Silencers in engine intake and exhaust lines are often required for reducing the sound pressure level (SPL) radiated from the open end of the system (a tailpipe outlet or snorkel inlet) below a specified maximum level. A silencer prototype that is satisfactory in this respect can be designed by direct SPL calculations on an acoustic model of the system by repeating the activities inside the rectangle with red dashed lines in Figure 1. However, it may not be convenient or feasible to validate a prototype that looks good on paper by SPL measurement, because it has to be mounted on the actual engine. A method of validation which does not require SPL measurement consists of computing first the boundaries of the ‘space’ of all possible prototypes that satisfy the SPL target. This space can be spanned by two acoustic parameters, whose measurement do not require the presence of the actual engine (see Duct Acoustics). Whether or not a silencer prototype satisfies the SPL target can then be determined from the disposition of the values of these parameters for that prototype with respect to the admissible design boundaries computed in the first step. This procedure is delineated in the flowchart of Figure 1 by the blocks outside the rectangle with dashed red sides.