band, overall, and A-weighted level varies

only by one or two tenths of a decibel over the entire wind speed range.

These data may be used to correct a measured broadband noise signal by

subtracting the tabulated value from the measured level.

This measurement was found to be very difficult to make for a number

of reasons. First of all, the frequency range was limited at the low end

by speaker response and at the high end by wind- induced noise. Secondly,

wind- Induced noise also precluded any insertion loss measurements at v=l4

m/sec. And, finally, uncertainties in the source level monitoring were found to be too great during a large portion of the testing to allow

accurate computation of the small insertion loss level differences. As a consequence of these difficulties, successful insertion loss measurements

could only be made for grazing flow and normal acoustic incidence. The

results are tabulated in Appendix C.

6. CONCLUDING REMARKS

Measurements of the wind-induced noise and acoustic insertion loss

have been made on ten microphone windscreens at wind speeds from 0 m/sec

to 14 m/sec for both grazing and normal flow incidence. The data show

that the windscreens are very different in the amount of

wind- induced-noise reduction they provide, with some achieving more than

25 dB reduction. In general the foam windscreens provided more wind- induced-noise reduction than the cage windscreens; however, the

cylindrical cage windscreen performed better than any of the foam

windscreens for normal flow incidence. For both grazing and normal flow

incidence, the A-weighted wind- induced noise levels for all the

windscreens covered a larger range (about 20 dB at 14 m/sec) than did the

overall levels (about 15 dB at 14 m/sec).

9
The insertion loss measurements prc /ed difficult to make because of

the small values (0.1 dB to 0.6 dB) encountered and, at least for the case of grazing flow and normal acoustic incidence, the data show no strong velocity dependence for any of the windscreens tested.

The wind- induced noise and insertion loss sound pressure levels have

been tabulated in the appendices of this report. These data and those in

the graphs and tables of this report provide a basis for studying the

noise generating mechanisms associated with flow incident upon a porous

sphere. For example, it was observed that the wind- induced-noise levels

for foam windscreens showed a clear dependence on porosity, and therefore

manufacturing variability. It was also observed, however, that this

dependence was reversed for the A-weighted and overall levels (increasing

porosity yielded increasing overall sound pressure levels but decreasing

A-weighted levels.) This reversal is believed to be associated with an increase in low-frequency (16 Hz), vortex-shedding noise as the foam

porosity increased.

Examination of the wind-induced noise spectra revealed a high-frequency (1 kHz to 5 kHz), velocity-dependent peak. Flow through

the windscreen pores seems to be the noise source for this peak. This

hypothesis is confirmed by the fact that the spectra for the microphone

with no windscreen show no high-frequency peaks; however, they do show a low frequency, velocity-dependent peak due to vortex shedding around the

microphone preamplifier (160 Hz at 10 m/sec). It was also for the

no-windscreen case that inflow turbulence effects seemed observable in

the frequency range below 20 Hz.

10
References

1 . Thome

U. s. Patent No. 588,034, August 1897.

2. Spotts

U. s. Patent No. 1,901,065, March 1933.

3. Phelps, William D„: Microphone Wind Screening. RCA Review, vol. 3,

1938, pp. 203-212.

4. Bleazey, John C.: Experimental Determination of the Effectiveness

of Microphone Wind Screens. Joum. Audio Eng. Soc., vol. 9, no. 1,

Jan 1969, pp. 48-53.

5. Ballard, Harold N., and Izquierdo, Mike: Reduction of Microphone

Wind Noise by the Generation of a Proper Turbulent Flow. U. S. Army

Electronics Research and Development Activity Report 262, DDC no. AD

455966, Feb. 1965.

6. Breeland, A. H., and Bonner, R. S.: Results of Tests Involving

Hemispherical Wind Screens in the Reduction of Wind Noise. ECOM

Report no. 5119, DDC no. AD 653005, April 1967.

7. Nakamura, Akira; Matsumato, Rimpey; Sugiyama, Akira; and Tanaka,

Tadami: Some Investigations on Output Level of Microphones in Air

Streams. Joum. Acous. Soc. Am., vol. 46, no. 6, part 1, March

1969, pp. 1391-1396.

8. Neise, W.: Theoretical and Experimental Investigations of

Microphone Probes for Sound Measurements in Turbulent Flow. Joum.

Acous. Soc. Am., vol 39, no. 3, 1975, pp. 371-400.

9. Oswald, Lawrence J. : The Wind Noise of Nose-Cone-Protected

Microphones. Proc. of Inter-Noise 76, Washington, D. C., April

5-7, 1976. pp. 141-144. 1976. pp. 141-144.

10. Kldtzsch, Von P.; and Bauer, H. J. : Das Wingerausch von Kondensatormikrophonen. Mitteilung aus der Leitsteile fur

Larmschutz in der Industrie im VEB RFT Meszelektronik und der

Technischen Universitat Dresden Sektion Informationstechnik, Bereich

Kommunikation und Meszwerterfassung.

11. Klbtzsch, Von P.: Zur Windgerauschdampfung von Windschirmen.

Hochf requenztechnik u. Elektroakustik, Leipzig 80 (1971), 1, s. 1-9.

11
12. Blomquist, Donald S.: An Experimental Investigation of Foam

Windscreens. Proceedings of Inter-Noise 73, Technical University of

Denmark, Copenhagen, August 22-24, 1973, pp 589-593.

13. Achenbach, Elmar: The Effects of Surface Roughness and Tunnel

Blockage on the Flow Past Spheres. J. Fluid Mechanics (1974), vol.

65, part 1, pp 113-125.

12
TABLE

I.

-

WINDSCREEN

PHYSICAL

CHARACTERISTICS

Porosity, B

CM

CM

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cd o ^ +-> cd o o o o o o o O 4-J cd s PM !Z CA o PM Pm Pm Pm Pm PM Pm Pm CA a

1

1 cd

s—rH 11 1

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cd cd cd cd cd cd cd cd cd •H o o a o o o o a o d

*H •H •H •H •H •H •H H •H T2

<u d e )M 9) u B d 0 d B d S d s d B d B C 0) CM 4) cd «) 00 <D cd a) cd ft) cd 6) cd 4) cd 8) cd 4) cd •H 00 >> 42 o rd cd 42 o 42 o 42 o 42 O 42 o 42 O 42 O rH cd H a- mm Pm O CM MM Cm mm PM MM CM MM CM MM CM MM PM MM tn CJ CA CA CA CA CA CA CA CA CA CJ

e

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w

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12

13
TABLE

II.

-

WIND-INDUCED

NOISE

EQUATIONS

<3+

CN

<3+

>

r— <3+

•KM

4-1 d

a) H

O

•H

>4-1

>4-4 03O

CJ

cn

u <3 CM ^3

0)

Cfl

I—

rH rH

n \ V 1 V V

fj £ > > >

£

cd Vl V V

P^4 > CM CM CM

li—li—Ir—li—It—| i—I rH OO rH

I OOOOOOOOO I I I I I I I I I o

X XXXXXXXXX

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CN

N'DOOUIliriO\-joo HOOMN^HiDO-J O' CO O CN >D (N in vO vO nii—i to (O n i—i cn cn ,—

X X X X X X X X X X X X

CM CN CN VO *<3 00 oo OV vO CO

tH vO O' cn *<3 CN CO 00 cn VO Oh O 00 1—

CN 00 vO OV oo cn vO 00 <3 CM m CN CN rH CN O' CN CM cn

CN

o rH O tH rH O O OrHO rH o oooocooooo

X X X X X X XXX

rH vO O' i—1 00 CN O O rH M -J CO >D N O' H O' O vO O O 00 r—I 00

QJV rl N H H CO m 00

CN vO O vO o cn

i—i cn

i i i i i i i i i

rH

d

<L> 4

CJ

03 X)d

•H&

CN CN CN CN CN CN r O O O O O O > I '—I 1—I I—I 1—I t—

H CN CN rH CN o o o o o «“I rH rH rH rH X XXXXXXXXX X MOdmo'>ooocNin n ‘noocN^j-ooOCT'O'in

cn -^rHCncnrHCNrHCnO

rH I—I i—It—li—It—I O' 1—| I—I OV

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CNCNCNCNCNCN ooooooCN CN CN CN CN o o o o

XXXXXX X X X X

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X X X X X X X X X X X X

o ov rH *<3 CN O <3* CN vOO vOo cn m m vO r^. O 00 rH o vO o cn o O VO O cn CO CN

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1

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CNOOOOOO CN CN CN CN CN CN , O

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m

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ood

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to a) d ai

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& o

03 O

33

d)

03

4-1 rd

00 rH

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1 r-i <3

ood

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d

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C3

03

0) -H > CJ d 03 -d ua

03

4-1 4-1 d d

ai oj •h 6 o ai •H >-i MH 3

<4H 03

03 d

O 0) o e

been

provided

to

four

significant

figures

for

computational

purposes,

however,

sound

level

to

ion

was

only

good

to

0.1

dB.
TABLE

II.

-

CONCLUDED

H Range v,

m/sec

<T

r—\ VI

2<

v<

14

VI

CM

1—1 1 1 —1 i—1 rH H i

1 i

1 rH rH H 1—

H iH rH CM CM iH —

—

1 —

1

1 o o O o o o o o o o O o o O o o

1 o O1 o1 o

1 o1 O

I—1 rH i—

rH 1—

1—1—1—

iH 1—rH r—

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rH r—r—rH r—rH CO> X X X X X X X X X X X X X X X X X X X X X

CO

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0 m rH 0 CO 00 rH o co co 00 CO

CM CO r—1 CM CM CM CM CM co CM CM CO m 1—

CM 00 CO 0 1—CM CN CM • • >

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<3 —

+ rH rH O rH O o o o —1 cD o tH cH o 1 1 O tH o —

1 o

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l r—

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00 (—

rH 0 t 1 rH <T CM 0 r—rH 00

II .J

1

1

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CM CM CM CM CM CM

4-1 O o O o o O o o O o O O O O o o o o o o O o

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s—1 rH r—

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rH in 00 0 o m o 0 r—

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i o CO CO CO r—

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M—

m m i—

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in CO c 1 1—1 i—

r—

rH 1 1 1 1 rH 1—r—r—

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ou

CM CM CM CM CM CM CM CM CM CM CM CM CM CM CN r—1 CM CM CM CM CM rH O O O o o O O o o o o O o O o o O o O O O o

rH rH 1 1 r—1 rH \ 1 1 1 1—1 i—

rH rH 1 1 1—1 I 1 r—

rH 1 1 r—1 r—

r—r—t—1 X X X X X X X X X X X X X X X X X X X X X X

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in m 00 rH 1 1 O rH co 00 CM CTN 00 0 rH CM in CO <y\ pH CO CM m <J\ ON a\ CM in CM O'! o CO 0 CO m m IT) o m co rH «H CM rH i—

^ 1 rH rH rH CM -d- CM

1

<di

1—

1

CM

L. J_

CM

i_

CMu LCN

i_

CM

0

*)

t) 0 0 o i &) 1 41 « <3 PQ o Q w IH o X H H) X 4) <1 CQ o Q W o x M 0 4) x) 0 0 0 0

qj •H O •H O

•H s m s m 3 o o

S3 s

1—

0 «

o >

•u pq *) T)

O- T) \—i 4) •H 4-J

S-l

r\ rH O 0 rH 00 pH « cd 4) 4) 0 4) > Q 4)>

£

1

a) X

6 <

v

u

0

•H

pH
16

Figure

1.

-

Photograph

of

windscreens

tested
Flow

Through

Pores

17

Figure

2.

_

Elements

of

wind-induced

noise.
18

Figure

3.

-

Commercially

available

polyester

windscreens

A)

9.5

cm

in

diameter,

C)

6.2

cm

in

diameter
19

Figure

4.

Windscreens

of

acceptable

porosity

for

one

manufacturer.

See

Table

I

for

physical

characteristics.
20

Figure

5.

-

Windscreens

specially

made

to

span

porosity

range

from

500

ppm

to

2200

ppm.

See

Table

I

for

physical

characteristics.
21

Figure

6.

-

Commercially

available

cage

windscreens

tested

in

this

study.

See

Table

I

for

physical

characteristics
22

Figure

7.

-

Photograph

of

experimental

apparatus.
23

Figure

S.

-

Block

diagram

of

data

acquisition

system.

Certain

commercial

equipment

is

identified

in

this

paper

in

order

to

adequately

specify

the

experimental

procedure.

In

no

case

does

such

identification

imply

recommendation

or

endorsement

by

the

National

Bureau

of

Standards,

nor

does

it

imply

that

the

material

or

equipment

identified

is

necessarily

the

best

available

for

the
Figure 9. - Windscreen Test Configurations, (A) Grazing flow and

grazing acoustic incidence angle, (B) Grazing flow and

normal acoustic incidence angle, (C) Normal flow and

grazing acoustic incidence angle.

24
OVERALL

SOUND

PRESSURE

LEVEL

VS

WINDSPEED

o

o

o

CD o

oo o

t> oco

CDLO oCD

ud7* 02 3d ga'i3A3i 3dnss3dd qnaqs ribddAo

25

figure

10.

Overall

wind

induced

noise

as

a

function

of

wind

speed

for

grazing

flow

incidence.
-WEIGHTED

SOUND

PRESSURE

LEVEL

VS

WINDSPEED

CE o O O o o o o

CD CO c—oz 3d aa‘"i3A3i

LO QND0S

M- Q31HD I 3M _

ti

CO

-J oo

CM

26

Figure

11.

—

A—

weighted

"wind—

induced

noise

level

as

a

function

of

wind

speed

for

grazing

flow

incidence
OVERALL

SOUND

PRESSURE

LEVEL

VS

WINDSPEED

o o

o

o

CD D

CD Oo

o

CD o

LO Ud 7* 02 3d 90 ‘ 13A31 3df)SS3dd ONA0S 11Bd3A0

-1 oo-3-

27

Figure

12.

-

Overall

wind-induced

noise

level

as

a

function

of

wind

speed

for

normal

flow

incidence.
(O\o

UJ

UJ

Q_CO

28

Fig.

13.—

A—

weighted

wind—

induced

noise

level

asa

function

of

wind

speed

for

normal

flow

incidence.
00

90

80

70

60

50

40

30

1 FREQUENCY, HZ

(a) Windscreen A - grazing flow

. - One-third octave band wind-induced noise spectra as a function of

wind speed for grazing flow incidence.

29
THIRD-0CTRVE

BAND

LEVEL

IN

DB

RE

20

piPA

00 1 1 r—i 1 1 1—i

1—i

1 i

' i ' i r

90

80

70

60

50

40

30

A 2 M/SEC

+ 4 M/SEC

X 6 M/S EC

<!> 8 M/SEC

10 M/SEC

X 12 M/SEC

z 1 4 M/S EC

J__j i i

FREQUENCY. HZ

(b) Windscreen B - grazing flow

Figure 14 . - Continued.

30
90

80

70

60

50

40

30

i t i i i i i i i i i i i i r i

1 1 1 1 1 1 r—i

1 1 1 r r a 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

<3> 8 M/SEC * 10 M/SEC

X 12 n/SEC

Z 14 M/SEC

(c) Windscreen C r- grazing flow

14. - Continued.

31
90

80

70

60

50

40

30

t

1 1 1 1 1 1 1 1 1 1 1—

1 1 1 1 1 1 1

1 i I 1 i i i i r A 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

<J> 8 M/SEC * 10 M/SEC

X 12 M/SEC

z 1 4 M/SEC

14. - Continued. 32
1 00

90

80

70

60

50

40

30

T T l l 1 I1 1 1 1 1 1 r

1 A 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

<5> 8 M/SEC * 10 M/SEC

X 12 M/SEC

Z 14 M/SEC

(e) Windscreen E t- grazing flow

- Continued, 33
THIRD-0CTAVE

BAND

L

EVE

1 AO r T i—i

1—i—i—

i

r

90

80

n 1 r l

1 1 r A 2 M/SEC

+ 4 fi/SEC

X 6 m/sec

<J> 8 M/SEC

10 M/SEC

X 12 M/SEC

z 1 4 M/SEC

(f) Windscreen F - grazing flow

Figure 14. - Continued.

34
•to

90

80

70

60

50

40

30

1

A

~T 1-

2 —1

1 1 M/SEC

+ 4 M/SEC

X 6 M/SEC

<!> 8 M/SEC * 10 M/SEC

X 12 M/SEC

z 1 4 M/SEC

FREQUENCY, HZ

(g) Windscreen G - grazing flow

14. - Continued,

35
90

80

70

60

50

40

30

t

1 1 1 1 1 1 1 r—i—i

1 1 1 1 1 1 1 1 1 1 1 1 1—i

1

1 1 1 i r A 2 n/SEC

+ 4 n/SEC

X 6 n/SEC

<J> 8 h/SEC

10 n/SEC

X 12 n/SEC

z 14 n/SEC

(h) Windscreen H - grazing flow

14. - Continued.

36
100

90

80

70

60

50

40

30

t i i i i i t i r~ t i i i i i i i i i | i

| 1 1—| , 1 1 1——r —

A 2 M/SEC

+ 4 0/ SEC

X 6 M/SEC

<J> 8 M/SEC

10 M/SEC

X 12 M/SEC

z 1 4 M/SEC

(i) Windscreen I - grazing flow

14. Continued* 37
10G

90

80

70

60

t 1 1—i

1 1 r~—1—

1 1 1 1 1 1 l

| A 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

o 8 M/SEC

10 M/SEC _ X 12 M/SEC

z 14 M/SEC

rrc tor

FREQUENCY. HZ

(j) Windscreen J - grazing flow

38

Continued
1 00

90

80

70

60

50

40

30

1

gure

(k) No Windscreen - grazing flow

14. - Concluded . 39
THIRD-OCTfiVE

BAND

LEVEL

IN

DB

RE

20

/*Pfl

00 1 1 1 1 1 1 1 1 1 1 1 1—i

1 1 1 1 1 1 1 1 1 1

1 i i i i i r~

90

A 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

8 M/SEC * 10 M/SEC

X 12 M/SEC

z 1 4 M/S EC

i I 1 i_ TuTT

Figure 15. -

FREQUENCY, HZ

(a) Windscreen A - normal flow

One-third octave band wind-induced noise spectra obtained as a function of wind speed for normal flow incidence.

40
] 00

90

80

70

60

50

40

30

1 1 1 1 1 1 i i i i I i i i I i 1 i

1 1 1 1 1 1 1 1 1 1 1 1 r ^ 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

<S> 8 M/SEC * 10 ri/sec

X 12 M/SEC

Z 14 M/SEC

FREQUENCY. HZ

(b) Windscreen B - normal flow

. - Continued*

41
90

80

70

60

50

40

30

ti—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r t A 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

<5> 8 M/SEC * 10 M/SEC

X 12 M/SEC

2 1 4 M/SEC

(c) Windscreen C - normal flow

15. - Continued.

42
] 00

90

80

70

60

50

40

30

t i I i i I I I I i I i i I i i i i i i i

1 1 1 1 1 r—t

t

1 r 2 M/SEC

4- 4 M/SEC

X 6 m/sec

<J> 8 n/sEC * 10 M/SEC

12 h/sec

z 1 4 M/SEC

I J I I I L. j 1 1 1 1 1 1 1 1 1 1 ^_| 1 1 1 1 1 font

-i—i—

. j .. .

i

FREQUENCY. HZ

(d) Windscreen D - normal flow

. - Continued . 43
100

90

80

70

60

50

40

30

—r~ A

i r~ 2 ~i—i—i—

m/sec

+ 4 m/sec

X 6 M/SEC

<J> 8 M/SEC * 30 M/SEC

X 12 M/SEC

z 1 4 M/SEC

FREQUENCY, HZ

(e) Windscreen E - normal flow

. - Continued.

44
100

90

80

70

60

50

40

30

1

1 1 1 1 1 1 1 i i i i r i i i i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 r A 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

<$> 8 M/SEC * 10 M/SEC

X 12 M/SEC

z 1 4 M/SEC

(f) Windscreen F - normal flow

. - Continued •

45
1 00

iW-^ J--frr

FREQUENCY. HZ

(g) Windscreen G - normal flow

Figure 15. - Continued. 46
1 00

90

80

70

60

50

40

30

1

i 15

t i i i i i i i i i i i i t i i i i i

1 1 1 1 1 1 1 1 1 1 r A 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

<J> 8 M/SEC * 10 M/SEC

12 M/ SEC

z 1 4 M/SEC

(h) Windscreen H - normal flow

Continued . 47
100

90

80

70

60

50

40

30

i r—

1 1 1 1 1 1 1 1 1 1—i1 1 1 1 1 1 1 1 1 1 1 r—i

1

'—

r A 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

8 M/SEC

10 M/SEC

X 12 M/SEC

z 14 M/SEC

FREQUENCY, HZ

(i) Windscreen I - normal flow

Continued • 48
cro_

a. o

CMLU

Qd

CDO

_J

LU

d>LU

CD2

cr

CO

LU>

cru

o

i CDQd

] 0 0 I I ! i i i—i—i—i—i—i—

90 -

1 I i S i I

1 1

i ! 1 1 1 1 1 ! T a 2 M/SEC

+ 4 M/SEC

X 6 M/SEC

O 8 M/SEC * 10 M/SEC

X J 2 M/SEC

Z 14 M/SEC

80

€0 -

FREQUENCY, HZ

(j ) Windscreen J - normal flow

Figure 15. - Continued . 49
THIRD-OCTAVE

BAWD

LEVEL

IN

OB

RE

20

/li PA

1 00 |- r~ r i— T T T T

ti—r -i

A 9 0/ oEC

+ 4 n/sE:

X 6 n/SEC

<5 8 n/SEC * 10 M/SEC

12 11/SEC

z 1 4 n/SEC

FREQUENCY, H7

(k) No Windscreen - normal flow

Figure 15. - Concluded • 50
Figure

16.

-

Dependence

of

overall

and

A-weighted

levels

on

windscreen

porosity

and

diameter,

v=12

m/sec.

SOUND PRESSURE LEVEL, dB re. 20pPa

—> w tn vj to o o o o o

51

110
APPENDIX A: Tabulation of Wind-Induced Noise Levels for Grazing Flow Incidence

This appendix contains a tabulation of the one-third octave band A-weighted,

and overall wind-induced noise levels (dB re 20 yPa) measured at grazing flow

incidence over the wind speed range 0 m/sec <y<_ 14 m/sec for each windscreen

shown in Figure 2 as well as for the microphone with no windscreen. Zeros

have been inserted in the tables when the signal-to-noise ratio was insufficient

to obtain meaningful data.
Table Al. - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen A

(Grazing Flow Incidence)

SPEED (M/SEC

OCT p A M

2 4 6 8 1 0 1 2

1 2 38 .

54.4 67.7 73.9 77.9 79.0

1 6 36.0 52.7 64.4 72.3 77.7 79.7

20 33.5 51.7 6 1.2 . 69 . 0 .. .

7 5.5 78.9

25 3 1.0 49.9 60 .

6 6 o 3 7 1.1 75.5

31 29.6 4 7 . 0 - 58.9 64 .

. - 6 8.2 - 71.6 —

40 27.2 44.6 57 . S 64 .67 .69.0

SO .0 4 2 . 6- — 5 3 . 6. 6 1 -^2 . 6 6 . 0- 67.5

63 .0 .0 50 .

58.2 64.7 6 6 ® 6

80 .0 38.2 - 48.4 55.3 6 1.5 64 .

1 00 .0 .0 46 .

52.0 57.0 60.7

125 .0 — 3 3 . 0- - 4 3 o 8 - 49 .- 4 . . 5 4.0 — 56.9

160 .0 29.4 4 1.3 47.2 5 1.7 54.0

200 .0 2 6 *~7 38.3 4 4-.

48.7 51.3

250 .0 2 1.6 34.4 4 1.6 45 • 6 48.2

3 1 5 .0 20.0 .0 39.3 - . - - .0 - 46.3

400 .0 .0 .0 36.5 .0 44.9

500 .0 . 0 - 25. 8 3 3.9 . 40.0 - - 43 , 6 - - 630 .0 . 0 23.0 3 S .6 .

00 42,7

800 .a .0 2 2 . 1 - 29 . 9 36.7 4 8.3

1 000 .0 .0 25.0 30.7 .0 40.8

1 250 .0 . 0- . — .0 2 7 . 3 - . 0 39.7

1600 • 0 .0 .0 26.6 .0 38 • 7 2000 .0 .0 . o - - 25 »- 2- - .0 —- - .0

2500 .0 © 0 .0 23.2 .0 .0

3 1 50 - -. 0- .0 —- .0 - - 22.2 e 0 . o - 4000 .0 .0 .0 20.9 .0 .0

5000 .0 .-0 -.0 .0 .0 --- .0

6300 .0 .0 .0 .0 .0 .0

8000 * ft .

0 . _ o .. 0 - © ty?“ . - -- - Q _ . ,0 ..

1 0000 .0 • 0 .0 .0 .0 .0

12500 .0 , 0 ... *0 .0 _ - - .0 -- -- . 0

AWT .0 23.5 36 .

44.0 48.0 52.5

LIN - 42 . 2 - 59 . 1 - 7 1^1 7 7.9 ___ 82.8 — 85 . 1 -

l 4

80.8.

8 1.3

8 1.2 . 78.

-7-6 e~2 7 1.4

6-9 e-5~

68 .

66.9

64.0

6 0 © 9

57.5

- 5 4 • 4- -

S 1 • 2

4 9.4 __ 47.9

47.0-

46.8

4 5.5

44.6

4 3.8

43.3

.. 43 .4 —

4 3.8

45^2

47.2

4 4 . 9 —

45.9

-4 6^0

43.0

4 0. 8

58.

87.3

53
Table A2

- One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen B (Grazing Flow Incidence)

SPEED (M/SEC ) OCT BAND 2 M 6 8 1 0 1 2 I M

1 2 37.7 55.3 ... . 6 7 . M 76 . M 8 1.8 83 . M .0

1 6 3 M • 6 52.6 66.2 7 M .9 82.3 86 . 1 .0

20 30.2 5 1 • 3 __ 6 3.6 73,0 80.7 87.5 • o 25 26.8 M 8 • 6 6 1.3 69.9 77.0 84 .

.0

3 l 2 M • 7 M 5 . 0 .... 59.8 6 7.7 . 7 M . 7 80.6 .0

MO 22.6 M 2 . M 56.2 66 .

73.2 78 ..0

50 .0 3 9.. M 5 2.3 6 3 .

7 1.2 77.0 , 0

63 .0 .0 50.9 6 1.4 69.6 74 .

.0

8 0 33.8 4 9.6 59.6 67 .

73 .

_ .0

1 00 .0 .0 M 7 * 5 58 . I 65.7 71.9 .0

1 25 ... ... . 0--— 30.

- ... MS . 2 - -- 56.7 - 6 4 . 3 - 69.9 - ...0

1 60 .0 30.0 MM .

55.2 63.2 68.7 .0

200 .0 - 30.6 - M 3.

— —5 M , 3 62.6 67.0 - . 0—

250 20.5 28.2 M 0 e 0 53.5 63.4 66 . 2 .0

3 15 • 0 34.2 —— 3 7 . 6 - 53.3 60 . 5— 65.6 — „o

MOO .0 37.2 37.3 4 8.

6 1.9 65 . 3 • 0

500 -- • 0 - 30.2 -- M 2.

M 6.0 60.7 64 . 8 -- . 0 - 63 C .0 29.8 M 6 • 3 M 7 • 7 55.0 62.9 .0

800 .0 35.8 - M 7 . 2 - M 7.5 53.0 59 . 5 - .0

1000 • 0 22.9 35.5 M 3 .M 50.3 57.9 .0

1 250 .0 -- —• 0 —— 30.3 39.1 47.6 54 .8 — - . 0

1 600 • 0 9 0 27.3 36 .

MM .

51.8 .0

2000 .0 -

. _ .0 —— 2 6*1 36.1 4 1 .9 52 . M .0

2500 • 0 .0 29 . M .0 M 5 • 7 52 . M .0

3150 .0 .0 .0 MO .

• 0— .0

M 0 00 .0 .0 .0 .0 36.7 .0 .0

5000 . «

_n . - . o . n 3 5^3 © 0 . 0

6300 .0 .0 .0 .0 .0 .0 .0

8000 .0 .0 .0 -.0 # o- -————« 0 .0

1 0000 .0 .0 .0 .0 .0 . 0 . 0

12500 .0 - . 0 - .0 —• 0 — . 0 - .0

AWT 11.9 38.5 M 9 • 9 55.0 6 M • 3 69.5 .0

LIN MO.M 59.1 7 1.9 80.8 87.7 92.6 .0

54
Table A3. - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen C

(Grazing Flow Incidence)

SPEED ( M / SEC ) OCT BAND 2 8 6 8 1 0 i 2 1 8

I 2 HO • 2 5 7.6 —

66.

- 7 3.7 79 .

8 1.3 82.6 -

1 6 39.2 53.9 65.9 7 1.6 77.7 8 1.7 83.3

20 36.3 5 1 . 9 6 3 . 6- 7 1.2 76.3 80.0 82.8 - 25 33.3 50 .

59 .

69.2 75.2 78 .80.3

3 1 _ -30 • 8 8 8 . 1 - 57 . 1 . — 65.8 72.9 7 6 . 6 - 7-7 * 5—

*10 28 • 3 8 5.7 55.7 63.

68 .

73.9 7 5.8

5 D . 0- 8 2.3 5 3*6 . 61-.1 65.9 6 9 . 7- -- 7 2 . 5 - 63 .0 . U 5 1.6 59.2 68.9 6 7 o 6 69.9

80 . 0 3 7 .8 9.3 5 7.2 - 6 3.3 - 66.2 6 8.3 -

I 00 . 0 .0 8 8 . 8 5 8.

6 1.1 6 8.6 67 e Q

» 25 • — .0 32.8 83.6 - 52 .

58 . 6 - 61 . 9 - 65.6 -

1 60 ® 0 29.3 HO . 7 89.9 5 5 ® 1 59.6 6 3 « 0

2 00 - -- .0 - 26.0 3 8.2 8 7 . 2 52.1 55.9 59.7 - 250 .0 22 . 8 3 8 . 8 83.8 89.6 53.6 57.7

3 1 5 .0 2 1 oS- - .0 8 1.0 8 7.7 51 . 2 - 56.6

<100 .0 .0 .0 37.9 85.2 88.7 58.8

5 00 — * 0. —- . 0 —

.0 35 . 0 - — 42.7 8 6.-7 - 53.2 - 630 • 0 .0 .0 32.6 80.3 88.9 5 1.5

8 00 ..... .0 . 0 .0 3J .0 37.8 8 2 . 6 - 8 9.3

I 000 • 0 .0 .0 3 1.5 37 . 3 8 1,5 87.3

1 250 .0 — • 0 .0 .28.3 36.1 39,7 85.9

8 600 .0 .0 .0 27.8 35.3 39 , 1 88.6

2000 - . 0 .0 .. ... .0 - 2 6.7 — . 0 - 38.

8 8.0 —

2500 a 0 .0 .0 26 ..0 38.3 88.2

3850 . . 0 ... .0 .... .0 25 .3 .0 .0 88,7

*1000 ®0 .0 .0 .0 .0 .0 88 . 8

5000 - 9 0 ---—.a—- -.0 .0 • 0 — . 0 - 85.6

6300 «0 .0 .0 ®0 .0 0 0 88,7

8000 „ Q_ • 0- - 6 0 .0 . o - - - .. ..0 8 2.7

1 0000 .0 .0 .0 .0 eO 0 0 82.2

1 2500 .0- . .. . o . .0 _ . ... . 0 - *0 _ . 0- - - 80.0

AWT .0 2 3.5 35.9 85.9 51.9 55.9 6 1.2

L l N H 8-. 3 60.8 7 1 .0 7 8 . 8 88.t 87.8 89.2

55
Table A4 . - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen D

(Grazing Flow Incidence)

SPEED ( M / SEC ) OCT BAND 2 8 6 8 10 I 2 1 8

1 2 39 . 1 5 2.9 __ 65.8 71.7 7 5.5 7 7.9 82 . 1

1 6 36.3 52 . 1 6 1.2 69.5 73.8 77.8 80 .

20 33.50.9 58.6 66 . 0 - 7 1.6 76 . 1 80.8

25 30.8 88.9 57 .

62 .

68.8 73.3 78.3

3 l 2 8 • 6 _- 8 6.1 _5 6 .8 6 1.5 65.8 7 0.6 75 . 8 . 80 26 .

83.0 58.5 60.8 68 .

67.9 72.6

so . 0- 40.7 5 l .9 . 5 8 62.8 66.7 69.9

63 • 0 • 0 88.7 55.6 6 1.3 66.0 69.3

8 0 — — . 0 - - 36 . 5 — 8 6.5 51.9 5 8.3 - 6 3.7 67.9

1 00 • 0 .0 88.7 89.0 58 .

60 . 1 65.8

I 25 - .0 —30*9 - 80.9 86.2 51.0 56.5 6 1 .-9

1 60 .0 27.8 38.8 83.0 88.2 53.0 58 . 1 200 .0 28.4 3 5 .7 80 . 3--- - 8 5.5 — 89.9 58 -8- 250 • 0 20.8 32.0 37.6 82.3 86.8 52.0

315 • Q —20.2 . 0 _ ... 36 *-2 - — .0 -- - 85.5 50 . 2-

8 CO .0 • 0 .0 38 . 3 .0 88.3 88.8

500 _ .0 .0 — , 28 . 8 - 32.6 39.0 83.7 8 7.9

630 .0 .0 22.6 3 1.7 37.8 83.0 87.8

800 — .0 .0 22.2 — 32.2 — - 37.3 -- 8 2.8 8 6.8

1 000 • 0 .0 25.7 3 1.5 37.5 82.6 86.3

1 250 .0 *o__ 20.0 29 U— 36.8 8 2.2 - 8 6.1

1 600 .0 .0 22 . 1 28 . 7 36.9 82.6 86 . 6

2000 - -- .0 • 0 -

.0 27.3 39.2 8 8.3 . 88 . 1 - 2500 • 0 .0 .0 26 . 1 39 . 6 85.8 89.9

3 150 o . o .0 3 9.6 8 6.6 50 . 7 HOOD • 0

_ . .0 .0 .0 38.5 86.8 51.8

5000 • 0 . *Q .0 - .a- 36.2 86.3 52.1

6300 • 0 • 0 .0 .0 33.9 85.

51.8

6000 . o ... .0— -.0 — . 0—- 31.3 8 2.7 _ 50.-8 -

I 0000 .0 .0 .0 .0 29 . 1 80.3 87.6

12500 . o * 0 . . o . .0 27.3 36.7 83.8

AWT • 0 2 1.9 35.0 82.2 50.2 56.9 61.8

LIN 8 2.3 58 . 0—

. 68.6 75.3 79.6 83.2 87.8

56
Table A5 . - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen E

(Grazing Flow Incidence)

SPEED ( M/SEC

)

| / 3 OCT BAND 2 M 6 8 1 0 1 2 1 M

1 2 3 5.3 52. 1 _6Z. i 6 8.9— 72.9 ... 76.0 78.9

1 6 3 M • 2 50.6 58 .

66.

70.8 75.5 77.9

2C 3 I .0 _— M 9 • M 56.63.5 68.2 73.7 -- 7 7 oM2 S 27.9 M 7 • 6 55.0 60 .

65.5 7 1.2 7 M . 6

31 2 6.4 MM m-6 -53.8 59 .1 62.8 6 8 ® 6 72.5 . MO 2M.M M2.

52.2 58.0 62.3 66 .

68 .

50 .a- 3 9 . 9. M 8 e 9 5 6.1— 6 0 » l— . 6 M » 8 6.6 . 6_

63 .0 .0 M 6 • 5 52 .

58 . M 63.5 6 6.6

80 .0-—3M .5

- M 3 .

M 9 * 1 - SM.7 - 6 1.1 6 M . 9 -

1 00 .0 .0 M l . 9 M6.5 5 1.0 57.6 6 1.8

125 .a-—2 9„M -38.9 M2 ^9 M7 .5 53.

58.6 —

1 60 .0 27.8 37.8 39.5 MM . 2 50 .

5 5 o M 200 - .0- .0 —. 0- -37.9 M | .8 M 8 e 5 - —52.0

250 • 0 .0 30. S 33.5 39 .MM • 9 M9 .

3 ! 5 .0- .0 — .0 -33.0 .0 - MM . M - M8 .

MOO .0 .0 .0 32.3 .0 M3.

M7 .

500 .0-——..a -2 a. s 30.8- 37.6 M3. 1 M7 .

630 • 0 .0 28.

29.9 36.8 M2.

M 8 o 6

800 .a - .0 -27.8 2 9 .6 -36.5 M2.

M&.9

1 000 .0 .0 28.5 3 5.8 37.8 M3 .50 .

12 50 --- .0 . 0 23.2 2.9.6- — 37-6 M 5 ® I 5 1.2 -

1 600 .0 .0 2 5.1 32.3 39 .

M 5 © 9 52 .

2000 • a~ .o

-22.M 33.5 M 1 .6 M7 .2 52 . M 2500 .0 .0 21 .M 31.5 M3.

M 9 © 0 53. M 3 I 50 .0- . 0

.0 3Q.-H - --MM . 2 50.6 SM .6 - M000 • 0 .0 .0 28 .

M3.50.9 55.9

50 00 — *0- .0 . o 25.-0

M 1 .M 50.3- 55.8 - 6300 .0 • 0 .0 22.5 39 .

M 9 .

55 . M 8000- —- .0 - * o__ * 0 20.3

36.5 -M6.9 53.8

1 0000 • 0 .0 .0 .0 32.7 M3 .

51.0

1 2500 — - - 9 Q .0 .0 .C- 2 9.0 39.9 M 7 . 1 AWT .0 18.7 35.9 M2.

52.7 59,7 65 .

LIN 3 9. M - -56. a —-6 5.6 — 7 2 . M. — 7 6 . 8 8 1.2- 8 M . 3-

57
Table A6 . - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen F

(Grazing Flow Incidence)

OCT BAND 2 M

S P E £ D ( M/SEC

6 8 1 0 1 2 8 4

1 2 35. M 51.1 59 .

66.9 72. I 75.2 78.5

1 6 33 . M M 8 • 9 56.

6 M . 7 69.9 7 M • 1 78.2

20 30.7 M 8 • 7 5 M . M 6 1 .M 67.5 72.2 76 .

25 28 .

*t6 . 6 53.9 57.6 6M .5 70 .

73.6

31 25.8 M3.

-52 • 5_... -5 8.3 - 6 1 .6 67 .

7 2.M ... MO 22.9 MO. 1 50.9 56.6 6 1.8 65.3 68 . M 5 Q

63

• 0 3&*4 4-7 .2 . 5 M - 59 *-& - 6 4.7 6 7.1_

.0 • 0 MM . 6 51.8 58 . M 63.2 66.2

- -- 80 .0 -—-33^3 M 1.8 — -M7.6 55.1 — 60.5 - 6 M . 7

1 00 • 0 .0 M 1 .5 M 5 • 6 50.9 57.62.3

125 • 0 2 7.9 35. 7 —-M2.

M7.6 5M • 1 59*1

1 60 .0 26.5 33 . M MO.

M 5 • 8 51.8 56.

20C .0 .0 — 30.9 -3 9.0 MM . 1 M 9 • 9 53 *M . 25C .0 .0 28.0 36.0 M2.

M 7 • 6 52.

3 15 .0 .0 .0 35.0 .0 M 7 . 5 — 5 2.0

MOO .0 .0 .0 33.8 .0 M 7 . 1 52.

500 .0 o 2 2.9- 32.5 39.5 M 7 • 3 52.7 - 63C • 0 • 0 22.0 3 1.8 39,2 M 8 . 2 53.8

800 .0 .0 - 2 3.8 3 1.5 39 .

M 9 .- 5 5.7

8 000 • 0 .0 25. M 33.6 M S . 7 5 1.5 57.0

1250 .0 .0 23.9 -3M.9 - M 2 « 9 5 1.6 — 5 8.3

1 600 .0 .0 2M.2 36.

M3.

50.9 57.6

2000 .0 — . 0 _ . .0 38 . M M5.3 5 1.1 — 5 7.0

2500 .0 .0 .0 37 . M M 7 * 2 52.5 57.0

3 150 .0 .0 — .0 3 5.2_ - -M8.0 53.5 57.9

M 000 • 0 .0 .0 33.6 M 6 • 9 5M .0 58.5

5000 »Q -- .0 . . o 3 8.2 -MM . 7 53.2 5 8.8

6300 .0 .0 .0 28.5 M2. M 51.2 58.0

8 0 00 - .,0 ... o .. . . .0-.-. - *0. 39 . H- M 9 . 0- 56.0

1 0000 .0 9 0 • 0 .0 35.5 M 5 • 8 53.2

1 2500 .0 _ • o __ .0 . .0 30.6 M 1 . 2 M 9 .

AWT .0 17.3 33.2 M6.5 56.2 63.3 68 . 9

L I N 39.

55.7 __ 63.3 70.6 76.0 80.2 83.9

58
Table A7 . - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 pPa) for Windscreen G

(Grazing Flow Incidence)

. SPEED ( M /SEC > o c T BAND 2 4 6 8 10 1 2 l 4

1 2 35.6 - —

7.4 — 57.4 — 64.

69.6 73.4 76 .

8 6 33.9 46.5 5 5 ® 0 6 2 © 5 6 7 o 9 73.1 7 S . 20 3 I .6 46.0 52.2 - 61.8 66.1 70 ® 5 74 . 2 S 27.9 44.6 50 o 9 57.9 65 .

68.7 72 . 31 2 4.6 -~ 4

* 7 - . -5 U 3 - 56.5 . 62.7 ..... . 67 * 8 71 , 40 22.3 39.4 49.3 55.3 6 1 .5 63.9 69 , SO .0—-36 • 3 ——

6 . 1 5 4 . 0— 59 . 9 — 63.8 67 . £ 3 . 0 .0 43.2 5 1,9 58 .

63 .67 . 80 .0 —-30.2 4 1 . 7 49.7 56*1 6 8.6 66 . 100 .0 .0 40 .

48.3 53 .

59 . 7 64 .

1 25 _ .0 ... 26.2 36.0 46*3 52 * 9 ..... 57.9 ......- 62 .

1 60 .0 25.0 34 .

46.52.4 57.3 6 i . 200 -—-a- -- 22 . 1 - 33.

4 6.8 .. 53.2 57.5 60 . 250 .0 20.2 32.

47 . 6 54.4 57 . 9 6 1 . 3 1 5 • a—— O- .0 49 . 2 55.9 59 . 4 - 62 . 400 .0 • 0 .0 49 . 9 56.9 60.

63 . 500 — *0 — . 0—- 34.4 5 i e 4 — 58.2 — • 6 1 . S - 65 . 630 .0 .0 33.9 53.5 59.8 63 . 1 6 6 . 800 — . 0- - .0— 3 4.4 S3 . 0 59.9 - 63.9 66 ,

1 000 • 0 .0 37 .

51.2 59,

64 , 4 67 . 125 a --- .0 - .0 — 3 8.5 49 . 5 - 58.9 c*

-E a 0 1 68 .

1 600 .0 .0 34 .48.6 57.0 63.4 68 . 2000 .0— .0 - 31.6 47.7 55 . 7 - 62.

67 . 2 5 00 eO .0 29 . | 46.5 55.4 60.9 6 6 . 3150 — . a— . 0 - - 26.0 4 4 e-C 54.5 60.2 65 . 4000 .0 .0 22.5 4 1.2 52.4 59.4 64 . 5000 • 0—

.a - .0 38.

49.8 57.5 63 . 6300 .0 • 0 .0 33.9 46.9 55.0 6 1 . 8000 - • 0 — .0 - - — .0 . —29 . 9. 43.4 5 2.3 59 .

I 0000 *0 .0 .0 27.9 39.9 48.5 56 ©

1 2500 • 0— .0 *0 * 0 . . 3 6*5 . 43*9 52 . Art T .0 17.9 44.

59.9 68 ®0 73.2 77 . L I N 39.4 5 3 . 1 - 6 1.8 69 • 8 7 5 . 6 - . 80.0 83 ,

6

48

5

3

85

P

l

8

1

l9

6

9

9

0

49

6

92

4

42

6

5

75

45

6

6

59
Table A8 . - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen H

(Grazing Flow Incidence)

SPEED ( M/SEC ) OCT BAND 2 8 6 8 1 0 1 2 18

1 2 35 .

50.6 60 .

67 .

72.8 76.7 78.8

1 6 33.5 88.6 57,

68.5 70.1 75 .

78.8

20 30.8 8 8.0 55.3 ..... 62.8 . 6 7 * 8 72.6 76.6

25 28 .

86.8 53 .58 .65.5 70.8 78.0

3 1 26.8 8 .8.5 , 52.

... 5 7.2 .... - 63.1 68.2 72.2

HO 23.8 82.2 5 1.2 57.0 61.2 65 .

69.5

50 .0 3 9 ._8 .... 8 8 .

55 .

6 0.5 ...

63.9 6 6.

63 • 0 .0 86.8 52.5 59.0 63.0 66 .

80 • 0 - 3 8 .- 2— 8 3.2 88.8 56.3 6 1 .6 _- 65.5

1 00 .0 .0 82.1 86.5 52.9 58.6 63.2

1 25 .0 2 9 ^0- ... 37 .

83.6 50.3 55.2 60 . 2- -

1 60 • 0 27.9 35.8 8 1.5 88.5 53 .

57.8

200 .0 .0 32.5 3 9.9 87.3 50.9 55.8

250 .0 20.8 29.8 37 . 1 83.0 89.0 53.7

315 .0 - 20^3 .0 - 36.5 • 0 - - 88.3 53.3

8 00 • 0 .0 .0 38 .

• 0 87.7 52.6

500 - .0 . 0 - —- 28 . 7 TM- 33.3 80.8 87.5 - - 52 . 5- 630 , .0 .0 23.3 32.6 39 .

87.7 53.

800 .0 .0 — 22 . 8 -- 3 1.7 80.0 — 8 8.1 - 53 . 8 .

! 000 .0 • 0 26.2 33.5 81.3 50.8 55.3

1 250 .0 . 0 - 2 2.8 3 I . 7 _ 82 . 1 - 5 1.7 56.8

5 600 .0 .0 22.8 35.3 83.0 5 1.3 58.

2000 • 0 .0 20.2 38.0 85.0 5 1.1 5 7.6 . 2500 • 0 .0 .0 33.5 87.0 52.

57.

3 150 - .0 .0 .0 32.6 86.9 53.8 57.6

8000 • 0 .0 • 0 32.8 86.9 58 .58.5

5000 • 0 .0 .0 29*8 85.0 53.9 59.0

6300 • 0 • 0 .0 26.8 82.1 5 1.7 57.9

8000 ..... .0 ,.0 .0 -.0 39.8 --8 9.3 - —56 . L ._.

1 0000 .0 .0 .0 • 0 35.3 86.2 53.3

1 2500 o 0 .0 .0 - .0 31.1 8 1.9 89.3

AWT • 0 20.8 33.9 85.5 55.9 63.8 68.7

L I N 39.2 . 55.5 6 8.3 .... 7 1.0 . 76.3 8 1.1 88.2

60
Table A9

One Third Octave Band Wind—Induced Noise Levels

(dB re 20 yPa) for Windscreen I (Grazing Flow Incidence)

SPEED ( M /SEC

OCT BAND 2 4 6 . 8 SO 1 2 1 4

1 2 37 * 7 . .. 5 4*5 —.6 6 ® 8 - -7 4 a 0 . .. 78*5 78.8 .. 81.3

9 6 35.2 52*5 6 3 a 8 7 2 o 5 76 .

78.9 8 1.3

20 32.2 50.

- 59 . 0 - 70 * 3 - — 75 « 2 78.3 - 80*8

25 30*0 48.4 57*6 64*5 71*9 75.4 7 8*3

31 2 8 . 8— 4 7 * 4 -.. - 5 6 & 8 ~— 6 3*8 67.4 72*3 7 5.8

40 26.3 44.0 54.6 62.3 66.2 68 9 8 72*2

50 .0 - 4 1-.-9 - 5 1.9 - - 5. 9 . 9 . 6 4 * 8— 6 6.-7 6 - 9.2

63 .0 .0 49.3 58 .

63 * 3 66*5 67.9

80 . n — - 38 . 0 - 47*4 - 55*8 - 6 1

- e. 4 64 * 4 66.

I 00 .0 • 0 45.3 52.8 58.5 6 S © 3 64 .

I 25 - * o — 32.7 43 . 0 - 5 0*1 55 . 6 - 58 * 3 - 61.8

8 60 • 0 30.3 4 1*8 48 .53.2 55.8 58 * 6

200 .0 — 2 7.3 38 .

- 45*8 5 0*8 5

o 3 . .. 56.5

250 .0 22.3 3 4.0 42.8 48.0 50 * 9 5 4 ® l 3 9 5 .0 -- 2 l .4 ~ - .0 - - 40*6 46.5 4 9.4 53.0

*100 • G 20.0 .0 38.0 *0 47.5 51.4

500 . 0 • . 0 26*4 -. - 35.4 — 4 2 * 0 - - 46*0 49.9

630 .0 .0 24 * I 33.4 40.3 44.9 49*0

800 - - .0 - - .0 - 23*3 38 *a_ - 38.5 4 3.1 — - 47.8

1000 .0 20 .

25.9 32.2 .0 4 3 ® S 47.5

9 250 .0 .0 - 20 ® 6 - 29.0 - *0 42*5 46.0

1 600 .0 .0 21.2 27.9 .0 40.3 44 * 4 2000 ... . .. *0 ... .0 - 20 * 3 -. 2 7-e 3 --.a 39*9 44 . 1 2500 .0 *0 a 0 *0 .0 *0 44*6

3 150 • Q-- .0 - *0 • 0 - .a- *0 ... 45.0

4000 .0 *0 *0 .0 *0 *0 45*5

5000 *0 —.—« 0 — *0- O 0 ® 0 — ----- 0 0 — 46*2

6300 .0 *0 *0 *0 *0 *0 4 5.1

8000 .0— . 0—— *0 —- —• Q .0 « 0 44*6

1 OOCO .0 *0 .0 .0 .0 .0 43.1

I 2500 —. .0 .0 —

- * 0 .. . 0 . .0 • 0 — 39.

AWT *0 25.8 36.5 44.9 49.

54 » 4 59*4

L i N 4 I * 2 . 5 8.8_. ... 6 9 * 8_ 7 8 . 0 ... —82 ® 6 8 4*6 87*3

61
table A10. - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 pPa) for Windscreen J

(Grazing Flow Incidence)

SPEFD ( M/SEC ) OCT BAND 2 M 6 8 10 I 2 1 M

5 2 Ml . 8 5M • 7 63.7 _ 73 .5 7 8 « 2 8 l . 8 83.9

I 6 38.9 5 3 « 0 6 3 ® I 7 2 c 9 77 © M 7 9 a 6 8 1.5

20 3 M . M 51.7 6 1.1 7 8 o 7 7 7.9 M 7 9 a M .... 7 9 ® M 25 3 1.9 M 9 « 8 59.0 7 1 « 0 76 ®0 78 © M 78. M 31 30.3 M 7 « 3 .. ... 57. M.. . .69 .8 .7 M a 6 77a 1 77 o 2

*10 30 a M MM® 1 55 » 3 6 8 © 7 73.7 76© M 7 7 o 5

50 .a MO . 9 52.8 66.8 7 2 . M 7 5a-6 .... 7 6.M

63 .0 .0 M8 .

6 M e 6 7 0 » 6 7 M a 5 73.0

8 0 .0 3 9 e 5 M 6 .

6 8 .9 ..... 69©0 73©M 7 3 a 3 --

1 00 »o .0 M 6 o 9 5 8 © 9 6 6 © S 7 0 © 9 71 ,8

.--125 - -.0 -—MM.O - M 6 © 3 - -55 © 9 63.3 6 8 © 3 6 9® 6 —

1 60 90 3 9 ® 8 M 7 e 5 5 3 o 8 59 .

6 M ® 5 67.3

200 ©0 - 36 ^-2 50 o 9 5 1 o 8 56 © 7 & 0 ® M 6 3. 8

250 .0 37*0 MM .6 50 © 2 5M©9 58 « 3 60,0

3 15 • 0

-3 2 © 9 M 1 ,2 —M 8 ® 9 — 5M.0 5 7 © 6 58 a 8

MOO e 0 26 e M 37 .

M 7 ® M 5 3 e 3 56 o 9 5 8.6

-500 .0 — ®-0—

30.6 M5a0- - 52.56 © M ... 59 » 2 - 630 .0 .0 27 .

M 1 a 9 50 a 2 55.3 59.0

800 ©0 .0 .... .0 3 8 © 0 M6-. 7 5 3 a M _ 57 » 2

1 000 .0 oO oO 36.6 M 3 a 5 51 aO 5 M « 9

I 250 .0- .0- — .0 .0 MO.O - M 8 o 6 . 52a l

1 600 .0 .0 .0 ©0 36.6 e 0 M 8 e 6

2000 - eO «0 - ©0 - - • 0 — » 0 9 0 M6.6

2500 oO o 0 .0 .0 .0 .0 .0

3150 .0 .0 ,0 ® 0 .0 .0 _ *0

MOOD .0 o 0 • 0 0 0 ® Q ®Q © 0

5000 • 0 - .0 _ .0 © 0- __ ,0. _ 3 0 - .0

6300 .0 ©0 .0 o 0 ®0 0 0 oO

8000 a 0 —

*o~. .0 ... .0 .... .... .0 ....... aO _

1 0000 .0 © 0 .0 ©0 aO .0 o 0

! 2500 • o

..... .0 ©0 sO Q 0 . . .0

AWT .0

_ 3 M e 3 M 3 9 9 5 I ®7 58a2 6 2 © 9 6 5 o 6

LIN MM.

5 9 ® 5 . 69®0 79 e 9 8 5 © 0 87.8 8 9 c 0 .

62
Table All One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Microphone With No Windscreen

(Grazing Flow Incidence)

SPEED ( M/SEC > OCT BAND 2 4 6 8 1 0 S 2 I 4

1 2 55.2 6 9.0 .8 5.2 88.6 92.1 . . 95,9 100.3

i 6 53.9 67.6 85.3 89.8 92.4 96 . 1 99 .

20 5 1.9 67 • Q. ...8 2.6 . ...9 1 a 2 . 93.0 . 96.0 98.7

25 50.2 66.4 78 .

90.6 93.6 95.6 97.7

3 8 47 ® 6 .. 6 6..4 .-7. 6.. 7 87 . 0 .. ......-,9 3.4 ... 95.7 97.8

40 44.3 65.7 74.6 84,2 9 1.7 96.0 97.8

50 .a 6 4 . a_ _ 7.4 *. 7- 8 1 *JH 8 8 . 2— 93.4 _ - 9 6 . 3- 63 .0 6 1.3 7 S .2 8 C » 4 95.9 90 .

94.2

8 0 ....... . .. -. o. 58 « 8_ 73.2 80.9 85.2 88 ® 0 9 1.3

1 00 • 0 58.9 7 1.0 80.0 84.8 87.4 89.6

1 2 5 .... - - --. 0 - 5 2 -8— . 6 8.4 - 7 8,4 . 3 4 . 3 - CD 9

KS 8 9 . 4. -

8 60 .0 48.7 65 ,

75.5 83.87.1 89 .

200 .0 - - 45.1 — 6 2 .

- 72.3 80.2 -- 85.3 88 ^9 - 250 • 0 4 1.5 59.8 69.8 77.5 83.3 87.7

385 .0 38.5 5 6- 9-3 - 67.6 . ... 75 .3 --.8 1 .6 8 6 o-O . 400 .0 34.6 52.5 64.7 73 . 1 78.8 83.5

500 .0 3 1 -8- —

--4 8 .9 6 1,3 - 7 0.4 - - 7 6.5 8 l .-2 - 630 • 0 29.0 45.5 57.4 67.2 74 .

79.5

800 .0 —- 2 8.1 4 3.5 53 . 0— 6 4.2 ——72.0 7 7.-7

9 000 .0 .0 42.4 50.7 6 1.5 70.7 77.3

1250 - -* a - *0 - 38.

- 47.9 __ -59 . S 69.2 77.1

1 600 .0 .0 35.6 45 .56.5 68.3 76 .

2000 .0 .0 .

. . 0 - 40.6 — 55.

- 66.9 7 8 »- 5 - 2500 .0 .0 .0 3 8 ® 1 52.5 65.6 75.9

3150 .0 *0 .0 - 3 6 »-4— 49.7 - . 63.9 75 ^9

4000 • 0 • 0 .0 .0 49 .

62.5 74 . 1 5000 - *Q.. -0 .0 . 0 —- 47.6 —-- 64.5 — 73 . 6 - 6300 • 0 .0 .0 .0 47.0 58.

76.3

8 000 . .... o - -0 - - „. 0 . ..... . .0 . —.0 -. - 5 3 . 9. . .7 1 . a-7-

1 0000 • 0 .0 .0 .0 .0 53.5 67 . 2

1 2500 . .0 0 ... . 0 ...... . 0 -- . ... . 0 . 51.6 . .... 6 3 . 7 - AWT .0 45.6 6 1 .0 70.8 78.3 84.3 90 .

LIN 59.6 75.7 9Q..S _97.4 IQ1...4 1 0 4 • 7 107.6

63
APPENDIX B: Tabulation of Wind- Induced Noise Levels for Normal Flow Incidence

This appendix contains a tabulation of the one-third octave band,

A-weighted, and overall wind-induced noise levels (dB re 20 yPa) measured

at normal flow incidence over the wind speed range 0 m/sec <v<_ 14 m/sec

for each of the windscreens shown in Figure 2 as well as for the microphone

with no windscreen. Zeros have been inserted in the tables when the

signal-to-noise ratio was insufficient to obtain meaningful data.

64
Table Bl. — One Third Octave Band Wind—Induced Noise Levels

(dB re 20 yPa) For Windscreen A

(Normal Flow Incidence)

SPEED ( M / SEC ) o

T QAMD 2 8 6 8 1 0 1 2 1 8

1 2 33.3 53.6 67.8 76.0 78.9 77.8 77.5

1 6 32.8 52.5 6 1.6 73.9 79.8 79 .

78.7

20 29.8 50 .

59.2 67,8 76.9 79.7 7 9.6

25 28 . 1 87.5 5 9 ; 1 64.0 69.8 76 .79 .

3 l 25 .

45 . 1 56.8 64 .Q 66.9 70.7 76.1

4 0 23.4 1.6 53.1 62.1 67.5 68.3 70.8

50 .0 39.3 8 9.4 58 .

65 .

68.1 69.1

6 3 • 0 39.1 87.2 55.8 6 1.9 66.1 68.3

8 U .0 37 .85.0 52 .

59.0 6 3.3 65.8

1 00 .0 .0 .C .0 56.0 6 G . 1 63.0

1 25 .c .0 .0 88.5 53.8 56 .

60.8

I 60 .0 .0 .0 86.1 .0 .0 58.8

200 . 0 .0 .0 48.7 89.8 53.2 . 0

250 .0 22 . 4 32.7 8 1.0 86.1 50 .

53.6

3 1 5 .0 .0 .0 38.9 . 0 89.2 52.8

4 00 . 0 .0 ;o 36.3 .0 47.6 5 1.8

500 .0 .0 25 .34.0 8 Q .7 86.2 50 .

6 30 .0 . 0 24.3 32.7 38 .88.6 49.3

800 .0 .0 28 .

32.0 37 .

82.9 47.5

i OCG . 0 • 0 25 .

32.2 37.8 82.7 86.6

I 250 .0 .0 20.8 29 .

36,1 82 . C 85.7

16 0 0 .0 .0 20.5 28.7 35 . 1 80 .

85 . 1 2 C 0 D • G .0 2 1.1 27.5 33.8 80 .

48.6

2500 Vo .0 .9 . 0 32.2 38.7 83.5

3 1 5 C .0 .0 .0 .0 32.0 38.1 43.1

4 COC .0 .0 .0 .0 3 1.1 37.5 82.2

5000 .0 • G . 0 .0 27.5 36 . 7 82.2

6300 .0 “VO .0 .0 .0 .0 83.3

8 QC 3 .0 .9 . 0 .0 .0 .0 8 1.7

1 u 0 0 0 .0 .0 .0 .0 .0 .0 80 . 8

1 2500 .0 .0 .0 .0 .0 • 0 . 0

' AWT

' VC i 9Vo " 33 . 0 43.3 ' 48.7 54.3 58.5

L I N 37.9 58.1 70.0 78 . 9 88.0 85.1 85 . 8

65
Table B2. - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen B (Normal Flow Incidence)

SPEED ( li / 5 E C ) 0 c T R A N D 2 M 6 8 1 0 l 2 1 M

1 2 35.8 M 8 .

5 M . 9 60 .

66.1 70 .73.1

1 6 36 . ! 50 . M 55 .

6 1.0 66 . M 70 .

73. M 2 n 3 1.5 50 .

56.9 60 . M 6 5.6 70 .

73.0

25 27.5 5 1.1 6 1.5 6 1.3 65 .

69.8 72.2

3 1 25.8 5 1.8 59 .

65 .

65 .68.1 7 > . 3

MO 23.

52.1 59.8 67 • 7 69 . M 69.1 7 C . M so .0 52 . M 6 1.0 66.3 o s CD 72.1 70.5

6 3

r\

• o 5 1.2 62.1 68 .

70.9 7 2.2 72.7

e c . C M 9 .

6 1.2 69 .

73.1 73.6 73.9

i c-o .0 .0 60. M 68 .

7 M . 8 76.5 75.8

1 ?s • 0 .0 59. 1 67.3 73.3 76.9 78.0

1 60 » 0 .0 57 . M 65 .

72.0 75 .77.8

2 0 0 .0 .0 55.0 6M .5 69.9 7 M . 3 76.9

2S J • c 29.8 52.0 62.6 69.

73.0 76.2

3 1 S .0 25 . M M 8 • M 60.7 68.0 72.6 76.0

MOO .0 23.6 M2 .57 . M 66.2 7 1.6 75 . M SOU .0 2 0*8 37 .

53.7 63.6 70.0 7 M . 8

630 .0 20.1 33.5 M 8 • 6 59 .

67 . M 73 .

800 • c .0 3 0.8 M3 .

55 .

63 . M 70. 1 1000 . 0 .0 30 . I MC • 5 5 1 . M 59 . M 66 .

S 2 S 0 .0 .0 28 . 1 38.1 MB. 1 5 M . 8 6 1.8

1600 .0 .0 27.5 36 . 7 .0 50 . 8 57.0

2000 .0 .0 26 . 7 . 0 .0 M 8 . 1 53.5

2 S 0 0 • 0 .0 .0 . 0 0 0 . 0 5 S . M

3 150 .c .0 .0 . 0 . 0 • 0 50.

M 00 0 .0 .0 . 0 .0 .0 . 0 51.1

5000 .0 .0 .0 .0 .0 .0 5M .0

6 30 0 .0 .0 .0 . 0 .0 .0 52.

8000 .0 .0 .0 .0 .0 .0 51.1

1 3000 • o' .0 .0 .0 .0 .0 50.5

8 2500 .0 .0 .0 .0 .0 .0 .0

AWT .0 3 1.3 5 1.7 62.0 69 . M 7 M • 6 79.0

L I N MC • 2 60.5 70.7 77.7 82 . M 85 . M 87 . M

66
Table B3 • - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen C

(Normal Flow Incidence)

SPEED ( N/SEc > OCT BAND 2 4 6 8 1 0 1 2 1 4

1 2 38.9 53.4 65 . 1 72.3 76.1 77.9 79 . 1

1 6 38.0 5 J .8 62.4 70.

76 .78.3 79 .

20 33.0 50.6 59 . 1 67 .

74.3 77.8 79.2

25 3 1.0 48.8 58.4 64 .

7 1.9 76.3 78.1

3 1 29 . G 47.1 56 . S 63.5 68.5 74.0 76.9

40 26.0 44.1 5 4*5 62.1 67 .

71.3 73.9

50 . G 4».6 52.3 59 .

65 .

69.9 72.4

6 3 • ii 4 1.0 4 9.5 58.1 6 3 .

63.0 7 1.1

80 .0 37.4 47.3 54 .

62 .66.0 68.3

1 00 .0 .0 46.5 .0 58.5 64.0 66.6

1 25 .c . 0 .0 49.8 56 . I 60 .

64 . 1

1 6 0 .0 .0 . 0 47.3 .0 .0 6 1.6

200 .0 .0 .0 45.6 50.9 55 .

.0

250 .0 2 1.8 32.4 4 1.6 47.5 52 .56 . S 3 1 5 .0 .0 .0 39 . 0 45.6 50 .

54.2

4 00 .0 .0 .C 35 . 8 .0 48.7 52.5

500 . 0 .0 24.3 32.9 40 . 4 46.6 5C . 8

6 3 G .0 2Q . 3 23.6 3 1.4 37.9 4 4.6 48.9

800 .0 * 0 24.0 30 . 8 36.2 42.4 46.8

1 000 .0 .0 24 .3 1.1 35 . 6 4 1.7 45.4

1 250 .0 .0 20.8 29 . 2 34.6 40.8 44.4

1 600 .0 .0 23.5 28.1 33 . 6 39.7 43.6

2000 .0 .0 .0 26 . 7 32.4 39 . 2 43.1

2500 .0 .0 . 0 . 0 30.4 37.5 42.4

3 1 50 .0 .0 . C .0 30 .

.0 4 1.4

4 0 00 To ".0 .0 .0 30.4 .0 40.5

500C .0 .0 . 0 .0 26.2 .0 38 . S 6300 . 0 .0 .0 .0 26 .

.0 .0

8000 • c .0 .0 .0 27.5 .0 .0

rod do .0 • 0 To .0 29.6 . 0 . 0

1 2 5 C 0 . 0 .0 .0 • 0 24 , 1 . 0 . 0 " A '.V T • e 22.2 34.0 43.7 49 .

55 . 1 58 .

L I N 42.7 58 .

68.8 76 .

8 1.6 8 4.7 86 .

67
Table B4 , - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen D

(Normal Flow Incidence)

SPEED { M/SEC 5 OCT BAND 2 M 6 8 1 0 1 2 1 M

8 2 3 M * 8 SC . 3 6 M « 9 72.9 7 M « 9 7M . 8 76.7

t 6 34.8 SO. 3 6 U • 3 70 o 9 75 .

75 • 3 77.3

20 30 • 8 49.7 57,

65 „ 8 72.3 75.77.5

2 S 29 . S 47.5 58 • 0 6 1,5 6 6.8 72.75 .

3 1 27® 7 MM . 7 56 .62.9 63.5 68.9 73 . M MO 24 » 8 M3.0 5 M ® 0 6 1.2 64,

65 .69.8

so .0 39 .

5 1.1 58 .

62 • M 6 5.

67 .

6 3 . D 39.3 M 8 .

55.5 60 .6 3.8' 6 7.0

6 0 « 0 3 6.3 MS . 6 S3 • 3 57.1 60 . S 6 M . M 100 .c .0 oO o 0 5 M . 3 57.7 6 1.8

i 25 • c . 0 .0 M8 .

S 2 .55 . 1 59 .

8 60 .0 » G .0 45,1 .0 . 0 57.1

200 . 0 . 0 .0 M 3 • M 47,1 5 1.0 .0

2 SC *0 2 1.2 32 . S 39 o 7 MM .0 M 8 . 2 52.5

3 1 S .0 .0 .0 37 . S .0 M7 .5 5 1.8

MGO .0 .0 .c 34,9 .0 M 5 • 9 50.7

SCO .0 9 0 2 M « 7 32.8 39.3 MM . 8 M 9 .

630 .0 «0 23.7 3 1.8 3 8*0 M 3 • M M 8 .

800 .0 .0 24, 1 3 1 . M 37 .5 M2.

M 6 .

1 000 .0 .c 2 M ® 3 3 1 • M 37 . S M2.M M 6 .

8 2 5 0 ® 0 . 0 .0 29 . S 36.

M 2 * 0 M 6 .

1600 • o ®G . 0 28 . 2 35 . M M2.

M6 .

2GG0 ®o ® o © 0 26 • M 36.0 4 2,0 M 7 . 1 250Q ®o ®0 • 0 25 . 3 3 S • 9 MM , 1 M 9 . 2

3 ISO .0 ®o . a 24 o 5 35.8 MM . 5 50.3

MOOO .0 .0 .0 23.0 33.7 M 5 ® 2 51.5

S 0 G 0 .c .0 .0 22 . 8 3 1.7 MM . E 51.3

6 30 0 .0 *c . c 2 M • 1 30 . 2 M 1 . M 50.9

8000 . 0 © G . 0 26.0 29 . 1 MO . 3 M 9 . 7

8 0 0 0 0 ®o .0 .0 28 . 2 29 . M 37.5 M 7 . 5

8 250 0 .0 . c .0 2 3® 2 23.9 28.5 39.2

A WT *0 18.6 3 1.8 M3 o 1 48,6 55.6 6 1.3

L I N 39,

56 • S 68.0 76.2 79,

8 1.3 83.9

68
One—Third Octave Band Wind— Induced Noise Levels

(dB re 20 yPa) for Windscreen E

(Normal Flow Incidence)

S P E E <"> (M/SEC ) H 6 8

48.3 62.3 69.2

Table B5

1/3 OCT B ANO

! 2

1 6

20

2 5

3 I HO

SO

6 3 80

100

1 2 S

1 60

200

2 SO

3 1 S

HOO

500

630

800

1 000

I 2 SO

1 600

2000

2500

3 1 SO

hooo

5000

6 3 00

8000

l 0000

I 2 S C 0

A i« T L I N

H 8 • 5 56 . R 65 .

H 7 • 5 SS. 1 62.0

HH . 8 55 .

59.

Q

H2 .

S3 .

58 .

4 G . 1 50.9 57.5

38 .

H 8 .

5 H • 3

4 1.1 H 6 .

52 .

3 H . ! H 3 • 3 H 9 . 1 .0 . 0 . 0

.0 .0 H 5 • 7 .0 .0 H 1 . H .0 . 0 HO . 9

20.9 29.

36.7

.0 .0 3 H . 7 .0 .0 32 .

.0 23.8 3 1.6

.0 23.3 3 1 .<j

.0 2 H • C 3 1.3

.0 2 H . 3 3 1.5

.0 .0 3 1 . H

• 0 . 0 3 1.5

• 0 .0 28 . 1 .0 .0 29 .

• G .0 29 .

.0 .0 25.0

.0 .0 23.7

.0 .0 2 H • 1 .0 .0 26.0

.0 .0 28 .

.0 . 0 23.3

1 8 . H 3 0.6 H 2 • 5

5 H • H 65 .

72.1

1 0 1 2 l R 73 .

72 .

75.6

69.9 7 3.2 75.1

68.5 72.8 76. 1 65 .

70.8 7 R . 1 6 1.9 67.9 72.5

6 1.3 6 R • 8 69.1

58 .6 R ® 1 66 . R 55 .

67.3 6 5.3

5 H • 2 58.8 63.6

5 1.6 56.8 60.8

H 9 « 8 5 R • 3 58 .

~

. 0 .0 56.9

HR . 1 R 9 • 5 .0

H 1 . 2 R 6 • 5 5 1.1

.0 R 5 • 8 50. R .a HR . 6 H 9 .

37.9 H 3 . 3 R 8 . 5

37.4 R 2 . 7 R 7 .

37 . H H2.H 4 7.9

37.8 H 2 • 9 48.4

37.7 R 3 . 7 49.2

39,5 RR . 8 50.2

H 1 . 1 R 6 , 2 5 1.3

H 3 • H H 8 , 5 52.7

H 2 • 5 50.

54 .

H 2 • 3 H 9 • 8 55.3

HO.H 50.0 55.7

37 .

H 8 • 5 55.3

35.2 R 6 • 3 54.2

32.2 R 3 . 3 51.7

2 H • H 33.7 42 .

52.0 59.1 64.5

75.5 7 9 . R 82.5

2

3 H • 7

3 H • 7 3 0.3

28.3

25.

22.9

.0

. 3

• 0

.0.c

.0

• G

• C

• U

.0

• C

.0

.0

.0

.0

.0

.0

.0

• 0

.0

.0

.0

.0

0

. 0

. 0

39.2

69
Table B6. - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen F

(Normal Flow Incidence)

SPEED ( M /SEc ) 3 OCT RAND 2 4 6 8 1 0 S 2 1 4

1 2 35 .

49.5 6 1.3 67.2 7 1.2 74.2 76.9

1 6 35.2 49.9 56.3 65 . B 70.4 74.4 77.2

20 29.9 48 . S 55 .62.3 69 • 5 73.0 76 .

25 28.0 46.4 55 .58.4 66.9 72.2 75.0

3 1 26 • 8 44.4 5 4 0 4 58 .

6 3.9 69.0 73.9

40 23.3 4 1.6 5 1.8 57 .

6 1.9 65 • 1 69.9

so .0 38 • 1 49 . 1 54 .

6 1.1 65.2 68 . 1 6 3 .0 36.4 45.9 52.9 58 . ! 64.1 67 .

BO • 3 33.9 42.8 49.4 55.9 6 1.1 6 6.1

100 .0 .0 .0 .0 52.5 58 .62 .

1 ?S .0 ® 0 .0 46.1 50 .55.3 60 .

l 6 0 • 0 .0 .0 40 » 8 .0 .0 58 .

200 .0 .0 .0 40.7 44.6 49.8 .0

2S0 .0 2 i .0 29 .

36 .4 1.8 47.4 52. 1 3 1 S .0 ® 0 .0 34.5 .0 46.9 51.9

400 . 0 .0 .C 32.9 .G 46.6 5 1.6

SOG .0 .0 23.6 3 5.9 3-5 . 1 46.3 5 1.5

630 "•o' » 0 23.4 3 1.9 39 . S 46.6 52.0

800 .0 • 0 24.8 33.2 40.0 4 7.4 52.2

3 COO • o’ .0 2 4.4 33.0 4 l .0 43.4 53.0

1 2 S G . G .0 20.2 35.6 42.1 49 .54.0

1 600 .c • 0 2 1.4 35 .

43.2 49.4 54. S 2000 .0 • G 2 1.4 36.1 45.0 50.5 54 .

2 SO 0 • c .0 .0 3 7.5 47.8 52.3 55.9

3 1 SO . 0 .0 .0 33.7 46.8 53.4 57.5

4000 .0 . 0 .0 32. 1 45 .

53.

58 .

SOOO .0 .0 .0 29.9 43.9 52 • 4 58.3

6 3 00 .0 .0 .0 27.2 4 1.4 50.9 57.8

8000 .0 • 0 .0 27.0 38 . 8 48.8 56.0

ICOOO . o

' .0 .0 28 • 3 35.2 46.2 53 .

1 2500 .0 .0 .0 23.4 26.9 37 . 8 44.4

AWT ® 0 3 6.8 32,2 4 6.0 55.7 62.5 67 .

L 1 N 39 . 7 55.

64.9 7 8,4 76.4 80.5 83.8

70
Table B7. — One—Third Octave Band Wind— Induced Noise Levels

(dB re 20 yPa) for Windscreen G

(Normal Flow Incidence)

SPEFO (M/SEC

OCT BAND 2 M 6 e ! 0 1 2 I M

1 ? 36.3 M 9 . | 6 C • M 66 . M 7 1.0 7 M . 0 77.5

1 6 3 M • 3 M 9 • 3 57 . M 65 . M 70.3 7 3 . M 77.5

20 30. M M 9 .

5 M . 8 6 M . 1 69 . M 72 . M 76.7

25 27.8 M 6 • M 55.0 59.8 67.0 7 1.8 7 M . 3

3 i 25.5 M 5 • 3 5 M . 1 59 .

63.0 69.7 73.2

MO 2 1.6 M2 .5 1.7 59 .

62.5 66.0 70 .

50 • 0 38.3 M 9 .

56 .

6 1.9 66.0 68 .

S J . 0 36.3 MS . M 55 . 1 5 Q • 6 6 5 . M 6 3.3

9 0 .0 32 . 1 M 1 . 5 5 1.3 58 . M 6 3.0 6 o . 8

1 00 .0 .0 . 0 .0 55 .6 1.8 6 M . 8

1 25 .0 .0 .0 M 6 . 1 52.9 58 .

62 . M

1 60 .0 .0 . 0 M3 .

.0 .0 60.5

200 • 0 .0 . 0 M3 . M M 9 • 5 55 .

.0

250 .0 • 0 29.1 M2 . 1 M9 .

55.0 59. 1 3 1 5 .0 .0 .0 M2.

50 .

56 .

60 .

MOO .0 .0 . 0 M3 . 1 5 1.2 56 .

60 .

500 .0 .0 29.5 M3 .5 1.2 5 6.6 6 0 .

630 .0 .0 29 . 1 MM . 2 5 1.5 56 . M 60 . M 800 .0 . 0 3 1.7 M3 .

5 1.3 56.2 60 . 1

1 000 .0 .0 3 1.9 M3.

50 .

56 .

59 .

1 250 .0 .0 29.6 MM . 1 50 . M 55 . 7 59.7

1 600 .0 . 0 29 . M MM . 2 50 .

55 .

59 .

2 0 G 0 • 0 .0 26 .

MM . M 5 1.0 55 . 3 59 . 1 2500 .0 • 0 2 M . 8 M2 . 9 5 1.9 56.5 60 . 0

3 l 50 • G • 0 22.0 MO . 7 5 1 . M 57.9 60.8

M DOG .0 .0 .0 38 . 7 M 9 • 7 57.2 6 1.6

5000 .0 • 0 .c 35 . M M 7 • 2 55 . 2 60.8

6 30 0 .0 .0 .0 31.7 MM . 1 52 . 9 59. 1 8 0 0 0 • c .0 .0 29 . 7 M 1 . 3 5 1.0 57.7

1 GOOO .0'" . o .0 .0 37 .

M 8 • 0 5 M . 9

1 2500 .0 .0 . 0 .0 28 .

39 .M 5 .

AWT .0 1 M . 1 38.7 53.9 6 1.8 67.5 7 1.7

L I N 39.6 55 .

6M .5 7 1.6 76.7 80.5 8 M . 1

71
Table B8 . - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 pPa) for Windscreen H

(Normal Flow Incidence)

SPEED (M/SEC ) OCT BAND 2 4 6 8 1 C 1 2 1 4

1 2 36.4 49.8 62 .

68.9 72.7 75 .

78.1

1 6 36 . 1 50 .

58 .

65 .

7 S . 3 74.3 76 .

20 3 1.5 49.3 57.4 63.1 69.4 74.0 77.4

25 29 .

47. 1 57 . 1 59 .

67.0 72.0 76 .

3 1 27.2 45.8 55 .

60 .64.1 69.5 74.0

4Q 24.1 42 .

53.7 58 .

63.6 66.9 70.7

50 .0 39 .

5 1.8 56 .

63.0 6 & • 8 69.8

6 3 3 7.9 48.1 5 q . 3 63.4 65.4 70.0

80 • 0 34 .

44.6 5 1.3 58.4 63.1 67.5

1 CO .0 .0 .0 .0 54 .

6 1.3 64.7

i 2 5- .0 .0 .0 45 .

5 1.6 56.9 62 .

i 6G .c • 0 .0 42.7 • C .0 58.7

2 G 0 .0 . 0 . 0 42.1 45.8 5 1.1 . 0

250 • 0 20.

29 .

36 .4?.8 4 8.0 52 .

3 1 5 .0 .0 .0 35 .

.0 47.4 52.

4 00 • 0 .0 .c 33.5 .0 46.4 51.3

5 CO . 0 • 0 24.3 32. 1 39.2 45.4 50.4

6 30 .0 .0 23 . 8 3 1.4 38 .

44.3 49.6

800 .0 ® 0 24 .

3 1.6 38.2 44.1 49.2

i oca .0 .0 25.

32.5 39.0 44.8 49.6

8 250 .0 • G 2 1.1 3 1.9 39.8 45 .

50.4

8 6 00 .0 • 0 2 1.1 33. 1 40.7 46.4 5 1.0

2000 .0 .0 2 1.4 32.4 4 1.8 4 7.4 5 1.7

2500 • 0 .0 .0 33.0 43.6 49.0 53.2

3 1 50 .0 .0 .0 29 .

44.8 50.3 54 .

4000 .0 .0 .0 27.8 42.6 5 1.1 55.7

5000 • 0 .0 .0 25 .

4 C • 4 50. 1 56 . 4 6 300 .0 .0 .0 24.6 38.2 48.1 55.2

8000 .0 .0 .0 25.8 35.6 46.6 54.2

1 0000 . 0 . 0 .0

' 28.

32.9 44.2 5 1.7

12500 .0 .0 . 0 23.5 24.8 36 . 1 42.9

AWT .0 17.4 32.8 43.9 53.60.0 65. 1 L I N 4Q. 6 56 . 1 66.2 72.4 77.4 8 1.2 84.9

72
Table B9 . One Third Octave Band Wind— Induced Noise Levels

(dB re 20 yPa) for Windscreen I (Normal Flow Incidence)

SPEED(M/SEC)

OCT B A N 0 2 M 6 8 8 G 5 2 1 4

i 2 3 6 • 0 53 .0 69.8 75 . M 78 . 1 78.4 80 .

1 6 3M . ? 5 1.7 65.6 73.9 78.8 79.2 80 . 1 20 3 1.5 5G . 8 6 3 . J 69.3 77.8 79.3 80.9

25 29 • M 49.3 60.5 64 9 8 72.8 7 7 o 5 80.9

3 1 27 .

M 7 « 6 5 9. i 6 5 e 3 68.3 7 2.1 77.8

MG 25 .

MM .5 5 6.7 6 4® l 68.8 69.9 73.3

50 .0 M I . 8 53 .

6 1.5 67.0 70.2 7 1.0

O J . J M 0 * 5 5 1,2 59.7 6 4.8 68.2 7 1.3

9 0 .0 38.3 M 8 • M 56 .

62.7 65 • 7 6 8.8

1 CC .0 • C M 6 „ M .0 59.5 63.7 66.9

1 25 .0 • 0 .0 5 1.3 56.7 59 .

64.4

l 60 .0 • 0 M 5 . 1 49 . I .0 . 0 6 1.0

2 0 0 . a .0 .0 47.

5 1.6 54.4 .0

2 5 0 .0 2 5.7 35.2 MM .0 48 .

5 S . 7 54.9

3 1 5 .0 .0 .0 Ml.] 47. 1 50 .

54.0

MOO .0 .0 .0 37 .

.0 48.2 52.3

5 00 .0 .0 25 .

3 M « 5 4 1.9 46.5 5 1.0

630 .0 .0 2 M . 2 32 .

39 .

4 M . 4 49.4

800 .0 .0 2M .5 3 1 . M 38 .

42.7 4 7.4

! 00G .0 .0 2 M . 2 3 1.3 38.1 4 1.9 46 .

1 2 5 G . 0 • 0 2 0.1 29.2 36.5 40.8 45.4

1 600 .0 .0 . 0 27.9 35 .

39 .

44.5

2 0 GO .0 • 0 . 0 25 • 9 33.7 39 .

43.8

2500 .0 • 0 .0 2 M . 5 32.3 37.7 42.7

3 5 50 .0 .& .0 23.7 32.5 38.3 42.4

M 000 . o .0 .0 2 ! . 8 32.5 36 . 4 42.5

5000 .0 .0 .0 2 1.5 3 5.2 33.9 4 1.3

6 3 00 .0 .0 .0 22.8 32 . 8 3 6.0 42.7

8 0 G 0 .0 .0 .0 25. 1 34 . 9 35 . 4 42.6

10 0 0 0 0 0 .0 . 0 20.1 38 . I 38 . 2 40.9

1 2500 .0 .0 .0 22 . 8 33.0 32.8 35. 1 A '.*! T .c 20 . 0 36 . M 45 . M 5 1.2 55 . 2 59.4

L I N 39.9 58 . 2 72 . M 79.

Q

83.9 85.4 87.6

73
Table BIO. - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 yPa) for Windscreen J

(Normal Flow Incidence)

SPEED ( M /SEC )

$ OCT BAND 2 4 6 8 1 0 1 2 1 4

i 2 39.4 48.8 58.3 6 1.2 64.1 68.1 70.

1 6 38.6 47.3 56 .

6 0.2 63.4 67.0 69 .

20 33.8 48.0 54 .

59 . 1 6 3.0 66.5 69 . G

25 3 1.9 4 8.Q 53.8 58.0 6 1.4 65.1 69 .C

3 I 29 . 9 46.8 54 .

57.3 59 .

63.8 67.1

4C 2 7 .

44.0 52.8 57.5 60.3 63.2 66 .

50 .0 40 .

50.4 55.5 59 . 1 62 . 1 64 .

6 3 • u 39.6 4 7.5 53.6 57.2 6 C » 5 63.1

8 C • G 37 .

46.3 52 .

57.8 60 .

63.2

! CO . 0 .0 4 6.4 .0 56 .

60 .

62.8

1 25 .0 . 0 .0 50 .

55 .

59.0 62.0

1 6 C .0 .0 .0 4 8.5 .0 .0 60.0

2 0 D .0 .0 .0 46.8 50 .

55.2 .0

2 5 0 . 0 24.7 36.1 43.3 48.5 5 1.8 54 .

3 1 5 .0 20 .

.0 4 1.3 46.7 5 1.3 53.9

HOG . 0 .0 . 0 37.7 .0 49.2 52 .

5 00 .0 .0 23.6 33.2 4 1.3 47.5 5 1.6

630 .0 .G 22.3 29 .37.1 44.0 49.8

800 • c .0 2 3.3 29 .

34.2 39.9 46.4

1 COO . 0 « 0 24.5 29 .

33.7 38.1 43.2

! 250 • 0 • G 20. 1 29 .

32.8 36 .4 1.1

1 600 .0 .0 20.5 28 .

33.5 36 .40.6

2000 .0 .G 20.

26.8 33.1 37.8 40.

2 5 U 0 . 0

" .0 . 0 26 . 3 32.3 37.2 4 1.9

3 150 .0 .0 .0 25 .

33.2 37.8 4 1.8

8000 • 6 • 0 .0 23.4 32 .

38.2 42.4

5000 .0 .0 .0 22.5 29 .

36 . 5 42 . 4 6 300 7b .0 .0 23.2 27 . 8 35 . 6 42.4

8000 . 0 . C .0 24 . 9 26 . 9 33.4 4 1.3

1 COCO .a . 0 .0 28.1 28.2 3 1.3 40.5

1 2500 .0 . 0 .e 23.0 22.8 24.4 34 . 1 A N T .0 2 1.2 34.3 44,4 49.1 54.2 58.0

L ! M 43.3 55 . 5 63.8 67 . 8 7 1.2 74.8 77.5

74
Table Bll. - One-Third Octave Band Wind-Induced Noise Levels

(dB re 20 pPa) for Microphone With No Windscreen

(Normal Flow Incidence)

SPEED { M/SEC »

- OCT RAND 2

' M 6 8 S Q 1 2 1 M

1 2 5M . 1 65 .

72.6 7 6 ® 8 8 0. M 83.7 85 .

1 6 53.7 6 M ® 8 7 1.9 76.8 80 • M 83.1 86 .

20 52.5 6 M . 9 7 8.5 75.8 79.6 83 . M 8 6.7

2 S 50. M 62.9 70.9 7 M a M 78.8 82.9 86 . S 3 i M 8 e 1 62.1 69.8 7 M . 9 78.8 82.2 86. 1 MO M 5 • 8 6 S . 3 69 .

7 M . 2 78 . M 8 1.6 85.2

50 .0 60.5 68.0 73 . M 77 • M 8 1 • M 8M . 5

6 3 . 0 59.8 6 1 . J 7 3.0 76.8 80.7 84 . M 80 .0 57 .

66.7 7 8.9 76.8 80 . M 83.5

1 00 .0 58.9 65 .

7 0® 9 75.

79.5 82.0

S 25 .0 .0 63.5 69 .75.3 78.9 82.

160 .0 . 0 6 1.6 68.9 7M .0 78.1 8 1.7

200 .0 « a 58 . R 67 .

72 . M 77.2 80 .

250 • 0 39 . 8 55 .

6 5.0 7 5 . M 75.9 79 .

3 1 5 .0 33.2 5 1.8 62.6 69.8 75 .

79 .

MOO .0 26. M M 6 .

59.0 67.3 7 3 o 5 78.0

500 .0 22 .

M 1 .5 55 • 2 6 M ® 3 7 S o 6 76.

630 .o' 20.3 36.6 50 • M 60 . M 68 o 5 7 M . 2

800 . c 25.6 36.8 M 7 .

57.8 6 M ® 5 7 8.8

10 00 .0 23.8 3M . 2 MM . 5 5M ®6 60.

68 .

I 250 .0 2 8.0 3 1.0 M 1 . 6 5 1 .0 55.9 6M .0

1 6 0 U .0 20 .

28.9 39 . M M8.9 52.6 6 8.0

2000 .0 © 0 27 .

36 .M6 • 9 50® 3 58.

2500 VC .0 . 0 . 0 .0 M8 .

.0

3 150 © 0 .0 .0 .0 .0 © 0 • o MOCO "Vo .0 . 0 .0 .0 .0 .0

5000 .c e 0 .0 .0 .0 . 0 .0

6300 "•0 . .0 . 0 .0 ® 0 .0 .0

8000 .0 .0 . 0 .0 • 0 .0 .0

1 GOCO .0 • 0 .0 .0 © 0 « 0 .0

1 2500 .0 .0 .0 .0 .0 .0 .0

AWT Vo M2 .55.8 6 M * M 7 S . 2 76.7 8 1.5

L I N 59.5 72.6 80.

84.9 89.2 92.9 96.2

75
APPENDIX C: Tabulation of Insertion Loss Measurements for Grazing Flow

and Normal Acoustic Incidence

This appendix contains a tabulation of the one-third octave band,

A-weighted, and overall insertion loss (dB re 20 yPa) measured for grazing

flow and normal acoustic incidence over the wind speed range 0 m/sec

< v<_ 12 m/sec for each of the windscreens shown in Figure 1. 'the

insertion loss was computed by subtracting the level measured with the

windscreen from the level measured without the windscreen on the microphone.

It will be noted that the numbers generally are only a few tenths of a decibel and that both positive and negative values were measured. ^hus

at some frequencies insertion gain, rather than loss, was measured.

76
Table Cl One-Third Octave Band Insertion Loss (dB) for

Windscreen A

(Grazing Flow - Normal Acoustic Incidence)

SPEED l M/SEe

OCT RAND 0 2 M 6 8 1 0

250 . 2 • 6 . 9 . 2 . 2 . M 3 1 5 . 1 . 7 . 9 . 2 . 2 « M MCO . 1 . 8 1 . 1 . i . 1 . 6

SCO - . 1 . M . 7 1 .0 . 1 630 .0 . M . 7 • 0 -.2 .2

800 .0 .5 . 5 .0 • 0 e 2

i 0 0 0 1 . 3 . 6 • 1 . D

1250 .0 . 5 . 6 . i - . 1 . 1

1 600 • c . M . 7 .0 .0 .1

2000 • c . M .5 .0 .0 . 1 2500 ~ • o

- — .3 . M .0 .0- . 1

3 1 50 .0 . 3 . 5 . 2 .0 • 2

M 0 0 0 • t .3 . M . 1 . 1 . 2

AWT .0 .5 . 7 .0 .0 . 2 L 1 N .0 . 7 ~ . 7 .0 .0 . 1

Table C2. - One-Third Octave Band Insertion Loss (dB) for

Windscreen B

(Grazing Flow - Normal Acoustic Incidence)

1/3 OCT RAND

250

3 1 5

MOO

500

6 30

800

1000

1 250

1600

2000

2500

3 150

M000

AWT

L I N

0

. M - • 1

O

t

2

12

0

10

10

0

0

2

0

I

SPEED ( M/5tr ) 2 M 6

• M . M .2

• I • 1 • 2

.1 .2 .M

• 1 • 0 • 1

• 0 • 0 • o

• Q • 0 • 2

.0 - » 1 * o .0 • l • o

. 1 .1 • 1

• 1 • o • 1

• 1 el • 1 .2 .2 .2

.2 .2 .2

• 1 *1 • 1

• 3 • 0 • 1

8

. 0

. 2

. 3

. 3

. 1

• 1

. 0

. 0

. 1

. 0

. 1

. 2

. 3

. 1

• 1

1 2

. 3

. 3

. 5

. 2

. 2

. I

. 0

. . 3

. 1

. 1

. 1

. 3

. 2

. 2

. 1

l 2

. 2

. 2

. 2 .0

« 1

. 1

. 0

. 1 .0

. 1

. 1

. 2

. M

. 1

. 0

77
Table C3 . - One-Third Octave Band Insertion Loss (dB) for

Windscreen C

(Grazing Flow - Normal Acoustic Incidence)

1/3 OCT BAND

250

3 1 5

MOO

500

6 30

8 0 G

1 000

125 0

1 600

2000

2500

3 1 50

M 0 0 0

A AIT L I N

SPEED l M/SEC > M 6 8

.1 .1 .0

.1 -.1 -.1

.1 .1 .1

.3 .0 .1

.1 .0 .2

.1 .0 .2

~ .2 ~ . 2 .0

. 0 .0 .0

.2 .2 .2

.0 .0 .1

• 0 • 0 • c .0 .0 .0

. o . o . 0

.1 .0 .1

.2 .1 .1

Table C4 . - One-Third Octave Band Insertion Loss (dB) for

Windscreen D

(Grazing Flow - Normal Acoustic Incidence)

1/3 OCT BAND C

2 5 0 • . 2

3 15 ~ . 1 MOO "*2

50 0 • C

630 .

800 • 0

10 0 0 “ . 2 1250 .0

1600 • 1 2000 ".1

2500 -.1

3150 .0

M000 — *1

AWT .0

LIN . 0

2 SPEED ( M/SEC

M 6 8 1 0

7 8

1 2

, 1

. 1

. 2

. 1

. 2

. 2

.0

. 2

• 2

. 1 .0

. 1 .0

. 1

. 1

1 2

. 1

. 1

. 3

. 1

. 2

. 2

- , 1

. 2

. 2

. 1

. 1

. 0

. 2

. 1

. 1
Table C5. - One-Third Octave Band Insertion Loss (dB) for

Windscreen E

(Grazing Flow - Normal Acoustic Incidence)

SPEEDIM/SEC)

1/3 OCT BAND 2 8 6 R 1 0 1 2 250 -a 2 • 1 .0 -

a 1 a I a 3 a i 3 l 5 “a 1 ~ a 1 - a 2 -.2 “ a 1 a 3 - a 1 8 00 “ a 1 - . 1

' - a ! .0 .0 a 3 . 0

500 ~ a 1 •o e I a 1 a 1 a 3 a 1 630 a 2 a 2 a 2 a 3 a 3 a 6

*

a 8

800 a 2 a 1 a 0 a 1 a 1 a 8 a 2

1000 * u -a ! a 0 a 2 a 1 a 8 . 2

1 2 5 G -a 1 .0 .0 a 0 a 2 a 8 a 2

1 600 "a 1 “a I “a l © l .0 a 2 .0

2000 “a 1 “ a l “ a 2 -a 1 .0 a 3 a 1 2500 “ a 1 “ a 1 - .0 — .0 a 0 a 3 a 1 “

3 150 -.2 .0 .0 .0 .0 a 2 a 1 H 0 00 a 1 a 0 .0 a 1 .0 - ,2 . 2

AWT .0 .0 .0 .0 • 1 a 3 a ! L I N • 0 a 3 .0 a 1 • 1 a 8 a 1

Table C6. - One-Third Octave Band Insertion Loss (dB) for

Windscreen F.

(Grazing Flow - Normal Acoustic Incidence)

S P FED (P/SEC ) M 6 8

.1 .0

-.1 -. 1

1/3 OCT P A MO

250

3 1 5

8 CO

500

630

800

1 OGf)

12 50

I 600

2000

2500

3 150

8 0 0 0

AWT

L I N

0 2

- . 2

-.3

- . 3 - . 2

.0

“ . 1 - . 1 " * 1 ~ . 2 “ .2

" .2

” .2

_

. 1 - . 2

• 1 — • 2

-•2 ~.3

- . 3 - . 3 - • 1 .0

.2 .2

.0 .

-a t -.1

- . 1 - . 1 - . 2 -.1

“ . 2 _

. 2

“.2 “ .2

" . 2 _

. 2 —

• 1 • I — • I — • 1 .2 “ . 1

1 - . 2

.0 .0

.2 .2

• C .0

1 1 “ . 1 - . 1 - . 1 - . 1 ~ . 2 ~ . 1 - . t ~ .2

“ . 2 . 0 ” e I *"• I - . 1 -.1 .o . o

10 12

-.2 *0

- . 2

- . 2

.0

. 3

.0

.0

a 0 a 0

-.1 - a 1 - . 1 “ . i ~ o I « 1 .0 - a 2 - a 1 - a !

a 0 - a 1

a 0 a C

79

(MN

«MO

O
Table C7, One-Third Octave Band Insertion Loss (dB) for

Windscreen G

(Grazing Flow - Normal Acoustic Incidence)

SPEED (M/SEC ) 1/3 OCT BAND C 2 M 6 8 1 G 1 2 250 . 1 -.2 -.2 - .2 -. 2 ~.2 ~ . 2

3 1 5 . 2 . 3 . 3 . 3 , M . 3 . 3

MOO . 1 • 1 I - . 1 .0 2 " . 1 500 * 1 . 2 . 1 . 1 . I . 8 . 8 630 ~ . 2 -.2 -.2 .0 .0 .0

800 ~ . i .0 .0 . i • 1 .0 .0

8 000 © 0 ~ .0 . i .0 • 1 “ . 1 12 5 0 . 0

" . 1 . 0 . 1 . 0 . 1 . 1

1 600 .0 .0 .0 .0 .0 .0 .0

2000 .0 .0 . 1 . I • I . 1 . 1 2500 • 0 .0 . t .0 . 1 a f — .0

3 1 50 • c .0 .0 .0 .0 .0 .2

M000 - . I n n

• 0

“ . 1 © 0 © L • u © u AWT .0 .0 • 0 .0 .0 . 8 .0

L I N .0 . M . t . i , I . S . i

Table C8 . - One-Third Octave Band Insertion Loss (dB) for

Windscreen H

(Grazing Flow - Normal Acoustic Incidence)

1/3 OCT BAND 0 2

SPEED (M/SEC

M 6 B I 0 1 2

250 " . 5 " . M

- - . 2 " . M “ . 2 - . 1 .5

3 1 5 . 3 . 3 • 5 .3 . 3 • 2 1 . 3

MOO " . 2 ~ . 2 - . 1 ‘ .2 “ . ! .0 . 9

SOO .0 .G .0 • 0 © i .2 1 . 1 630 ~ . 2 • 0 -o2 “ .2 .0 .0 .9

800 .0 .0 1 .0 .0 .0 .8

I 000 .0 .0 .0 .0 . s .0 1 .0

1 250 ® is . 0 . 0 .0 . 0 * 1 1 . 0

I 600 .0 .0 .0 .0 .0 .0 • 8

2000 . 0 . 1 . 0 . 8 . 1 . 1 . 9

2500 .0 " .0 . 0

- .0 • 1 . 2 . 9

3 1 50 .0 .0 . 0 .c .0 . 2 . 9

M 000 • 0 .0 .0 1 1 .0 . 5

AWT .0 .0 .G .0 .0 . t . 9

L I M . 0 . 3

- . 0 .0 . I • 1 . 9

80
Table C9 . - One-Third Octave Band Insertion Loss (dB) for

Windscreen I (Grazing Flow — Normal Acoustic Incidence)

SPEED « M/SEC ) 1/3 OCT BAND 0 2 4 6

250 " . I .0 . 7 " . 1 3 1 5 . 3 o 3 l . 3 . 4 400 . 0 1 1 .

“ . I 500 • i . 2 ! . i • 2

630 .0 2 . 9 . 1 8 C 0 .0 .0 1 . 0 . 8

1 000 »

.0 1 . 0 . 8

1 2 5 0 • 0 . 1 1 . 0 . 1

8 600 .0 .0 . 9 .0

2000 • i « ! . 9 . 8 2500 . 0 . 2 o 9 . 2

3 l 50 .0 • 0 . 7 • 2

40Q0 • 0 .0 .5 .0

AWT .0 . 1 I .0 . 1 L I N • 1 . 4 8 .0 . 8

Table CIO. - One-Third Octave Band Insertion Loss (dB) for

Windscreen J

(Grazing Flow - Normal Acoustic Incidence)

OCT BAND 0 2

SPEED ( M/SEC ) 4 6 8 1 0 1 2 250 1 . 2 . 6 . 4 . 5 . 3 • 3 1 5 . 3 . 2 . 8 .6 « 5 . 2 © 400 • 1 . 3 • 6 .6 . 4 . I • 500 • ! .0 .5 .5 . 2 . 2 • 630 . 4 .5 . 7 .9 . 7 .5 • 809 “ . I . 2 .5 .5 . 3 • 8 «

8 0 00 . ! . 2 . 6 . 5 . 3 . 3 • 1250 • 3 . 4 . 7 .6 • 6 . 4 • 1600 .0 .0 . 3 . 3 . 2 • 0 9 2 0 0 0 .2 . 2 • 6 .6 .5 . 3 • 2500 . ! . 1 . 4 .5 . 2 . 2 9 3 1 5C . 3 . 3 .5 .5 . 5 . 3 • 4000 . 3 . 3 . 4 .5 . 4 . 4 © AWT . 2 . 3 • 6 .6 . 4 . 3 • L I N © 1 .5 . 6 .5 . 4 . 2 •

8 ]

3

2

2

2

4

2

2

40

3

!3

4

3

8
NBS-114A (REV. 7-73)

U S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC ATION OR REPORT NO.

NBSIR 79-1599

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Microphone Windscreen Performance

5. Publication Date January 1979

6 . Performing Organization Code

7. AUTHOR(S)

Robe rt N. Hosier and Paul R. Donavan

8. Performing Organ. Report No. NBSIR 79-1599

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2004450

11. Contract/Grant No. EPA-IAG-D7-41087

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of Noise Abatement and Control

U. S. Envi rTnmental Protection Agency

Washington, V ^ 20460

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Measurements were made of the wind- induced noise and acoustic insertion loss of ten

microphone windscreens. Eight of these were reticulated polyester spheres of varying

porosity. The other two were the metal cage type typically used in long term outdoor

noise monitoring. The measurements were made under laboratory conditions for normal

and grazing flow incidence at wind speeds up to 14 m/sec. The data show that the

windscreens are very different in the amount of wind- induced noise reduction they

provide, with some achieving more than 25 dB. In general the foam windscreens

provided more wind- induced noise reduction than the cage windscreens; however, the

cylindrical cage windscreen performed better than any of the foam windscreens for

normal flow incidence. The insertion loss measurements proved difficult to make because of the small insertion losses encountered. However, data for grazing flow and

normal acoustic incidence show no strong velocity dependence for any of the windscreens

tested. Examination of the wind- induced noise spectra provided significant insight

into the noise generating mechanisms associated with flow around a porous sphere.

Effects of inflow turbulence, self-generated turbulence, mean flow, flow through the

sphere, and flow incidence angle were identified. Test results are provided in the

form of curves and tables for easy use in evaluating potential wind noise levels in

outdoor measurements.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Acoustics; flow; insertion loss; microphone; porous sphere; sphere; spheres; wind

noise; windscreen; windscreens; windscreen performance

18. AVAILABILITY fT Unlimited

For Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc., U.S. Government Printing Hfice

Washington, D.f . 20402, SD Cat. No. Cl 3

Order F rom National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

81

22. Price

USCOMM-DC 29042-P74