The Certification agencies of law rule a minimum level of external noise. The most widely used of a family of curves which relates to the measurement of sound pressure level. A – Weighting is applied to instrument – measured sound levels in order to account for the relative loudness tolerated by a human ear, as the ear is less sensitive to low sound frequencies. It is done by arithmetically adding a table of values, listed under the octave or the third – octave bands, to the measured sound pressure levels in dB. The measurements of the octave band so formed are normally added (logarithmically) to get a single A – weighted value describing the sound whose unit is written as dB(A). Other weighting sets of values are B, C, D and now Z is also there.
The noise level rating is measured by A – weighting which is then reduced by a factor called NRR (Noise Reduction Rating) which is measured with the help of the hearing protectors sold in the U.S. are required by the U.S. Environmental Protection Agency (EPA). NRR is an estimate of the reduction of noise at the ear when these protectors are worn correctly. The National Institute for Occupational Safety and Health (NIOSH) has also developed de – rating formulas to lessen the NRR factor. While the NRR are designed to come in use with C – weighted noise, that means the lower frequencies are not de – emphasized, other ratings (such as, NRR (SF) and NRSA) are determined to use it with A – weighted noise levels, which allow lower frequencies de – emphasized. The compensation of 7-dB between C and A weighting was mandated by U.S. EPA to be applied when the NRR is used with A-weighted noise levels though recommended by NIOSH. This thereby, lessens the noise level rating of a car. The vehicles have many sources of noise namely the brakes, tyre’s contact patch with road surface, engine, driveline, brakes, and even, wind. Using better engines and silencers of fine qualities is also recommended.