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

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

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

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 (%)

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

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

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

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

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

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

Box plot of PTS data

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

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

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

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

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

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

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

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

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

Constant vs. variable threshold Constant threshold Variable threshold

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

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)

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

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

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

Questions?