Diffraction of sound definition2/18/2024 ![]() It can be a parameter to determine the effectiveness of sound-absorbing materials. Noise reduction coefficient (NRC) is a single number that also ranges from 0 to 1 and is used to represent the percentage of sound energy absorbed by the surface. So we usually use a single number rate called NRC to simplify the term of α. As α varies with frequency, it won’t be straightforward to be understood by common people. It can be measured using the room method according to American standard ASTM C 423 or international standard ISO 354. The sound absorption coefficient of material ranges from 0 (absorptive) to 1 (reflective), and it varies with frequency. The sound absorption coefficient (α) is the ratio of the absorbed sound energy to the incident sound energy. Now we will talk about absorption and the essential parameter associated with it, sound absorption coefficient. It is used to measure the amount of sound scattered away from the specular reflection direction.ĭid you notice that when we discuss reflection above, we say that some of the energy will be reflected? Why only some of it? Because some of the other energy will be absorbed by the material. The scattering coefficient describes the degree of scattering due to the roughness or irregularity of a surface. This value ranges from 0 to 1, where 0 means a fully specular reflecting surface and 1 means a fully scattering surface. It is based on their structure.Īccording to ISO 17497-1:2004, the scattering coefficient is the value calculated by one minus the ratio of the specular reflected acoustic energy to the total reflected acoustic energy. Some materials will have different scattered sound energy. In diffusion, there is a lot of scattered sound energy since the reflection will be spread into all directions. The sound energy that is reflected at angles other than the incident angle is called as scattered energy. Those terms are synonymous with reflection and diffusion, respectively. ![]() Please don’t be confused when you encounter specular reflection and diffuse reflection in other articles or books. We call this phenomenon as acoustic diffusion. ![]() In results, the reflected sound is scattered at many angles. Thus, the reflection angle will vary according to which surface the incident sound hits. In this case, the incident sound hits many different surfaces each of them has a distinct normal line. When a sound wave strikes a rough or irregular surface, it actually follows the physics of reflection as we have discussed above-the sound wave will bounce back at the same angle as its incident wave. If you observe how the sound hits a rough surface, you may discover that there are many reflected sounds and each of them goes in a different direction. ![]() However, in the real world, we may find that all reflections do not always seem to behave that way. For reflection in a flat surface, the reflection angle equals to the incidence angle. The same applies to the reflection angle. The angle between the direction of the incident sound wave and the normal line is then what we define as the incidence angle. Let’s see the illustration below to help you imagine how the sound bounces after it hits a surface.ĭraw a line perpendicular to the surface that starts from the point on which the sound wave is incident. If you ever learn physics of sound in middle school, you may still remember that the reflected sound will have the same angle as the incident sound. When a sound wave strikes a surface, we can confidently say that some of the energy will bounce back in a different direction. When a sound wave propagates in a closed space and strikes one of the surfaces, it may undergo one or more of the following behaviors: (1) reflection, (2) absorption, (3) diffusion, (4) diffraction, and (5) resonance. It depends on how you control those behaviors so that the sound waves can act the way you want. ![]() Some behaviors can improve or degrade sound quality inside a room. To create high sound quality for music venues or speech listening rooms, we need to learn the science behind sound behaviors. ![]()
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