Abstract
This paper presents the experimental results of attenuation(ATT)characteristics of some lined and splitter ducts. A rectangular air passage with one or more inner surfaces covered with a fibrous acoustical material is used for the mesurements of ATT. The experimental apparatus is schematically shown in Fig. 1 and 2. The flow resistance of some acoustical materials versus volumetric density is given in Fig. 3. Typical distribution of local sound pressure intensity in a cross section is shown in Fig. 4. The noise reduction rate for the distance along the axis of the duct can be calculated from the integrated mean value of the sound pressure distribution for each section plotted against the distance as shown in Fig. 5. It is shown that the sound pressure linearly decreases with logarithmic distance Z. Fig. 6 to 8 show the experimental results of ATT for various thicknesses of acoustical material, where a dotted line shows the calculated value from the Bruel's formula. The ATT of a lined duct depends primarily on the thickness and the flow resistance value. It is also shown that the peak value of ATT moves to the side of lower frequency with increase of the thickness and flow resistance of the acoustical material. The number of sound waves in an acoustical material can be determined theoretically from the flow resistance value, showing that the number is larger than that of sound in air, because the velocity of sound decreases in the acoustical material. Fig. 9 and 10 show the reduction of wave length in the material as a function of fill-up ratio multiplied by the flow resistance value. The ATT of a lined duct can be expressed by Eq. 3, in which K is defined as the absorption coefficient of the acoustic material. Fig. 11 shows the variation of K with the flow resistance of the acoustical material. Similar experiments were performed for a splitter duct by varying the thickness of acoustical material. A peak of ATT also exists at the wave length of sound nearly equal to the interval of arrangement of the acoustical material as shown in Fig. 14. Fig. 15 and 16 show the variation of the coefficient K of a splitter duct of types D_A and D_B with the flow resistance of acoustial material. Again, the ATT can be estimated from the flow resistance of the acoustical material. The coefficient K are compared in Fig. 17 for lined duct, splitter duct of type D_A and D_<A′> in which each duct is divided by a steel plate along the center line. A practical dissipative type muffler for a boiler fan was designed and constructed based on these results as shown in Fig. 18. The attenuation characteristics of muffler were measured as shown in Fig. 19, where a dotted line shows the designed value. (A chain line and points show the attenuation characteristics of the muffler for an induction fan. ) It is shown that the attenuation characteristics of the dissipative type mufflers can be estimated well by the method described here.
