Norsonic nor118, Instruction Manual

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58 Chapter 11
About sound power measurements
59 Norsonic Nor118
User Guide
About sound power
measurements
S
     from sound pres-
sure levels using the Nor. e method is described
in    –   
     –   and
requires measurements of the -weighted sound pressure
level at four or more positions located on a hypothetical
measurement surface of an area  which envelopes the
source.
To facilitate the location of the microphone positions on
the measurement surface a hypothetical reference box shall
be defined. When defining the dimensions of this box, ele-
ments protruding from the source which are not significant
radiators of sound energy may be disregarded.
e measurement surface on which the microphone
positions lie envelopes the source as well as the reference
box.
e location of the source under test, the measurement
surface and the microphone positions are defined by a
coordinate system with the horizontal axes x and y in
the ground plane parallel to the length and width of the
reference box. e characteristic dimension d
0
is shown
in the Fig. on the next page.
One of the following two shapes shall be used for the
measurement surface:
a
a rectangular parallelepiped whose sides are parallel to
those of the reference box; in this case the measurement
distance d is the distance between the measurement
surface and the reference box.
b
a hemispherical surface or partial hemispherical surface
of radius r ;
For sources usually mounted and/or to be measured in
rooms or spaces under unfavourable acoustical condi-
tions (e.g. many reflecting objects and high levels of
background noise), the selection of a small measurement
distance is appropriate and usually dictates the selection of
a parallelepiped measurement surface. For sources usually
mounted and/or to be measured in large open areas under
satisfactory acoustical conditions, a large measurement
distance is usually selected and in this case the hemispheri-
cal measurement surface is preferred.
For measurements on a series of similar sources (e.g.
machines of the same type or a given family of equip-
ment), the use of the same shape of measurement surface
is required.
A test report shall always be made and the construction
of the reference box, the size and shape of the measurement
surface, as well as the measurement distance d or the radius
of the hemisphere r , shall be described in the test report.