Basic studies of a pneumatic percussive rock drill, operating without a drill steel in an anechoic chamber, showed that the noise distribution consisted of maximum intensities at each extremity of the machine and peak values within the vertical longitudinal plane of the drill, A study of the positions adopted by operators relative to their hand-held machines revealed three very common attitudes, from which the average position of an operator's ear was determined to be 30 in. behind the drill exhausts, at an angle of 20 degrees to the machine axis.
The basic frequency spectra of a selection of hatid-held paeumatic percussive drills were found to be essentially similar. The alterations occurring within the spectrum of the test drill through variation of air pressure or nature of rock were determined. When drilling, there was no apparent connection between the overall sound pressure level and any of the operating variables.
Two measuring positions, A and B, were selected for determining the individual contributions of seven major sources to the overall noise intensity. At Position A, the order of importance was exhaust processes, bit noise, mounting noise, percussion action, rifle bar impact, valve noise and pawl noise.
Various techniques were tested during abatement studies, including the attachment of four small exhaust mufflers, the use of rubber and felt jackets, and the substitution of spheroidal graphite cast iron as the material of drill fabrication. None of these approaches was outstandingly successful. However, exhaust hoses significantly reduced the exhaust noise without increase in backpressure, and nylon internal canponents resulted in considerable attenuation. By combining all muffling devices, a reduction in intensity from approximately 117 dB to 98.5 dB at the Operator Position was achieved, with a decrease in drilling rate of approximately 45 per cent.
Analogy investigations into the percussion process suggested that east noise is produced when the masses of the piston and drill steel are equal.