Low Frequency 201215th Conference on Low Frequency NoiseStratford-upon-Avon, UK, 22-24 May 2012

Enhanced Perception of Infrasound in the Presence of Low-Level Uncorrelated Low-Frequency Noise

Dr M.A.Swinbanks, MAS Research Ltd.

MAS Research Ltd.

The Problem: 2008 T.H.Pedersen* (DELTA) Emphasized that Different Resolutions of the Same Wind Turbine Spectra lead to Different Conclusions

Data:March 201048 dBA Leq@ 1500ft

(Following Convention of Pedersen 2008)

“……. it can be seen that a direct comparison of the hearing threshold and the spectrum of

the wind turbine is not meaningful……” *

“ …….. have been discussed with a number of researchers (Henrik Moller, Aaborg University, Torsten Dau, Danish Technical University, Hugo Fastl and Geoff Leventhall)

and solutions have been sought for without result.” *

Spectra:

1/1 Octave

1/3 Octave

1/6 Octave

1/24 Octave

1Hz FFT

0.1Hz FFT

Pure Tone Hearing Threshold

1Hz 10Hz 100Hz 400Hz

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Figure 1

MAS Research Ltd.

Figure 2

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Pedersen proposed Weighting the Frequency Spectra with the Inverse Hearing Threshold (HT-Weighting)

So Hearing Threshold becomes a Straight Line at 0dB

Integrating Spectra of Differing Resolution over 2 Lowest Critical Bands i.e. < 100Hz, & 100-200Hz yields 2 Consistent Values which can be Compared to Threshold.

But still does not define Frequency where Threshold is Crossed

MAS Research Ltd.

Present Author Proposed Using a Running (Cumulative) Integration

Condenses All Spectra to a Common Rising Curve, which Intersects 0dB Threshold at Well-Defined Frequency

75% of Perceived Energy lies within -6dB of Threshold

At 0dB Intersection, total Perceived Energy equals Energy of Perceived Sine Wave at Threshold

Figure 3

1 10 100 5001 10 100 5001 10 100 500

Figure 4

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Mean Energy (rms Level) fails to take account of Peak Levels & Crest Factor

Examine Time Domain response to quantify Peak Levels re Threshold

Simulate Infrasonic Impulse ~ NASA 1989, for Upwind-Rotor Turbine in Wind-Gradient & Shadowing

Filter Time-Series with Simulated Hearing Response

Figure 5

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+ve Peak for Pure Tone

–ve Peak for Pure Tone

Figure 6MAS Research Ltd.

Evaluate Cumulative Spectra for Waveforms which match Pure Tone Limits

Indicates Sound is Perceptible at rms Levels lower than 0dB Pure Tones

Results Consistent with 1982 NASA Hearing Tests for Noisy Impulsive Turbines

Figure 7

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Present Investigation: Introduce Non-Linear Time-Domain Threshold at Output of Hearing-Filter

Consider 3 Alternative Implementations of Threshold

Figure 8

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Output Amplitude of Alternative Thresholds, for Sine Wave Input

Signal goes toward Zero below Threshold Level

Threshold Characteristics Normalized to Yield -6dB Output just below 0dB Input Level

3.16 10 31.6 100 316 Frequency Hz

Figure 9

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1/3rd Octave Input Spectra for Numerical Simulation of Threshold Interactions

(1) Infrasonic/Low Frequency Gaussian Random Signal, Band-Limited 15-25Hz,

Amplitude -12dB re Threshold

(2) Additional Gaussian Random Noise, 50Hz-90Hz

Amplitude Varied from -20dB to +6dB re Threshold

Effect of Progressively Increasing Noise Component

(a) (b)

(c) (d)Figure 10

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Progressively Increasing the Amplitude of the Higher Frequency Noise causes Fixed Amplitude Low-Level Signal to Appear in the Output

Figure 11

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For Maximum (+6dB) Audible Noise, Threshold Signal Output Compares Closely to Linear Signal prior to Threshold, when Filtered 10Hz - 30Hz

Note: Maximum Signal Amplitude is well below Threshold Level

Figure 12

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*Yasunao Matsumoto, Yukio Takahshi, Setsuo Maeda, Hiroki Yamaguchi, Kazuhiro Yamada, & Jishnu Subedi. An

Investigation of the Perception Thresholds of Band-Limited Low Frequency Noises: Influence of Bandwidth. Journal of Low Frequency Noise Vibration and Active Control, 2003

2003 Japanese Laboratory Investigation of Perception Thresholds

Three Infrasonic/Low Frequency Test Spectra, with Roll-Offs at 10Hz, 20Hz & 40Hz

Test Subjects could Identify Them Separately, Despite their Differences being well below the Hearing Threshold *

Suggests Higher Frequency Audible Noise Enabled Discrimination

Multiple Low-Frequency Impulses Measured Indoors in March 2010 at a Modern, Upwind-Rotor Windfarm. 6 Separate Turbines can be Identified

Figure 3

Multiple Low-Frequency Impulses Measured Indoors in March 2010 at a Modern, Upwind-Rotor Windfarm. 6 Separate Turbines can be Identified

Figure 3

Figure 13

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Application of Time-Domain Procedure to Infrasonic/Low-Frequency Wind-Turbine Noise Measured in the Bedroom of a House

Peak Sound Pressure Levels 88-90dB, rms SPL 77dB, Crest Factor ~ 11-12dB. Directly Downwind of 6 Separate Wind-Turbines, nearest at 1500 ft, farthest at 1.2 miles. Attenuation of -3dB per Doubling of Distance.

Peak Level ‘4’ 88dB SPL

Figure 14

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Lift-Force Impulse acting on a Turbine-Blade represents an Impulsive Acoustic Dipole. Sound radiated is proportional to Time-Derivative of Dipole, so has both +ve & -ve components

Wind Turbine Spectrum Indoors, 0.1Hz Resolution. Compare to Acoustic Spectrum from Idealised Periodic Impulsive Lift-Forces.

1 10 100 1 10 100

(a) (b)

Figure 15

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0.1Hz Bandwidth Spectra for Simulated Hearing-Response to Wind Turbines (Indoors).

Red Curve: Ideal Linear Response Prior to Threshold

Green Curve: Actual Threshold Output

(a) Threshold at -8dB (10% Young Adults). Comparison of 3 Different Threshold Implementations,

(b) Threshold at -12dB (2.5% All Adults)

Figure 16

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Simulated Time-Domain Hearing-Response after Threshold, Filtered 5Hz-20Hz

a) Threshold at -8dB (10% Young Adults)

b) Threshold at -12dB (2.5% All Adults)

(+/- RMS Amplitude for -12dB Shown)

Red: Ideal Linear Response before Threshold

Green: Actual Threshold Output

Figure 17

Octave Band Wind-Turbine Sound Pressure Level Inside House (Red)

compared to 1982 NASA Perspective*

MAS Research Ltd. * D.G.Stevens, K.P.Shepherd, H.H.Hubbard, F.W.Grosveld, Guide to the Evaluation of Human

Exposure to Noise from Large Wind Turbines, NASA TM83288, March 1982

Summary & Conclusions

Methodical Assessment of the Audibility & Perception of Infrasound & Low-Frequency Noise :

1. Derive Initial Estimate of Transition to Audibility, by Equating Cumulative HT-Weighted Spectrum to Equivalent Energy of Single Sine-Wave at Threshold

2. Modify Estimate by allowing for Increased Crest Factor

3. Allow for Non-Linear Threshold Interaction leading to Enhanced LF Perception, induced by Higher Frequency, Just-Audible Noise.

Results are consistent with

1. 1982 NASA Perspective of Infrasound/Low-Frequency Perception within Buildings

2. 2003 Japanese Laboratory Investigation of Infrasound/Low-Frequency Perception Thresholds

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Results Indicate:

Impulsive Infrasound from Modern Upwind-Rotor Wind Turbines can be Perceptible Indoors, under Low, Audible Background Conditions