Supersonic Jet Sound Research At Riccarton

Press, Volume XCII, Issue 27739, 17 August 1955, Page 12

Supersonic Jet Sound Research At Riccarton

Jets which simulate speeds between 1500 and 2000 miles an hour are operating in Christchurch this month. They can discharge the equivalent of 300 cubic feet of free air every minute at a pressure of 1601 b a square inch from half-inch nozzles, and their shrill shriek; unlike any other known sound, is sufficient to cause pain or permanent damage to unprotected ears at close range. It is this noise, rather than the power of the jets which is the subject of the research—New Zealand’s contribution to the projects now being undertaken by the Commonwealth Advisory Aeronautical Research Council. TTie Government made a grant for this scheme in 1949 and, although preliminary research was started immediately, the development of the.present project, which is the only one of its kind in the world, did not begin in earnest until 18 months ago. At that time, Mr D. C. Stevenson joined the staff of Canterbury University College as senior lecturer in mechanical engineering, specialising in aeronautics. Originally, he had been working on high-speed flight at the Royal Aircraft Establishment, Farnborough, and then transferred to the Australian Aeronautical Research Laboratories at Melbourne. Now he is supervising this work in association with Mr J. G. M. Williams, who is one of the first candidates in New Zealand for the new degree of master of engineering. Fundamental as this research is, it will have practical applications almost immediately. Once the nature of the noise has been measured and analysed, there will be tests with new types of silencer. The small-scale apparatus produces sound analagous to that 300 feet behind a Comet aircraft on ground running. This noise can be harmful to humans. It has been established that jet noise can cause mental fatigue in the tail planes of aircraft. The investigations should also throw new light on jet performance.

Research Equipment A jet turbine is not used in the tests. An air compressor some distance away feeds the pipeline which rises up to 25ft metal tower to the aperture where different jets are attached. This structure has been erected on the site of the new engineering school at Riccarton. It is almost ideal for the purpose. There is clear land for almost half a mile without reflected interference from buildings or trees, and in that distance the noise dissipates quickly, so that residents in the neighbourhood will

hear little more than a small highpitched whine of much less volume than a train whistle.

The four brass jets themselves are worth more than £5OO, although each is a quarter the size of a petrol funnel. Externally, they are all the same size with a half-inch nozzle, but the internal contours have all been varied, corrected to 1-10,000 th of an.inch, and the design of each involved approximately 10,000 calculations. They were made in the workshops of the School of Engineering and the Canterbury College Industrial Development Department. # It is proposed to measure the sound intensity of the jets from all angles. A special high-fidelity microphone, which would be ruined by one drop of rain or other moisture, will be used. For tests in the “near field” it will be attached to a swinging boom within three feet of the jet, and in the “far field” it will be mounted on a 25ft aluminium staff, fitted with levels.

A large area round the tower has been pegged at 15 degree intervals for a -radial distance of 105 ft, dovering an arc of 165 degrees. The staff is mounted ■ on these pre-set pegs. Sound tests can thus be made over a range from immediately in front, to almost immediately behind the jet. There is spe f^ a l interest in preliminary results at the 45-degree angle, where a peak m high-frequency sound exists. Measurements of Sound measurement of sound intensity will be in decibels. Ordinary, conversation rates at about 60 decibels, and ion U } in^. 100’ When sound rises to }2O decibles, it becomes unpleasant, at 140 decibles it can cause pain and permanent damage. A ram-jet at close quarters can record 160 decibles—enough to cause death. The test rig at Riccarton can produce 140 dqcibles in the near field, but the research engineers wear specially-designed ear m SP ers ? nd work in short spells. The microphone'will transmit noise to a sound-level meter at the base of the tower, and it will be then fed into an electronic octave-band analyser to find the sound frequency range. A jet engine, such as the Avon, which powers Canberra, Valiant, and Hunter aircraft, develops about 10.0001 b and it gives a thrust-out about 100 horsepower in noise. This research may help to reduce noise trouble. It will almost certainly provide new silencer designs. With the reduction in noise will come greater protection against human ear damage and structural fatigue failure from this cause.