Document mpMR26qGwLMKVmzp0nzEnvR44

640 JEROME R. COX, JR. fork through a certain area it is only necessary to multiply the value of tensity by the number of square feet in the area. For instance, if the tuniji produces a sound intensity of 1 ^watt per square foot at the surface of thetical sphere surrounding the source and the area of this sphere is 10.^ feet, the total power radiated by the tuning fork will be 10 ^watts. Since the surface area of a sphere is 4 times the square of the radius total power radiated by any simple source in free space will be: Sound power = 4 irr2I Note that the intensity, I, measured r feet from the source must vary i-if as the square of this distance in order that the sound power radiated by thej remain unchanged. This is the well-known inverse-square law of acoustic^ this law it follows that the intensity must drop to one quarter for each doubj1 the distance. In terms of intensity level each doubling of distance from a.scj equivalent to 6 db drop in the level. This can be checked by noting that ,do the value on the intensity scale by four (Table 2b) is equivalent to a 6 db, the intensity level. C. SOUND IN A ROOM WITH NONABSORBING WALLS . .n - In a room with hard, sound-reflecting walls the radiation of soundiig different from that shown in Figure 10. Whenever any obstacle is placed' path of a sound wave, a reflected wave is produced. The intensity of this, wave will-depend.upon.the composition of-the obstacle,-Hit-is"hard"aHflTr<|, the intensity of the reflected wave will be practically equal to the incidentj When a sound source is completely enclosed in a room with hard^Ir,.-,, sound will be reflected back and forth from wall to wall many, many time$ sound that undergoes these multiple reflections is said to reverberatesiliP'ft.m room. Su6h a reverberant condition is shown in Figure 11 where, as b . thickness of the rings indicates the intensity of the sound. Near the soureij direct sound predominates, -but at points closer to the walls the multiple,rcfljsS generate a rather random sound field. This situation is completely differciur1 a sound source in an empty space (Figure 10). In the reverberant parf-A sound field the sound waves come from all directions and the averagb) tensity.Js^jo^unjiojm..-The-part...of-the.-field.-tbat4s-close-torthe`^'rif!M and resembles the field in Figure 10 is called the direct or free field! T,A'''"'''r? the field that is characterized by sound travelling in all directions,' the J..TjS3p> Tandom part, is called the reverberant sound field. A 'rt In the reverberant field the reflected sound is of primarvimportaac jb sound-radiated-directly-from-the source is practically negligible. The -i ejgT between sound intensity and sound pressure shown in equation 2 no' ImcivJ since it is derived for free-field conditions. The equivalency between aim'll sure level and intensity level (Table 2) will also be invalid because in rove||f sound fields the intensity level is 6 db less than the sound pressure leve, ,!'S NOISE AND THE CONSERVATION OF HEARING 641 llJrA graphical representation of the radiation of sound from a simple source in a || reverberant room. i,*- . D. THE ABSORPTION OF SOUND loft, porous material, with a vast number of tiny but interconnected air likely to absorb a large share of the sound that falls on it. The sound S|onverted into heat, but at normal intensities the resulting temperature Significant. ^pateria-l-can-absorb-more sound-than-falls upon-it. The pleasant picture mg material sucking sound from the air is totally inaccurate. A mateffiwill absorb sound as well as does an open window is optimum. Special Hnd arrangements are required for small local improvements over this umfabsorption and even these "superabsorbers" are no better than an open ijwhen an average absorption over a large area is considered. ||lj6arly foolish, therefore, to enclose a noisy marine with an absorbing Mvin an effort to reduce the sound inside the enclosure. The best sound *$.!'!,v` - ii fipn already surrounds the machine: empty space1 This is the situation that |n-;the tuning fork example shown in Figure 10. The-sound source is com- |8rrounded by empty space and, therefore, no reflected sound waves are K E. THE ABSORPTION COEFFICIENT t of the sound that falls on any material is absorbed andpart is reflected. lf|jfethfi-snnnrl- is reflected, the material is nonabsorbing,and4s-'likely-to-have S-ffimpervious surface like that of metal, bricks, concrete, or plaster. If very ilfethe sound is reflected, the material is absorbing and is likely to have a ffii'Ous surface like that of carpeting, glass wool, or snow. The fraction of ent sound intensity that is absorbed by a surface is called the absorption