In reality, however, sound radiates out spherically from its source. This means that if a wave were to propagate in a straight line from its source, it reflects in the way pictured in Figure 4.15. The law of reflection states that the angle of incidence of a wave is equal to the angle of reflection. Hard surfaces reflect sound more than soft ones, which are more absorbent. Sound that is not absorbed by objects is instead reflected from, diffracted around, or refracted into the object. The material of walls and ceilings, the number and material of seats, the number of persons in an audience, and all solid objects have to be taken into consideration acoustically in sound setups for live performance spaces. Even hard, solid surfaces absorb some of the sound energy, although most of it is reflected back. Thick, porous materials can absorb and attenuate the sound even further, and they’re often used in architectural treatments to modify and control the acoustics of a room. The amount of attenuation depends in part on the air temperature and relative humidity. (See Section 4.2.1.6.) The attenuation of sound in the air is due to the air molecules themselves absorbing and converting some of the energy to heat. A general mathematical formulation for the way sound attenuates as it moves through the air is captured in the inverse square law, which shows that sound decreases in intensity in proportion to the square of the distance from the source. The diminishing of sound intensity is called attenuation. Sound absorption is the conversion of sound’s energy into heat, thereby diminishing the intensity of the sound. In enclosed spaces, absorption plays an important role. In the real world, there are any number of things that can get in the way of sound, changing its direction, amplitude, and frequency components. Figure 4.13 Sound radiation from a loudspeaker, viewed from top 4.1.7.1 Absorption, Reflection, Refraction, and Diffraction You can see that there is some sound behind the loudspeaker, resulting from reflection and diffraction. However, because the loudspeaker partially blocks the sound from going behind itself, the sound is lower in amplitude there. The area in front of the loudspeaker might be considered a free field. In this figure, sound is radiating out from a loudspeaker, with the colors indicating highest to lowest intensity sound in the order red, orange, yellow, green, and blue. A free field is partially illustrated in Figure 4.18. Sound in a free field can be pictured as radiating out from a point source, diminishing in intensity as it gets farther from the source. A free field is an idealization of real world conditions that facilitates our analysis of how sound behaves. An environment with no physical influences to absorb, reflect, diffract, refract, reverberate, resonate, or diffuse sound is called a free field. Sometimes it’s convenient to simplify our understanding of sound by considering how it behaves when there is nothing in the environment to impede it.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |