A broadband noise prediction scheme for low-noise centrifugal blowers

In this paper we outline a semi-empirical procedure for predicting the broadband sound power spectrum radiated by any member of a family of geometrically similar, centrifugal blowers. The prediction scheme is based on the use of a normalized sound power spectrum that is obtained by measuring the sound power radiated by one member of the blower family. In the present instance, a lownoise blower design has been identified by conducting a large scale parametric study of blower noise as a function of geometrical design parameters and operating point; the resulting optimum design served to define the geometrical parameters of the blower family considered here. The prediction procedure allows calculation of either 1/3-octave band spectral levels or overall A-weighted levels. In addition it is possible to calculate "noise surfaces," i.e., plots of A-weighted sound power level versus design parameters and operating condition; in this way it is easy to identify optimally quiet blower designs that satisfy specified pumping requirements. It has been found, for example, that large diameter impellers of relatively small width operating at low speeds tend to result in the lowest noise level at a given pumping requirement. INTRODUCTION "Sound laws" can be used to predict the overallsound power level radiated by centrifugal blowers when the power level of a geometrically similar reference blower is known [1]. In this paper we present a prediction scheme for blower noise spectra that is firmly based on an existing parametric representation of blower noise spectra. In particular, it will be shown that a parameterized spectral representation proposed by Maling [2] may be combined with measured reference data to yield noise predictions for a complete family of geometrically similar blowers. In addition, Maling's approach has been extended here to allow for impeller width variations in the scaling procedure. A key result of the present work is the definition of a noise surface that allows one to identify the low-noise blower designs that will meet a given pumping requirement. DEVELOPMENT OF A NOISE PREDICTION SCHEME The scheme presented here allows the prediotion of the 1/3-octave band sound power levels in the frequency range 100 Hz to 10 kHz for various blowers having 78 Emission: Noise Sources different geometries and operating speeds, but all fulfilling a specified pumping requirement (i.e., a particular combination of pressure and flowrate). As a result, the overall A-weighted sound power level can then be calculated and plotted versus the corresponding point of rating, 1/>, and the aspect ratio, a (impeller width over diameter), to produce a noise surface. The lowest noise level can then be identified along with the corresponding blower geometry and the required operating speed. The scheme is based on measurements of the aerodynamic and acoustic performance of a low-noise reference blower that is treated as a representative member of a family of geometrically similar blowers. The aerodynamic data consist of a discrete set of operating points at which acoustical measurements were conducted using the reference blower. For each operating point, nondimensionalized generalized flow and pressure coefficients, I/>g and lj/g, were calculated according to equations (1) and (2) [2]: Q I/> g 1C D3N a' (1)