Strength and Elasticity
Tests comparing 26 perforated and un-perforated metal samples ranging from .020″ to 3/4 ” proved that perforated material has different strength and stiffness depending on the direction of loading. Because they account for more than half of the perforating industry’s production, tests were performed on round hole 60° staggered patterns. Designers can use the information provided in the study to determine what perforated material thickness will provide the strength and stiffness equal to that of un-perforated material. Performed by O’Donnell and Associates.
16 samples with hole diameters ranging from .040″ to .187″, tested at 9 frequency levels, show that a Shielding Effectiveness of 40 dB provides 99.000% attenuation of the electromagnetic (EMI/RFI) radiation while a Shielding Effectiveness of 92 dB, the highest Shielding Effectiveness found in the tests, provides 99.997% attenuation. The 60° staggered pattern samples used in the tests were of a variety of metal types. The tests proved that there are many perforated patterns that designers can choose from to meet their design requirements. The report notes that if the structure is not tightly sealed, leakage is greatest along contact surfaces between two parts. Performed by Dash, Straus & Goodhue, Inc.
Liquid and Gas Pressure Loss
Tests performed on various thin gauge perforated metal samples measured pressure loss at a series of velocities and angles. The samples were inserted onto a uniform, non-swirling, perpendicular, velocity liquid- and air-flow streams. Pressure loss for ambient liquid and air-flow was then measured at a series of velocities and angles and reported as inches of mercury and water column loss for each flow. The data collected can be used to determine pressure loss for any liquid or gas density by using the ratio of the anticipated liquid or gas density to the tabulated density as a multiplier of the noted loss. Performed by Boyle Engineering Laboratories (Liquid) and by Penn State University (Gas).
There are two principal acoustical applications for perforated metals – as a decorative facing for acoustical material and as part of a tuned resonant absorber. When used as a decorative facing for material that is designed to absorb, reflect, or scatter sound, perforated material needs to be so “transparent” that the sound waves pass through it – without being diminished or reflected. Because the objective of many acoustical applications is to remove or reduce sounds that occur only in a narrow range of frequencies, in applications where sound frequencies are below 1000 Hz, the amount of material required to absorb the sound may be impractical. In cases such as these, it is possible to design a sound absorption system that is “tuned’ to those targeted frequencies in which perforated metal plays a critically active role. By employing such a system, the designer can reduce the thickness of the absorbing layer, save space and lower cost.
In a resonant sound absorber, the air motion in and out of the holes in the perforated metal sheet oscillates in response to an incident sound wave. The preferred frequency of oscillation is determined by the mass of the air in the perforations and the springiness of the trapped air layer. At that frequency, the air moves violently in and out of the holes and, also, back and forth in the sound absorptive layer, where the acoustic energy is converted by friction into heat and is, thereby, removed from the acoustical scene. It is the interaction between the thickness of the perforated sheet, the size and number of the holes in it, and the depth of the trapped air layer, that determine the target frequency and thereby, the thickness of the absorbing layer required to remove the sound.