Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 109

1 For whole space: I = 1/4π(50)2 = 1/104 π (W/m2), then

 Since we may assume r = 50 m is in the far acoustic field, Lp ≅ LI = 75 dB as well (we have also assumed ρc ≅ 400 rayls).

 For half space: I = 1/2π(50)2 = 2/104 π (W/m2), then

and L_p ≅ L_I = 78 dB also.

 It is important to note that the sound power radiated by a source can be significantly affected by its environment. For example, if a simple constant‐volume velocity source (whose strength Q will be unaffected by the environment) is placed on a floor, its sound power will be doubled (and its sound power level increased by 3 dB). If it is placed at a floor–wall intersection, its sound power will be increased by four times (6 dB); and if it is placed in a room comer, its power is increased by eight times (9 dB). See Table 3.2. Many simple sources of sound (ideal sources, monopoles, and real small machine sources) produce more sound power when put near reflecting surfaces, provided their surface velocity remains constant. For example, if a monopole is placed touching a hard plane, an image source of equal strength may be assumed.

Table 3.2 Simple source near reflecting surfaces^a.

Intensity	Source	Condition	Number of Images		Power	D	DI
I		Free field	None		W	1	0 dB
4 I		Reflecting plane	1		2W	4	6 dB
16 I		Wall‐floor intersection	3		4W	16	12 dB
64 I		Room corner	7		8W	64	18 dB

^a Q and DI are defined in Eqs. (3.53), (3.58), and (3.60).