丸のこの騒音に関する研究 : 空気力学的に発生する空転騒音について

The aerodynamic noise produced by idling circular saw blades was investigated experimentally. All blades used in this series of measurements were un-slotted, carbide tipped blades. They were 305 mm in diameter, and 1 mm, 1.25 mm and 1.65mm, respectively, in thickness. Carbide tips were 1 mm thick, and had 10 and 15 degrees in hook and clearance angles respectively. The number of teeth chosen was 4, 12, 24, 48 and 80. The tip speed of the blades, which was 34.4 m/sec, 45.5 m/sec, 68.6 m/sec and 89.4 m/sec, was altered by using different pulleys on the moter (Table 1).
The saws were tested in an anechoinc chamber whose external dimensions were 7.7 m^2 by 2 m high and which provided inverse square acoustic propagation for frequencies above 125 Hz. A microphone was placed 30 degrees to and 1 m from the end of the saw drive shaft as shown in Figure 1. Its height was 80 cm above the floor. The linear scale overall sound pressure level and 3 percent-octave band spectra of the noise which was generated by each blade tested was measured by a precision sound level meter, frequency analyzer and level recorder.
The results, based on the data presented, are as follows ;
1) The sound power increases as the 4.8th power of the blade tip speed (Fig. 2).
2) The sound power generated increases as the 0.5th and 1.4th power of the number of teeth and the blade thickness, respectively (Fig. 5 and 6).
3) Strouhal numbers based on the frequency of maximum sound pressure level, the carbide block thickness and the tip speed were between 0.12 and 0.25 for the range of Reynolds numbers based on blade radius tested. The Strouhal number of each blade was constant for the range of Reynolds numbers tested and increased with decreasing carbide block thickness (Fig. 4).
It is conjectured from these results that tip speed is the dominant factor in the noise production, that the noise sources of idling circular saws are some combination of monopoles and dipoles and that the dipole sound generation process is the major source of noise produced by saw blades.