6.4.1 Acceleration spectral density (ASD) The ASD shall be within ±3 dB (range ½ × ASD to 2 × ASD) of the specified ASD levels as shown in the appropriate Figures 2 to 5. The initial and final slope should not be less than those shown in Figures 2 to 5. 6.4.2 Root mean square value (r.m.s.) The r.m.s. of the acceleration at the reference point over the defined frequency range shall be that specified in Figures 2 to 5 ± 10 %. NOTE With respect to the low frequency content it may be difficult to obtain ±3 dB. In such cases it is only important for the test value to be noted in the report. 6.4.3 Probability density function (PDF) Unless otherwise stated, for each measuring point the time series of the measured acceleration(s) shall have a distribution with a PDF which is approximately Gaussian and a crest factor (ratio of the peak to r.m.s. values) of at least 2,5. NOTE Figure 6 shows the tolerance bands of the cumulative PDF. 6.4.4 Duration The total duration of exposure to the prescribed random vibration in each axis shall not be less than that specified (see 8.2 and 9.2).