Low Frequency th Conference on Low Frequency Noise Stratford-upon-Avon, UK, May 2012 Enhanced Perception of Infrasound in the Presence of Low-Level Uncorrelated Low-Frequency Noise Dr M.A.Swinbanks, 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 dBA 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 Sound Pressure Level dB/bandwidth ] Figure 1 MAS Research Ltd.

Figure 2 MAS Research Ltd. 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, & Hz 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

Figure 4 MAS Research Ltd. 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 MAS Research Ltd. +ve Peak for Pure Tone –ve Peak for Pure Tone

Figure 6 MAS 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 MAS Research Ltd. Present Investigation: Introduce Non-Linear Time-Domain Threshold at Output of Hearing-Filter Consider 3 Alternative Implementations of Threshold

Figure 8 MAS Research Ltd. 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

Frequency Hz Figure 9 MAS Research Ltd. 1/3 rd 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 MAS Research Ltd. Progressively Increasing the Amplitude of the Higher Frequency Noise causes Fixed Amplitude Low-Level Signal to Appear in the Output

Figure 11 MAS Research Ltd. 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 MAS Research Ltd. * 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, 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

Figure 13 MAS Research Ltd. 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 MAS Research Ltd. 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.

(a)(b) Figure 15 MAS Research Ltd. 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 MAS Research Ltd. 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 NASA Perspective of Infrasound/Low-Frequency Perception within Buildings Japanese Laboratory Investigation of Infrasound/Low-Frequency Perception Thresholds MAS Research Ltd. Results Indicate: Impulsive Infrasound from Modern Upwind-Rotor Wind Turbines can be Perceptible Indoors, under Low, Audible Background Conditions