Resonant curves of loaded felt pads in compressional vibration give values for the damping and dynamic stiffness which show that felt in vibration is quite different from an equivalent perfect spring with viscous damping. The un-symmetrical resonance curves and the fact that, within experimental error, the stiffness is inversely proportional to the square root of the thickness indicate a decrease in stiffness with increasing amplitude. The increase in stiffness with pressure is so large that the ratio of dynamic elastic modulus to pressure (stiffness over mass) does not vary greatly between pressures of three to one hundred pounds per square inch. In this range the natural frequency of a mass on a felt pad is determined by the thickness of the pad rather than by its area and static load. An analysis of the performance of felt in reducing vibration transmission shows that its effectiveness is limited in most cases to frequencies above forty cycles per second. Practical installations have been studied and tables drawn up to give the acoustical engineer “handbook” information for the solution of problems in vibration reduction.

In order to provide engineering information for the use of felt in vibration isolation, the dynamic stiffness and damping of three widely different grades of felt were measured by means of resonance curves. The dynamic stiffness, which was much greater than the static stiffness, decreased with amplitude so that the resonance curves were unsymmetrical. Except for pressures below 6 p.s.i., the stiffness modulus of felt increased with static pressure at a rate which made their ratio nearly constant. The natural frequency of a mass supported by a felt pad is, therefore, nearly independent of the static pressure load on the felt. Curves are given showing natural frequencies for 1 inch thicknesses of soft, medium, and hard felt for a range of static pressure between 1.5 and 100 p.s.i. It is shown that, except at low pressures, there is little difference in the natural frequency of hard or soft felt, and that there is a low limit of about 20 c.p.s. for 1-inch felt.

Results are given for a research program leading to the design of an anechoic treatment for the test chamber of the ASD high intensity sound facility. The acoustical requirements for 96 percent absorption coefficient at normal incidence for a frequency range of 50 -7000 cps and for sound pressure levels up to 160 db and the mechanical requirements for a collapsible treatment presented novel problems in design. The requirements were met by a treatment, six feet thick, composed of six layers of absorbing material irregularly spaced with the acoustical resistances per layer increasing from values for layers at the incidence sound side to higher values for layers near the room surfaces. Tests in the high intensity impedance tube facility designed for the program showed that the normal incidence absorption coefficient of the treatment was 96 percent or higher over most of the frequency range from 50 -7000 cps at sound pressure levels from 130 to 160 db. Fairly satisfactory results were also obtained for a five layer treatment subsequently designed. Etched polyurethane foam supported by wire screens was initially chosen as the layer material because of its resistance to damage in small scale life tests at particle velocities corresponding to a sound pressure level of 160 db. In subsequent accelerated life tests in the siren facility at North American Aviation, Columbus, Ohio, there was no significant damage to a four by four foot specimen of material after 105 hours at sound pressure levels between 165 and 170 db except for failure of the supporting wire screens at about 78 hours.This technIeal dooments-y repot has bow rvilwed and is approved for pub3lietion.

The performance of the anechoic room of the Parmly Sound Laboratory has been measured to determine the characteristics of rooms of small volume with wedge-covered walls. The inverse square law holds to within ±1 db to 6.5 feet from 60 to 24,000 c.p.s., although the theoretical cut-off frequency of the room is 115 c.p.s. At 100 c.p.s., the radiation resistance of a loudspeaker placed near the walls was found to be 18 percent above the free-field value, and the radiation reactance was approximately 5 percent above the free-field value. The differences disappeared two feet from the wall. The wall transmission has been measured as a function of frequency, and the wedges themselves are found to act similarly to a thin plate in optics, having a maximum transmission at 110 c.p.s.