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Development of Improved Noise Metrics and Auditory Risk Assessment Procedure June 22, 2009 Won Joon Song and Jay Kim Mechanical Engineering Department University of Cincinnati

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Contents Description of noise metric Correlation study between the noise metric and PTS data Methodology to determine spectral NIHL threshold SPL EARM curve and its implementation strategy to NIHL research

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Noise group data Provided by collaborators in SUNY Plattsburgh 23 noise groups –18 noises of 100 dBA overall SPL –Additional 5 noises 3 noises of 90 dBA overall SPL: G-47, G-48 and G-56 2 noises of 95 dBA overall SPL: G-57 and G-58 Complex or Gaussian noise –20 complex noises –3 Gaussian noises: G-61, G-47 and G-57

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Noise exposure data Provided by collaborators in SUNY Plattsburgh Continuous 5-day exposure to 9-16 chinchilla subjects (23 groups, 275 chinchillas in total) Obtained at 6 frequency points of 0.5, 1.0, 2.0, 4.0, 8.0 and 16.0 kHz –Auditory evoked potential (AEP) measurement TTS (dB): right after exposure PTS (dB): after 30 days of recovery –Mechanical damage OHC loss (%) IHC loss (%)

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Basic designs of noise metric 6 basic designs reflecting time-frequency characteristics of the noise 14 metrics derived from basic designs Type 1: Equivalent SPL Type 2: Kurtosis Type 3: Maximum SPL Type 4: Dynamic SPL Type 5: Modified equivalent SPL Type 6: Normalized weighted exposure time 1/3 octave pressure time history

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1/3 octave pressure time-history : Implementation of AWT Pressure time-history 1/3 octave pressure time-history A special version of analytic wavelet transform developed at UC Signal analyzer to obtain a set of 1/3 octave time histories from a single time history Useful tool to study spectral noise-to-hearing loss relationship

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Noise metric calculation procedure Pressure time-history Spectral noise metric 1/3 octave time-history @ 6 frequency points 0.5 kHz1.0 kHz2.0 kHz 16.0 kHz8.0 kHz4.0 kHz T-F decomposition by AWT

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Noise metric description Type 1: Equivalent SPL Time average of 1/3 octave sound pressure Type 5: Modified equivalent SPL Time average of 1/3 octave sound pressure above threshold

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Noise metric description Type 2: Spectral kurtosis Statistical quantity representing impulsiveness of a signal β = 79.81 β = 2.99 ≈ 3 G-63: Complex type G-61: Gaussian type

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Noise metric description Type 3: Max. SPL Top 95% value of the 1/3 octave SPL distribution histogram Type 4: Dynamic SPL Weight to the dynamic fluctuation of 1/3 octave SPL Type 6: Normalized weighted exposure time Application of ‘3 dB exchange rule’ to each sampling time interval

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Box plot of PTS data

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Noise vs. frequency correlation Noise correlation: frequency by frequency Frequency correlation: noise by noise Noise metric surface PTS surface (median)

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Example of noise metric vs. median PTS: Complex noise (G-63) Spectral kurtosis Median PTS

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Example of noise metric vs. median PTS: Gaussian noise (G-61) Spectral kurtosis Median PTS

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Correlation study with 18 noises Correlation scale: Noise correlation Frequency correlation

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Correlation study with 23 noises Noise correlation Frequency correlation Correlation scale:

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NIHL threshold SPL determined by statistical process Scatter plot of Leq-PTS (1.0 kHz) C.I. Threshold SPL (1.0 kHz) Threshold SPL curve

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NIHL threshold SPL curve construction 0.5 kHz 1.0 kHz 2.0 kHz 8.0 kHz 16.0 kHz 4.0 kHz

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Improved noise metric with variable threshold Type 5a: Modified equivalent SPL with variable threshold Time average of 1/3 octave sound pressure time history above frequency-dependent thresholds

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T-F decomposition Variation of the equivalent sound pressure Constant threshold Variable threshold Type 5a Type 5 Type 1Conventional Time averaging

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Constant vs. variable threshold Constant threshold Variable threshold

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Equal auditory risk metric (EARM) curve construction Scatter plot of Lem-PTS (1.0 kHz) Linear regression line C.I. EARM curves Lower prediction bound

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Interpretation of EARM curves Iso-PTS curves (10,20,30,40,50dB) Slow development of PTS Fast development of PTS PTS-free zone NIHL threshold metric curve (0 dB PTS curve)

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Application of EARM curve to NIHL study: Noise reduction level NIHL threshold metric curve (0 dB PTS curve) Noise metric curve (G-47) Recommended spectral noise reduction level to protect the auditory system Noise reduction level required at 8.0 kHz No noise reduction required at 2.0 kHz

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Application of EARM curve to NIHL study: PTS prediction Noise metric curve (G-47) PTS at 4kHz (estimated by interpolation) 0 dB PTS at 2.0 kHz

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Validity checking of EARM curve prediction Properly-estimated Predicted PTS at 8.0 kHz Measured PTS at 8.0 kHz Overestimation at 0.5, 8.0, and 16.0 kHz Acceptable PTS prediction band

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Questions?

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