The influence which several factors relating to the

No abstract

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 Pe (%) at which a fiber assembly has the maximum sound absorption, i.e., 1.00, and the thickness T (cm) is shown as follows :where a' is a constant which is decided by fiber fineness and b' is another constant. If T is constant, the relation between Pe and fiber fineness d (denier) is shown as follows :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 Pe (%) and T (cm) is shown as follows: S = a Tb x 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.

This article presents the results of an investigation into the relation between the normal incident sound absorption coefficient and the apparent density of glass wool boards used as a sound absorbent.(1) Only one kind of glass wool boards 64 kg/m3 in apparent density and 2.5 cm in thickness out of 20 kinds made by 3 manufacturers has given a sound absorption characteristic belonging to the mixed type. The sound absorption characteristics of the other kinds of glass wool boards have been shown by experiment to belong to the viscosity-resistance type.(2) The relation between effective apparent density Dea (kg/m3) at which a glass wool board gives 1.00 in sound absorption coefficient and the thickness T (cm) is :Dea=aT -b where a and b are constants fixed by the manufacturer.(3) The relation among apparent density Da (kg/m3), thickness T (cm), distance d (cm) between the center of a sample and the rigid wall, and frequency f o at which the sample gives the maximum sound absorption coefficient is expressed by the following empirical formula : f0= (c/4-aD3a'T)d-' where a and t are constants and c is the speed of sound.

The acoustic impeadance density of fabrics has been measured by the tube method with the following results:1) The acoustic reactance shows the negative value at low frequencies and the positive value at high frequencies. 2) If the rear air space of the fabric is deep, the acoustic reactance is zero at low frequency.3) The reactance of a pliable fabric having only a limited number of interlacing points is zero at high frequency. 4) A stiff fabric having many interacting points shows a large value of resistance. 5) The value of resistance of a fabric is nearly constant if the depth of its rear air space is kept in a range of 6 to 10 cm. 6) The value of resistance of a pliable fabric is constant, irrespective frequency.The value of resistance of a stiff fabric increases with an increase in frequency. The relation between the acoustic impeadance density and the sound absorption coefficient has been studied with the following results:(1) Irrespective of the mechanism of sound wave absorption, the absorption coefficient of a fabric which has a rear air space is calculable by Pariss' formula.(2) The impeadance density of a fabric of the viscosity resistance type is calculable by Paris' formula and its normal incident absorption coefficient is calculable without complicated figuring.