Glass wool is a common sound absorbing material, often used in KTV, opera house, conference room, studio, recording studio, broadcast room and other places.  The bulk density of a material is the weight of an object per unit volume.  

For centrifugal glass wool, its fibers, muscles, particle size, diameter and solid density are not the same, under the influence of these factors, its bulk density will be very different, so as to have different effects on the sound absorption coefficient of the material.  For example, the absorption coefficient will be different with different fiber diameters.  

The study shows that for a certain fiber arrangement, the determining factor of sound absorption is the sum of fiber surface area. Under the condition of other factors being equal, the smaller the fiber diameter is, the larger the total surface area is, and the larger the average sound absorption coefficient is.  Under certain conditions, the increase of bulk density will increase the absorption coefficient of low frequency, but the increase of bulk density will not significantly improve the absorption coefficient, and the optimal bulk density value can often be obtained from experiments.  

In the low-frequency region, with the increase of bulk density, the sound absorption coefficient increases a lot, which is especially obvious in the curves of the samples of 32kg/m3 and 48kg/m3, from 100 hz to 630 Hz, the latter is 0.15 higher than the former on average.  At high frequencies, the effect is minimal.  It can be seen that the change of bulk density does have a greater impact on low frequency and a smaller impact on high frequency.  When the bulk density is too high, the sound absorption performance of middle and high frequency tends to decrease.