ISO 3744:2010 pdf free download – Acoustics – – – Determination of sound power levels and sound energy levels of noise sources using sound pressure一 Engineering methods for an essentially free field over a reflecting plane.
6.6 Operation of source during test
The sound power or sound energy emitted by a source, whether stationary or moving, can be affected by the load applied, the running speed, and the conditions under which it is operating. The source shall be tested, wherever possible, under conditions that are reproducible and representative of the noisiest operation in typical usage. The specifications given in a noise test code, if any exists, shall be followed, but in the absence of a noise test code one or more of the following modes of operation shall be selected for the test(s):
a) source under specified load and conditions;
b) source under full load [if different from a)];
c) source under no load (idling);
d) source at maximum operating speed under defined conditions;
e) source operating under conditions corresponding to maximum sound generation representative of normal use;
f) source with simulated loading, under defined conditions;
g) source undergoing a characteristic work cycle under defined conditions.
The source shall be stabilized in the desired operating condition, with any power source or transmission system running at a stable temperature, prior to the start of measurements for sound power level or sound energy level determination. The load, speed and operating conditions shall either be held constant during the test, or varied through a defined cycle in a controlled manner.
If the sound power or sound energy emission depends on secondary operating parameters, e.g. the type of material being processed or the design of cutting tool, those parameters shall be selected, as far as is practicable, that give the smallest variations and that are typical of normal use. If simulated loading conditions are used, they shall be chosen such that the sound power levels or sound energy levels of the source under test are representative of normal use.
7 Reference box and measurement surface
7.1 Reference box
In order to facilitate the selection of the shape and dimensions of the measurement surface, the reference box shall first be delineated. The reference box is a hypothetical surface defined by the smallest right parallelepiped that just encloses the source under test. When defining the dimensions of the reference box, elements protruding from the source which are known not to be significant radiators of sound may be disregarded.
The locations of the reference box, the measurement surface and the microphone positions for measurements, are defined with respect to a co-ordinate system with origin 0 in the ground plane, shown in Figure 1. The point 0 is the middle point of a box consisting of the reference box and its images in the adjoining reflecting plane(s). The horizontal axes x and v of the co-ordinate system also lie in the ground plane, parallel to the length and width of the reference box. The characteristic source dimension, cIa, used to determine the dimensions of the measurement surface, is shown in Figure 1 for reference boxes on one, two and three reflecting planes.
In general, the type of measurement surface may be selected based on the shape and size of the noise source, with a goal of having the distances from each microphone to the noise source be roughly equivalent. In addition, measured power levels tend to be the lowest and most accurate when most sound energy passes through the measurement surface at normal incidence. Thus, for a relatively small source, a hemisphere may be preferable; for a long box-like source, a parallelepiped may be preferable; and for a tall, but not wide or long, source, a cylinder may be the preferred measurement surface. However, because different measurement surfaces have different requirements for the minimum distance between the microphones and the source, other considerations, such as the amount of background or reverberant noise in the test room and size of source relative to the usable measurement space of the test environment, may enter into the choice of an appropriate measurement surface.
NOTE For purposes where the sound power or sound energy levels are compared with limit values, it may be possible to reduce the uncertainty due to measurement reproducibility, if the associated noise test code specifies the size and shape of the measurement surface.