A noise attenuation unit (10 ' ) connectable to a fluid flow path of an engine comprising: a housing consisting essentially of a first housing portion (50 ' ) and a second housing portion (52 ' ) mated together with a fluid-tight seal to define an internal cavity , wherein the housing defines a first port and a second port (24 ' ) which are both in fluid communication with the internal cavity ; a porous noise attenuating material dimensioned for a tight fit within the internal cavity and seated therein, wherein the porous character of the noise attenuating material causes noise pressure waves to attenuate by interfering with themselves and does not unduly restrict or interfere with the fluid flow, the porous noise attenuating material comprising a plurality of stacked plugs (20 ' , 20 ' a, 20 " ) of porous material sandwiched between a first seat and a second seat of the housing , wherein one of the plurality of stacked plugs (20 ' ) has a bore therethrough that is larger than pores of the porous material characterised in that , another one of the plurality of the stacked plugs (20'a, 20") has a depression in a major surface thereof that is larger than pores of the porous material, wherein the depression is cone- or funnel- shaped and the combination of the plug with the bore and the plug (20'a, 20") with the depression is stacked to form a hollow portion forming a cone- or funnel- shaped bore passing partially through the porous noise attenuating material in general alignment with the first port and the second port (24 ' ); wherein fluid flow from the first port to the second port through the internal cavity is only through the plurality of stacked plugs (20', 20'a, 20"), the bore and the depression . a housing consisting essentially of a first housing portion (50 ' ) and a second housing portion (52 ' ) mated together with a fluid-tight seal to define an internal cavity , wherein the housing defines a first port and a second port (24 ' ) which are both in fluid communication with the internal cavity ; a porous noise attenuating material dimensioned for a tight fit within the internal cavity and seated therein, wherein the porous character of the noise attenuating material causes noise pressure waves to attenuate by interfering with themselves and does not unduly restrict or interfere with the fluid flow, the porous noise attenuating material comprising a plurality of stacked plugs (20 ' , 20 ' a, 20 " ) of porous material sandwiched between a first seat and a second seat of the housing , wherein one of the plurality of stacked plugs (20 ' ) has a bore therethrough that is larger than pores of the porous material characterised in that , another one of the plurality of the stacked plugs (20'a, 20") has a depression in a major surface thereof that is larger than pores of the porous material, wherein the depression is cone- or funnel- shaped and the combination of the plug with the bore and the plug (20'a, 20") with the depression is stacked to form a hollow portion forming a cone- or funnel- shaped bore passing partially through the porous noise attenuating material in general alignment with the first port and the second port (24 ' ); wherein fluid flow from the first port to the second port through the internal cavity is only through the plurality of stacked plugs (20', 20'a, 20"), the bore and the depression . <EOS>