The power spectrum level of a sound at a specified frequency is the power level for the acoustic power contained in a band one cycle per second wide, centered at this specified frequency. The letter n is the designation number for the band being considered. The width of the band and the reference pressure must be specified. The band pressure level of a sound for a specified frequency band is the effective sound pressure level for the sound energy contained within the band. The width of the band and the reference power must be specified. The band power level for a specified frequency band is the acoustic power level for the acoustic power contained within the band. The A, B, and C curves can be regarded as very rough approximations to the contours of equal loudness at 40, 70, and 100 phons respectively in order to compensate for the reduced sensitivity of the ear to very low and very high frequencies. Weighting curves for sound level measurements. Leq (the equivalent continuous noise level) is the level which, if it were constant for the stated period, would have the same amount of acoustic energy as the actual varying noise level. Therefore L 0 is the maximum noise level during any period and L 100 is the minimum. 5 min L 90 of 80 dB(A) means that for the 5-min period of measurement for 90 per cent of the time the noise exceeded 80 dB(A).

Percentiles are expressed as the percentage of time (for the stated period) during which the stated noise level was exceeded, i.e. The most common of these are percentiles and equivalent continuous noise levels. It may therefore be necessary to derive indices which describe how this happens. It will, however, be apparent that most noises change in level with time. 42.1.7 Noise indicesĪll the previous discussions have concerned steady-state noise. Narrow-band analysis uses a VDU to show the graphical results of the fast Fourier transform and can also display octave or one-third octave bar graphs. This is the fastest of the methods and is the most suitable for transient noises. One-third octave filters are commonly used for building acoustics, and narrow-band real-time analysis can be employed. It may be necessary to record the noise onto tape loops for the repeated re-analysis that is necessary. If further resolution is necessary one-third octave filters can be used but the number of required measurements is most unwieldy.

Db sound reference