Page 217 - The Master Handbook Of Acoustics
P. 217

192   CHAPTER NINE



                                                              Effect of Density of Absorbent
                         1.0
                              4"                              Glass fiber and other materials come in var-
                                3"
                         0.8       2"                         ious densities from the flimsy thermal insu-
                       Absorption coefficient  0.6  1"        boards used widely in industry. All of these
                                                              lation batts to the semi-rigid and rigid
                                                              have their proper place in acoustical treat-
                                                              ment of spaces, but the question right now
                         0.4
                                                              is, “What effect does density—the packing
                                           Thickness of
                                           material
                                                              of the fibers—have on sound absorption
                         0.2
                                                              coefficient?” In other words, is the sound
                          0
                           125  250   500  1 kHz 2 kHz 4 kHz  able to penetrate the interstices of the high-
                                                              density, harder surface material as well as
                                    Frequency - Hz
                                                              one of the flimsy kind? The answer appears
                     FIGURE 9-8
                                                              in Fig. 9-10, which shows relatively little
                   The thickness of glass-fiber sound-absorbing mater-  difference in absorption coefficient as the
                   ial determines the low-frequency absorption (den-
                   sity, 3 lb/cu ft). The material is mounted directly on  density is varied over a range of almost 4 to
                   a hard surface.                            1. For very low densities the fibers are so
                                                              widely spaced that absorption suffers. For
                                                              extremely dense boards, the surface reflec-
                                                              tion is high and sound penetration low.

                        1.0                                   Open-Cell Foams


                        0.8      3"  2" 1"                    Flexible polyurethane foams are widely
                       Absorption coefficient  0.6  0"  Air space  machinery, aircraft, and in various indus-
                                                              used in noise quieting of automobiles,

                                                              trial applications. After a slow start, foams
                        0.4
                                                              are finding some application as sound

                        0.2
                                                              including recording studios and home lis-
                                           Thickness of       absorbers in architectural applications,
                                           material = 1"      tening rooms. Figure 9-11 is a photograph
                          0                                   of one form of Sonex, a foam product con-
                                                                                  4
                           125  250   500  1 kHz 2 kHz 4 kHz
                                                              toured to simulate the wedges used in ane-
                                     Frequency - Hz
                                                              choic rooms. These are shaped in male
                     FIGURE 9-9
                                                              and female molds and come in meshed
                   The low-frequency absorption of 1-inch glass fiber-  pairs. This material can be cemented or
                   board is improved materially by spacing it out from
                   the solid wall.                            stapled to the surface to be treated.
   212   213   214   215   216   217   218   219   220   221   222