Introduction to Acoustics

1072 Part H Engineering Acoustics
Part H 25
mined. The sound intensity method makes it possible to
measure the transmitted sound power directly. In contrast
one cannot measure the incident sound power in
the source room using sound intensity, since the method
gives the net sound intensity. If the intensity method
is used for determining the transmitted sound power it
is not necessary that the sound field in the receiving
room is diffuse, from which it follows that only one
reverberation room is necessary. Thus sound intensity
is suitable for field measurements of transmission loss.
There are international standards both for laboratory and
field measurements of transmission loss based on sound
intensity [25.69, 70].
Figure 25.25 shows the results of a round robin
investigation in which a single-leaf and a double-leaf
construction were tested by four different laboratories
using the conventional method and the intensity-based
method. Apart from the fact that only one reverberation
room is needed the main advantage of the intensity
method is that it makes it possible to evaluate the
transmission loss of individual parts of the partition.
However, to be reliable each sound power measurement
must obviously satisfy the condition expressed
by (25.29). There are other sources of error than phase
mismatch. If a significant part of the absorption in the receiving
room is due to the partition under test then the net
power is less than the transmitted power because a part of
the transmitted sound energy is absorbed or retransmitted
by the partition itself [25.71]. Under such conditions
one must increase the absorption of the receiving room;
otherwise the intensity method will overestimate the
transmission loss because the transmitted sound power
is underestimated. In contrast, the conventional method
measures the transmitted sound power irrespective of
the distribution of absorption in the receiving room.
Deviations between results determined using the
traditional method and the intensity method led sev-
References
25.1 A.D. Pierce: Acoustics: An Introduction to Its
Physical Principles and Applications, 2nd edn.
(Acoustical Society of America, New York 1989)
25.2 F.J. Fahy: Sound Intensity, 2nd edn. (E & FN Spon,
London 1995)
25.3 ISO: ISO 3745 Acoustics - Determination of sound
power levels of noise sources using sound pressure
- Precision methods for anechoic and hemianechoic
rooms (ISO, Geneva 2003)
25.4 J.-C. Pascal: Mesure de l’intensité active et réactive
dans differents champs acoustiques, Proc. Rec.
Devel. Acoust. Intens (CETIM, Senlis 1981) pp. 11–19
eral authors to reanalyze the traditional method in
the 1980s [25.72] and point out that the Waterhouse
correction [25.73], well established in sound
power determination using the reverberation room
method [25.74], had been overlooked in the standards
for conventional measurement of transmission loss. Recent
results imply that the Waterhouse correction should
be used not only for the receiving room but also for the
source room [25.75].
25.4.5 Other Applications
The fact that the sound intensity level is considerably
lower than the sound pressure level in a diffuse,
reverberant sound field has led to the idea of replacing
a measurement of the emission sound pressure
level generated by machinery at the operator’s position
by a measurement of the sound intensity level,
because the latter is less affected by diffuse background
noise [25.76]. This method, which involves measuring
three components of the intensity at a specified
position near the source, has recently been standardized
[25.77].
In principle, sound intensity may be used for measuring
sound absorption in situ. As in measurement of
transmission losses the incident sound power must be
deduced from a spatial average of the mean square pressure
in the room on the assumption that the sound field
is diffuse, and the absorbed sound power is measured by
integrating the normal component of the intensity over
a surface that encloses the specimen under test [25.2].
In practice, however, this is one of the least successful
applications of sound intensity, partly because of the assumption
of diffuse sound incidence and partly because
estimation errors in the absorbed power will be translated
to relatively large fractional errors in the resulting
absorption coefficients.
25.5 J.A. Mann III, J. Tichy, A.J. Romano: Instantaneous
and time-averaged energy transfer in acoustic
fields, J. Acoust. Soc. Am. 82, 17–30 (1987)
25.6 F. Jacobsen: Active and reactive, coherent and incoherent
sound fields, J. Sound Vib. 130, 493–507
(1989)
25.7 F. Jacobsen: A note on instantaneous and timeaveraged
active and reactive sound intensity, J.
Sound Vib. 147, 489–496 (1991)
25.8 P.J. Westervelt: Acoustical impedance in terms of
energy functions, J. Acoust. Soc. Am. 23, 347–348
(1951)