Sound Absorption Characteristics of Fiber Assemblies

Abstract

The influence which several factors relating to the make-up of a fiber assembly have on sound absorption characteristics was investigated by measuring the normal incident absorption coefficients of fiber assemblies from 250 to 2, 000c/s at intervals of 1/3 octaves. The results obtained are:(1) There are two other types of absorption characteristics besides the well-known viscosity resistance type (I). One is a fibrous resonance type (II), of which the absorption characteristics show resonance absorption at low frequency but which, in a high frequency range, belongs to type (1). The other is an intermediate type (III), which is between (I) and (II).(2) The absorption characteristics of a fiber assembly belong to type (I) if the fibers are arranged parallel to the direction of the propagation of the sound wave. The air in the fiber assembly plays a part in the absorbing action. If the fibers are arranged so as to divide the air space in the assembly into small sections, the value of its absorption coefficient is high.(3) It is experimentally estabished that the absorbing mechanism of a fiber assembly comes mainly from the frictional action between the surface of fibers and the air in the assembly. Fiber assemblies are equal to one another in their absorption characteristics if the fibers are the same in total surface area, even it they differ in length or fineness.(4) To increase the absorption coefficient of a fiber assembly in a low frequency range, it is better to increase its thickness than to reduce its porosity. The thickness of a fiber assembly has an effective value which increases the absorption coefficient to a maximum for a certain frequency and a certain porosity degree.(5) The relation between a certain frequency (f) and a certain effective porosity (Pe), which porosity increases the absorption coefficient at that frequency to a maximum value, is shown as follows: f=K(100-Pe)-1.3 where K is a constant which is decided by the kind of fiber material, its fineness, the fiber orientation and the thickness of the fiber assembly. If K is obtained experimentally at a certain frequency, the value of Pe for every frequency is calculable by the above equation. There is the most effective porosity (Pme) giving the greatest value among the maximum absorption coefficients in Pe of all frequencies.The larger the total surface area is, the greater Pme is and the lower the frequency is.