Aural Nondetectability limits

[b1200N]

Can anyone explain the aural nondetectability section of Mil Std 147C.
From table IV the aural nondetectability at 2500Hz at 20m is 28dB and at 30m is 23dB, the AD decreases with distance, while most of the table shows the AD increasing with distance.

Thanks

 

[IRstuff]

A close examination of Table IV in MIL-STD-1474C or Table 2-II in MIL-STD-1474D shows similar anomalies at almost all frequencies at 500m and nearly half of the frequencies at 100m.

I can only guess that the tables are based on actual measurements and that they either changed microphones or amplifiers at those ranges, or the data was simply too noisy and they picked some osrt of envelope as the spec limit. This is partly borne out in the graphs for non-detectability

TTFN

 

[GregLocock]

Could be temperature inversions or other environmental effects bending the rays.

The boss of the first NVH group I ever worked in had spent a couple of years measuring this stuff for the military - if there were no surprises I guess it would have taken a month! Cheers

Greg Locock