THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

366 14. Machinery Noise ControlEquation (14.7) is less accurate than Equations (14.4)–(14.6) for predictingperformance characteristics but it is sufficiently accurate for estimating purposes.These fan laws state mathematically that when two fans have similar design configurations,their performance curves are similar, and at the equivalent point of ratingon each performance curve, the efficiencies should be equal. In order to apply thefan laws, it is necessary to have the test data for one fan in the same design series.The applicability of the fan laws is restricted to cases where all linear dimensionof the larger or smaller fan are proportional to the fan for which there are test data.14.5 Electric Motors and TransformersElectric motors convert electrical power into mechanical power. Some of the fundamentalnoises occurring in electric motors are caused by rotational unbalance,rotor/stator interaction, and slot harmonics. The electromagnetic force betweenthe armature and the field magnet, or rotor and stator, gives rise to vibration. Noiseis also generated by the excitation of natural frequencies of the motor structure,air resonance chambers, and the movement of air itself. The intensity of the noiseis typically a function of rotational speed and motor type.As a means of estimating the noise output of a motor, the following expressioncan be used to obtain the total sound power level in the bands 500, 1000, 2000,and 4000 Hz:whereL w = 20 log hp + 15 log n + K m dB (14.8)hp = rated horsepower (1–300 hp)n = rated speed of motor, rpmK m = motor constant = 13 dBOther more involved techniques have been developed, which entailed differentmotor constants for each of the octave bands of interest (Magrab, 1975; Webb,1976).Transformers exist for the purpose of stepping up or stepping down voltages.Their changing magnetic field causes deformation of the transformer coil, occurringat the alternating current (AC) frequency and higher harmonics, especially atthe twice the AC frequency. This results in the characteristic frequency of 120 Hzin the United States and 100 Hz wherever 50-Hz AC current is used.14.6 Pumps and Plumbing SystemsPump noise arises from both hydraulic and mechanical sources, namely, cavitation,pressure fluctuations in the fluid, impact of mechanical parts, imbalance, resonance,misalignment, and so on. The hydraulic causes are, however, the predominant noisegenerators. Pumps will generate even more noise if they are not operated at rated