CIRCULAR-SAW AERODYNAMIC NOISE

The aerodynamically induced noise of an idling circular saw turning at high speed is investigated both experimentally and analytically. The origin of the noise is shown to be fluctuating lift forces acting on the teeth. The sources at the teeth radiate incoherently as baffled dipoles and account for the generated noise. The analysis predicts that the generated sound power will scale on tooth area, and this prediction is confirmed for gullet width to tooth thickness ratios greater than about two, thus establishing tooth area as the second most important scaling parameter following that of saw rotational speed. The fluctuating lift force on a tooth is shown experimentally to increase with the square of the saw rotational speed, while the generated sound power is shown to increase with saw rotational speed to a power slightly greater than five. The analysis shows that the source radiation impedance increases with the frequency of the radiated sound, which in turn increases essentially linearly with saw rotational speed. The fifth power dependence of radiated sound power on saw speed in thus explained as being due to the source location on the teeth at the perimeter of the blade. An instrumented tooth on an artificial saw blade was used to measure the aerodynamically induced local pressure fluctuations. These pressure fluctuations were used to estimate the expected sound at 1·0 m from the saw on the axis of rotation by making use of the analysis. In the analysis it is assumed that the sources at the teeth radiate incoherently to contribute to the observed sound. Fairly good agreement is demonstrated between measurement and prediction. © 1992.