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(various environments same rotating speed and backpressure) Because of the complicated nature and interactions of the BPF and especially the flow noise, determining successive rankings may require some very accurate mock-up geometry, or may only be possible with a prototype or a serial product cabinet. In different cabinets, the rank order is subject to change see Table 1, which compares weighted noise power results for Cabinet A (a prototype as in Figure 1), Cabinet B (a mock-up system for fan ranking), and a pressure box according the ISO 10320 standard. The rank order of the fans is not only dependent on the backpressure. Airflow noise spectrum in the mentioned prototype cabinet. BPF is easily calculated from the number of blades and rotating speed.įigure 4. The BPF peaks are similar in Figures 2 and 3. The shapes of the airflow spectra are similar in Figures 2 and 3. The shapes of the spectra in Figures 2, 3 and 4 give evidence that the blade passing frequency and flow noise can be treated and attenuated separately. In Figure 4, a spectrum was measured by using relatively silent airflow through the structure. Single fan spectrum (one B 5 with the same backpressure and rotating speed as in Figure 2).Īirflow also makes noise in a cabinet without a fan. Base station spectrum (a third generation cabinet prototype with six B 5-type fans as the main noise source).įigure 3. In any case, if the BPF is totally eliminated from the overall noise level, the total value decreases only by approximately 3 dB.įigure 2. Blade passing frequencies (BPF) and harmonics are sharp peaks, flow noise is approximately 10 dB lower. The spectrum of the cabinet product is not the same shape as that of the fan alone see Figures 2 and 3. An axial fan’s inlet side is especially sensitive, but in some cases a radial fan will produce less noise with a fingerguard (a thin wire web to protect people from being injured by rotating blades).įan noise has two main components: blade passing frequency (and its harmonics) and flow noise. Placing the fan too close to any obstacle can increase the noise by many dBs. Choosing a silent yet powerful fan is essential to total noise when the cabinet layout is fixed. The fan noise is proportional to the cabinet-induced total noise. The curves have been measured with the same airflow and backpressure. Fans A5 and B5 each have 5 blades and fan C7 has 7 blades. Figure 1 illustrates the noise levels of 127-mm axial fans from three manufacturers. Axial fans with equal interface dimensions and the same electronic power produce very different noise levels. When the pressure load crosses the capacity of the fan, noise increases significantly, but airflow decreases. In axial fans, the correct operating range is an important factor. Noise powers of three manufacturers’ (A, B and C), 127-mm axial fans with same backpressure and airflow. Thus, in compact and aggressive designs with high backpressure, mixed flow or radial fans can be less noisy.įigure 1. Axial fans generate a greater volume of flow, but less pressure. Ordinarily, axial fans are used at the first stage to assist free convection, or the cabinet layout is designed from the beginning for forced convection. In this paper, noise measurements of air-cooled cabinets are analyzed, and possibilities to reduce the noise are discussed.
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Fans are commonly used to enhance the cooling of electronics by improving heat transfer due to higher air velocities. The spaces available are limited, and the power and electronics packaging density are increasing. Modern environmental requirements demand low noise levels for telecommunication cabinet products, especially when located in offices or urban areas.