ASA - ANSI/ASA S12.68
American National Standard Methods of Estimating Effective A-Weighted Sound Pressure Levels When Hearing Protectors are Worn
|Publication Date:||20 August 2007|
This standard specifies a choice of three methods for use with hearing protector attenuation data to estimate the effective A-weighted sound pressure levels when a hearing protector is worn. The three methods, the Noise Level Reduction Statistic for use with A-weighting (NRSA), the Noise Level Reduction Statistic, Graphical (NRSG), and the octave-band method are presented in order of increasing complexity of use and potential accuracy. Furthermore, the standard specifies in the case of the NRSA and the NRSG that values will be presented for both the 80th and 20th percentiles, indicated as NRSA80 and NRSA20, and as NRSG80 and NRSG20, to reflect the range of attenuation that can be anticipated.
The NRSA specifies an attenuation value, the Noise Level Reduction Statistic for use with A-weighting, determined from the octave-band attenuation data of a hearing protector in an ensemble of 100 representative noises, which may be directly subtracted from an A-weighted noise assessment to estimate L'A, the effective A-weighted sound pressure level when the hearing protector is worn.
The NRSG specifies an estimated noise level reduction value deduced from a graph (or, alternatively, an arithmetic interpolation done by a spreadsheet) that relates the protection in a given A-weighted exposure to the difference between the C- and A-weighted sound pressure levels of the noise. It requires two noise measures (A- and C-weighted), instead of the single measure (A-weighted) necessary for use with the NRSA. The NRSG is determined by applying the octave-band attenuation data for a hearing protector to an ensemble of 67 noises that span a broader range of spectral types than used for the NRSA computation.
The octave-band method specifies a procedure for directly applying the octave-band attenuation data of a hearing protector to a set of octave-band measurements of the noise. The computation includes a correction that is a multiple of the standard deviation in order to adjust the prediction for the desired protection performance.