Uncertainty budget for ISO 3744

In general, the calculation of sound power level, based on ISO 3744, may be considered as a function of numerous parameters, some of which represent sources of uncertainty. In the expression below, these components are represented by the symbol(delta).
All the quantities are expressed in dB.

NPL has been conducting research into the evaluation of some of these components of uncertainty. The links in the table go to summaries of the NPL research which contain estimates of A-weighted values of these delta terms to be used in the uncertainty budget.

A calculator is included here to provide a simple illustration of how varying uncertainty values effects a declared sound power level, and to show how concentrating effort on reducing particular contributions can lead to significant reduction in the total values

Uncertainty Calculator (pop-up window)

(requires Javascript)

Parameters for uncertainty budget (based on ISO 3744)
symbol	Description
LW	Sound power level of the noise source
Lp	Surface sound pressure level
K1	Background noise correction
K2	Environmental correction factor
S	Area of the measurement surface
	delta slm, Allowance for any error in measuring instrumentation
	delta rep, Allowance for any error resulting from variation in operating conditions of the noise source
	delta boun, Allowance for any error due to sound reflection from the test environment boundaries
	delta mic, Allowance for any error due to finite number of microphone positions
	delta met, Allowance for any error due to variation in meteorological conditions
	delta angle, Allowance for any error due to angular difference between measurement surface normal and sound emission direction
	delta imp, Allowance for any error due to impedance of the surroundings into which the source is emitting sound energy
	delta A, Allowance for any error due to the use of applying A-weighted bandwidth filter

Note that delta boun has been included but may not appear in the proposed ISO 3744 since it is considered negligible.

NPL research into sound power measurement uncertainty >>