Google Patents AU2013216620A1 - Article comprising light absorbent composition to mask visual haze and related methods AU2013216620A1 - Article comprising light absorbent composition to mask visual haze and related methods The pressure drop required to drive a unit flow through the material can be related to the viscous losses of the propagating sound waves inside the porous absorber and denoted as flow resistivity. Sound absorbers are used to dissipate sound energy and to minimize its reflection. Sound energy is dissipated by simultaneous actions of viscous and thermal mechanisms. A simple schematic of surface-wave interactions is shown in figure 2.įigure 2: surface-sound interaction- absorption (left), reflection (middle) and diffusing (right) The absorbed sound may either be transmitted or dissipated. When a considerable amount of the reflected sound is spatially and temporally scattered, this status is called a diffuse reflection, and the surface involved is often termed a diffuser. The reflected sound may be almost completely redirected by large flat surfaces or scattered by a diffused surface. Sound waves striking an arbitrary surface are either reflected, transmitted or absorbed the amount of energy going into reflection, transmission or absorption depends on acoustic properties of the surface. active control works well for low frequencies hence, the combination of two methods may be utilized for broadband noise reduction.įigure 1: Noise Control Mechanisms Sound Absorption Since no input power is required in such methods, Passive noise control is often cheaper than active control, however the performance is limited to mid and high frequencies. Passive noise control refers to those methods that aim to suppress the sound by modifying the environment close to the source. The latter method is called active structural acoustic control (ASAC). second method is to use flexible and vibro-elastic materials to radiate a sound field interfering with the disturbances and minimize the overall intensity. First by utilizing the actuators as an acoustic source to produce completely out of phase signals to eliminate the disturbances. There are two methods for active control. Modifying and canceling sound field by electro-acoustical approaches is called active noise control. Industrial machinery, air/surface transportation and construction activities are assumed to be main contributors in noise production or so called "noise pollution". The importance of noise issue could be well understood by looking at regulations that have been passed by governments to restrict noise production in society. As an example in control valves, the vibration caused by flow instability occasionally defects the feedback to the control system and resulting in extreme oscillations. Moreover, high level noise and vibration lead to structural failures as well as reduction in life span in many industrial equipments. Noise can interfere with sleep and speech, and cause discomfort and other non-auditory effects. Loss of hearing is only one of the effects of continuous exposure to excessive noise levels. There are many cases and applications that reducing noise level is of great importance. 5.1 Wave equation in rigid porous absorbents.5 Acoustic modeling of Porous Absorbents.4 Physical Characteristic Properties of Porous Absorbers.3.2 Non-Porous Absorbers ( Absorbing Resonators ).
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