Waves in fluid and solid media 63
In this equation |ITn| is the absolute value of the normal sound intensity and I 0 is the
reference value for the intensity, equal to 10-12 watt/m^2. When the normal intensity in a
measurement situation is negative, the level is expressed as (–) XX dB. In the same way
we define the sound power level
0W 10 lg (dB),W
L
W
⎡⎤
=⋅⎢⎥
⎣⎦
(3.32)
where the reference value W 0 is 10-12 watt. In a plane wave field we may use the simple
relationship between sound pressure and particle velocity to write
200d,p
WS
ρc=∫ ⋅
v (3.33)
where the wavy symbol above p indicates an RMS-value. The expression is also valid for
an ideal spherical wave field and is the basic equation used in several ISO standards for
determination of sound power in a free field. The term signifies a sound field without
reflections, e.g. in anechoic rooms, in ducts having an anechoic termination etc.
(References on the subject are given at the end of the chapter.)
Using intensity, one is not dependent on having a free field to determine the sound
power. The common procedure is first to define a closed measuring surface around the
source. One then divides this surface into smaller subareas Si, thereby measuring the
normal sound intensity by placing an intensity probe normal to each of these smaller
surfaces. In this way one determines an average intensity value, both in space and time,
for each surface and finally sums up the result using the expression
(^) n.
N
Ti i
i
WIS=∑ ⋅ (3.34)
N is the number of subareas used and it should again be noted that the space average
ITin is a signed scalar value.3.4 The generation of sound and sources of sound
Up to now we have described acoustic waves and wave motion without touching on how
waves are being generated. In one way or another we shall have to feed energy into the
system to start a wave motion, mathematically expressed; the right hand side of the wave
Equation (3.5) cannot be equal to zero everywhere.
Sound generation is normally linked to processes involving mechanical energy,
thereby resulting in a transformation of a part of this energy into acoustical energy.
Concerning building acoustics the most common processes occurring are: 1) Buildings
elements or whole constructions are excited into vibrations due to impacts, by friction, by
sound pressure etc. Due to the fact that they are in contact with the surrounding medium
(air) they will transfer this motion to the medium, and a sound field is generated due to
this volume displacement. 2) Liquid flow or gas flow results in pressure and velocity
variations in the medium and/or one has turbulent flows interacting with solid surfaces.