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Near Field

Note: The information below refers equally to open inlet, open outlet and casing breakout noise.

To obtain free field measurements from a noise producing source it is necessary to be at a distance of at least 2 x the longest side.  Also most standards refer to the noise source as being a point in the centre of the fan outlet or inlet. If we are within 2 x the longest side we are considered to be in the near field range.

When a spec calls for a sound level not to exceed an amount at 1.0 metre it should be noted that this will fall into the near field range if the longest side of the inlet or outlet (or indeed the casing) exceeds 500 mm.

If we were taking a measurement inside the outlet duct the average sound pressure level can be calculated from 

SPL = SWL-10Log(A).

(Log to the base 10)

(A = cross section area square metres).

As we get nearer to the open fan outlet within the near field range we would expect the measured sound pressure level to change from the free field level to the in duct level. There will also be end reflection corrections and directivity factors that come in to play. It should also be noted that if the area of the outlet is less than 1.0 m2 the in duct SPL level is greater than the SWL level and vice versa.

Centrix uses an empirical formula for open inlet and outlet noise that will produce an average SPL at any distance from the source when in the near field range. A constant of 2 dB is used to ensure a slightly higher figure than the in duct SPL alone when relatively close to the outlet and also yields a 2 dB increase over free field when only just entering the near field range.

A different formula is used for breakout noise when in the near field range and a large difference can be expected between the Near Field and Free Field sound levels

Remember that sound pressure levels produced by Centrix and those supplied by most other fan manufacturers are averages. Actual readings taken at any location will always be influenced by directivity factors i.e. readings taken pointing at the outlet face will be higher than readings at 90 degrees to the outlet flow.