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Aviation History
1953
1953 - 0764.PDF
758 FLIGHT, 12 June 1953 AERONAUTICAL ACOUSTICS . . . combined effect was a high-frequency noise, greatest in a direction direcdy ahead of the aircraft. An engine widi an axial compressor gave a similar pattern at maximum engine speed, but there was much less pronounced high-frequency noise at lower engine speeds. Before considering possible forms of treatment for noise abatement, the speaker continued, it was pertinent to examine just how the noise decreased with increasing distance from the aircraft. The term "noise" had to be discarded as a physical characteristic, for noise was a subjective feature that could not be precisely denned. An appropriate analysis must refer instead to sounds formed of an aggregation of tones and different frequencies- Grouping the frequencies into consecutive bands, usually octaves, and taking in each octave the sound intensity (or pressure) level per octave in decibels above the conventional datum, enabled an objective pattern to be established. Combining the intensity level in decibels into a loudness level in phons led to a subjective presentation which could be appreciated. As an example, one could assume that at a distance of 100ft from an acoustic source the sound intensity in each of eight octaves was 100 decibels, i.e., the spectra was flat. This emission was known colloquially as "white" noise. The total intensity would be 109 decibels and the loudness level 125 phons. At 200ft the total intensity would have fallen to 103 decibels by application of the inverse square law of energy radiation, which for sound gave a drop of six decibels each time the distance from the source was doubled. At 400ft the total intensity would have fallen to 97 decibels, and at 1,000ft to 89 decibels, made up of 80 decibels in each octave. The corresponding loudness level would be 101 phons, so that in 900ft there had been a drop of 20 decibels in total intensity and 24 phons in loudness level. To these decays could be added the effects of the atmosphere through which the sound was travelling, assuming first that there was no absorp tion by the terrain, nor any absorption, reflection or screening from high objects. It was a feature of the transmission of sound that the higher the frequency the larger the effect of any imposed condition. The atmosphere affected the transmission of sound by absorption, by dissipation caused by turbulence, by refraction caused by wind, temperature and density gradients, and by the translational effects of wind. There were also other effects which could hardly be forecast. Mr. Hayhurst then considered in detail the effects of atmospheric absorption, wind, and the terrain, and then gave his personal views on the problem of noise abatement. The fitting of ground silencers to piston- engined aircraft would be extremely difficult, as equally effective treat ment of airscrews and exhausts would be necessary. Jet aircraft, he stated, offered greater scope dian piston-engined types, since the sources of noise were more compact. Investigations were taking place to find whether mobile detuners for exhausts and intakes were practicable; this type of device had already been used in the United States, but had been limited there to single types of aircraft, and the arrangement had not been really suitable for everyday use in a busy maintenance area. The use of obstacles offered wide scope, but there was the limitation that the obstacles screened only a certain sector around an aircraft. The greatest advantage then would be obtained by having the aircraft as close to the obstacle as possible, but one could not completely surround the aircraft because of the need to disperse the slipstream or the jet efflux. A good form of obstacle was provided by a running-up pen. It had been shown that reductions of up to 25 phons could be obtained in this way, a reduction which represented a real measure of noise abatement. There still remained attendant problems to be explored, for example, the manoeuvring of aircraft into and out of the pen, and the effect on the airflow into the intakes of jet engines might present serious difficulties, but as the advantage of a running-up pen was equally effective for all types of aircraft, further investigation was certainly warranted. Trees had also been considered as a means of screening noise. They would certainly help, provided that they could be planted in the right place and to the necessary thickness, that they retained their leaves throughout the year, and that they did not infringe the safety clearances required on approach and departure lanes for aircraft. The variation of noise level around an aircraft, already mentioned, could also be utilized. By so orientating the aircraft that the directions along which the lowest noise levels were found fell on the areas where noise might be disturbing, a measure of alleviation would have been achieved. At the same time, this method might be limited by having to face the aircraft either into wind or so that die jet efflux was directed away from particular parts of the maintenance area. These, die speaker BELGIAN PRINCE AT HAWKERS FOLLOWING the visit of the Duke of Edinburgh to Hawker Aircraft, Ltd., as described in Flight of May 29th, the company was again honoured on June 4th, this time by a visit from Prince Albert of Li^ge, brother of King Baudouin of Belgium. The Prince and his party arrived at Dunsfold during the morning and visited the control tower before inspecting the Cygnet, Tomtit, Han, Hurricane, and finally a Hunter F.2 (Armstrong Siddeley Sapphire). His Royal Highness sat in the Hunter's cockpit, vacating it to allow Neville Duke to take off. Meanwhile, he saw a display by the Cygnet, Tomtit, Hart and Hurricane. By this time Neville Duke had attained a level of about 40,000ft, and within a few seconds a terrific supersonic bang was heard. Later he made several runs and rolls above the airfield. Having congratulated the Hunter's pilot, Prince Albert and his party took lunch with the directors of the company. concluded, were the available methods. They had their limitations, but they did admit some possibility of alleviating noise. The general discussion on the ground-noise papers of Mr. Fleming and Mr. Hayhurst began with a question from Professor Richards con cerning a type of ground muffler fitting around both jet nozzle and intake to give a 30-decibel reduction; surely diis was impracticable in busy traffic conditions, and also on the grounds of expense. But we did understand more about where the noise came from, he continued, and so it should be possible to design effective ground silencers. Mr. Briscoe, of the M.C. A., stated that the annoyance caused by the noise had not been measured; this was especially important in connection with the civil helicopter which would have to serve centres of population. The noise level from an aircraft should not be louder than that due to a bus passing along a street, i.e., about 85 decibels. Mr. King (Metropolitan-Vickers) mentioned the wide frequency band (100 c.p.s. to 10,000 c.p.s.) encoun tered with jet engines, and suggested for reasons of economy in space that low-frequency and high-frequency absorbing systems might be combined in cascade. The subjective nature of noise was referred to by Mr. Rendall (R.A.E.), who said, also, that information on noise radiation was particularly wanted. Professor Cave-Brown-Cave (late of Southampton University), spoke of the effect of noise on the nervous systems of the victims: the best approach was to explain the reasons for the noise, to point out that it was inevitable, and to state that active efforts to reduce the noise were being made. Other suggestions made by speakers were for co-operation between engineers and aviation medicine experts, and for the elimination of noise of particular frequencies. Replying, Mr. Fleming agreed that the nuisance value of noise differed with people and circumstances, and could be measured only by using a large number of observations. It might not be very important, however, necessarily to give an exact figure to this nuisance. Particular designs of silencing ducts, incorporating splitter vanes, would certainly be efficient but might not prove economic. Mr. Hayhurst emphasized that the difficulty in constructing efficient test houses was different from the problem worrying the airport authorities. A smaller reduction in noise would be acceptable for airports, akhough, in view of the complaints received, the airport authorities would have to take definite steps towards noise abatement. The operators also should give thought to this now. DISCUSSION on DAY'S PAPERS After tea, a general discussion on points arising from the day's acoustic topics was held. Professor Richards, answering a speaker who suggested that low-frequency vibration would not only cause passenger discomfort but be damaging to electronic and other instruments, pointed out that this type of vibration had been at its strongest witii the piston engine and that the introduction of propeller turbines and turbojets should give a longer life to instruments and hydraulic pipes. In any case, the distribu tion of amplitude and frequency of the noise around a jet had been studied and critical equipment should be suitably located. Discussing helicopter noise, Dr. G. Hislop (Fairey Aviation Co.), emphasized that permission to operate helicopters to the centres of cities was important and he wondered if some of the figures of noise limits which had been mentioned (e.g., 85 db at 150ft from the rotor) were not too strict. During the B.E.A. night mail operations, nearby houses had been subjected to 90-100 db and no complaint had been received. The tolerable noise level differed widi frequency: Dr. Hislop mentioned that a Comet passenger had reported that the cabin appeared quiet, but that the noise had a deadening effect on the speech frequencies. Perhaps, on this count, it would be undesirable to relax the standards laid down at die high frequencies, as had been suggested. In an amusing contribution to the discussion, Mr. H. Caplan (British Aviation Insurance Co.) claimed tiiat the best units of measurement of noise were the number of complaints received per month. His company were, naturally, very interested in the noise problem (especially in "bangs"), but he regretted that he had doubts about certain complicated mathematical equations. Mr. Woodward-Nutt (Ministry of Supply) diought that the sonic bang story had been "oversold," that supersonic speeds low down would not be with us for a very long time and that flying discipline could deal with die problem. Mr. Warren disagreed, stating that sudden noise was always objectionable and the bangs might have serious results when nerves were tensed by some other cause. Prof. Richards confirmed that many Southampton people had been "put out" by bangs recendy heard there. GERMAN RESEARCH PLANS W EST GERMAN radio has been devoting much time to publicizing the Goettingen conference of the Society for Aeronautical Science. On May 28th a report was broadcast that Dr. Seebohm, the Federal Transport Minister of the Bonn Government, had stated that it was proposed to set up a research nucleus which would be concerned with training a young cadre of specialists. The various Laenders (provinces), he added, should set up suitable institutes as well as university chairs for aero nautics. Young engineers would be given three- to five-year courses in the first phase of the programme; the second phase would be concerned with reorganizing the experimental side. It was the special task, in diis phase, to keep alive the seven re search institutes which had never ceased to exist in law. Efforts should be made to retrieve lost equipment and the valuable aviation research libraries which had been taken abroad by the occupying powers.
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