To determine the most suitable porosity of a fiber assembly used as a sound absorbent, we measured the sound absorption coefficients of fiber assemblies having air spaces behind them by the standing wave method and investigated their sound absorption characteristics. The results:
(1) Sound absorption characteristics change from a viscosity-resistance type to, successively, a mixed type and a resonance type as they decrease in porosity. There are two types of absorption characteristics of the resonance type. One is a fibrous resonance type, which is for a thick sample. The other is a board resonance, for a thin sample.
(2) The relation between the most suitable porosity P_e; (%) at which a fiber assembly has the maximum sound absorption, i.e., 1.00, and the thickness T (cm) is shown as follows: (100-P_e; )=a'T^-b' where a' is a constant which is decided by fiber fineness and b' is another constant. If T is constant, the relation between P_e; and fiber fineness d (denier) is shown as follows: (100-P_e; )d^1/2=C where C is a constant decided by T.
(3) The relation between the total surface area S (cm²) of fibers constituting a fiber assembly of porosity P_e; (%) and T (cm) is shown as follows: S=aT^b×10⁴ where a and b are constants.
A fiber assembly which meets this equation has the maximum sound absorption coefficient at a certain frequency, if it has no back air space, or at an optional frequency if it has a back air space suited to the frequency.