A common method for increasing acoustic absorption in applications varying from concert halls to exhaust mufflers is by the use of perforates. Perforates typically consist of perforated sheet metal backed by a layer of bulk absorptive material and/or a cavity volume. Whether used as just a protective sheet over a layer of bulk absorbing material or as a tuned resonant absorber, perforates offer distinct advantages over porous or fibrous materials.
Applications for perforates include appliance or kitchen surfaces due to clean-ability; engine or burner enclosures for temperature resistance; and silencers, mufflers, or fan cowls by virtue of durability. Perforated panels can also be tuned to provide maximum acoustic performance within a specified frequency band. For example, the enclosure for a power transformer is designed for acoustic performance at a frequency of 120 Hz.
The percentage of open area on the perforate surface determines the acoustic behavior of the surface. For perforates with large open area (>40%), particularly if backed with a bulk absorber, the surface behaves as a bulk absorber. The controlling mechanism for this system is flow resistance through the surface. For perforates with smaller open area (<40%), the surface displays reactive behavior, and each opening acts as a resonator. The controlling parameters for this system are the size of the fluid plug in each opening and the backing volume. These parameters determine the resonance frequency of the resonator and thereby the frequency range of effectiveness for the surface.
Developing hardware configurations using perforates to increase acoustic absorption typically begins by building analytical models of the perforate layers. The basic goal for an analytical model is to predict the normal incidence absorption coefficient versus frequency. Inputs to the model include: hole geometry, hole spacing, and sheet thickness (for the perforated sheet); complex propagation constant and thickness (for the bulk absorber layer); volume depth (for the cavity volume); and, of course, the fluid properties such as for air. An example of a model output for two perforate configurations is shown in the following figure.
Analytical models often deviate somewhat from real-world conditions. For example, the hole geometry is represented as perfect in the model whereas the process for actually punching a hole through sheet metal is not perfect. Consequently, verification measurements are needed to fine-tune and qualify the design of the perforated panel before committing to full-scale construction.
The most accurate and efficient experimental setup involves impedance tubes. Samples are constructed of the most promising configurations as predicted by the model. Tests are then conducted to measure normal incidence absorption coefficient versus frequency and to study the sensitivity of design parameters on performance. The ease of testing in an impedance tube offers quick, inexpensive iterations to the best design. Below is shown an impedance tube test sample for a perforated panel with absorptive layer and air volume backing.
Scantek, Inc. offers for sale or rent several impedance tubes from BSWA. The impedance tubes vary in diameter for different frequency ranges.
Scantek also offers consulting and testing services whereby we construct analytical models of perforate absorber layers per the customer’s application. We then build prototype samples and test the samples in our impedance tube facility. Measurement results are compared to model predictions. Design parameters are selected to achieve desired frequency response and maximum acoustic absorption.