BRUEL & KJAER 4204 - Sound Insulation Measurements

CHAPTER 2
Operation
13
To obtain a logarithmic scale, the following reference values are introduced: p
0
= 20 µPa,
W
0
=10
–12
W and A
0
= 1 m
2
and eq. (2.3) becomes:
(2.4)
and setting ρc to 400 Nsm
*
:
(2.5)
Eq. (2.3) to (2.5) are only valid if the sound pressure squared is averaged over the entire room.
However, this averaging is normally only done over the central part of the room and to
compensate for that, the so-called Waterhouse correction is added. Introducing the Waterhouse
correction, eq. (2.5) becomes:
(2.6)
where:
λ = the wavelength at the centre frequency of the band
S = the surface area of the room
V = the volume of the room
2.4 Sound Insulation Measurements
The reference sound source may be used for power-based measurements of sound insulation.
To perform this, the alternative method, which is described in detail in the Technical Review
No.3, 1980, can be used. An updated measurement arrangement is shown in Fig.2.3.
* ρc = 414 Nsm
–3
for an ambient pressure of 1013 hPa and an ambient temperature of 20°C. This results in
a constant of 6.2 instead of 6 in eq. (2.5).
10log
10
A
A
0
------
10log
10
W
R
W
0
--------
10log
10
–
p
2
p
0
2
--------
10log
10
+
4ρcW
0
A
0
p
0
2
-----------------
=
10log
10
A
A
0
------
L
w
L
p
– 6+=
10log
10
A
A
0
------
L
w
L
p
– 10log
10
1
λS
8V
-------
+


– 6+=