low frequency 2012 15th conference on low frequency noise stratford-upon-avon, uk, 22-24 may 2012...
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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
So
un
d P
ress
ure
Le
vel d
B/b
an
dw
idth
]
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, & 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
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 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
MAS Research Ltd.
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
MAS Research Ltd.
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
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, 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
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.
1 10 100 1 10 100
(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
1. 1982 NASA Perspective of Infrasound/Low-Frequency Perception within Buildings
2. 2003 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