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Aviation History
1960
1960 - 0843.PDF
FLIGHT, 17 June 1960 .851 What is a PNdb ? PNdb130 _ '"THE decision to use "perceived noise decibels" as a measure -*• of aircraft noise is an encouraging advance in the Ministry of Aviation's jet noise policy. The limitations of the misleading common-or-garden decibel, which were recognized elsewhere more than 18 months ago Flight, November 7, 1958), have at last been officially acknowledged. In the Commons debate on jet noise on June 3, Mr Rippon, Parliamentary Secretary, said: — "I can inform the House that it has been decided to adopt 'perceivednoise' expressed in decibels, or PNdbs, as the most suitable unit at present available for the relative assessment of annoyance caused byaircraft of different types. "We accordingly introduced on June 1 a revised limitation of 110perceived noise decibels measured in the main built-up areas under the take-off paths at London Airport to which operators of jet aircraft willbe required to adhere by day. This limitation is broadly equivalent to a sound pressure level of 97 decibels. "The hon Member for Hayes and Harlington said that he wouldprefer these measurements to be in noys. That is the same measurement as the perceived noise decibel. The only difference is mathematical.Noy is arithmetical. The perceived noise decibel is logarithmic, as is the decibel measurement to which w: are now accustomed. In future,anyone who wishes to do so will be able to translate the measurements of perceived noise decibels into noys, if he is so minded." The perceived noise decibel, PNdb, was invented in 1958 by the Port of New York Authority's acoustical consultants, Bolt Beranek and Newman.* Its scientific basis was explained in Flight for August 14, 1959 (page 10, "Noise in the Jet Age"). One reason why the db is so misleading is that the loudness of, say, a lOOdb sound at 4,000c/s is nearly twice that of a lOOdb sound at lOOc/s. But the db was shown by the tests to be even more misleading when noise, as distinct from loudness, was con- sidered. Briefly to recapitulate, the PNdb was the outcome of an *"Studies of Noise Characteristics Of The Comet 4 Jet Airliner AndOf Large Conventional Propeller-driven Airliners," prepared by Bolt Bzranek & Newman Inc for the Port of New York Authority, October1958, Sections D-3 to D-16. (Also similar volume for the Boeing! 01-120.) 120 110 extremely thorough series of tests by Bolt Beranek and Newman on people's reactions to the noisiness, as distinct from the loudness, of sounds from aircraft. These tests were described in detail in the report. The PNdb is an empirically derived measure of noisiness which takes into account the effect on the human ear of sounds in different frequency bands, since it is these that influence noisiness. The relationship of the PNdb to the db (which, being a measure of sound pressure only, can just be read off a gauge) depends on the type of aircraft, its distance from the listener, power settings, and so on. A typical PNdb/db relation- ioo ship for various jet and piston aircraft was illustrated in a graph on page 640 of Flight for May 6, and some tabulated figures were given in our issue of November 7, 1958. The PNdb is not, with respect to Mr Rippon, the same measure- ment as the noy (another term, sometimes rendered as N, invented by Bolt Beranek and Newman). But these units are directly related, as may be seen from the adjoining scale. The virtue of N is that, being arithmetical and not logarithmic, it is more easily understood by non-technical people. When noise is doubled, Ns are doubled and so on. For example, it is not readily apparent that 110 PNdb is roughly twice the noise of 100 PNdb. This is apparent from the equivalent figures for N, which are 125N and 64N. (Incidentally the Ministry's 110 PNdb limit is 2 PNdb less strict than New York's.) In continuing to urge the case for N rather than PNdb, Flight has in mind the following passage from the foreword of British Standards, BS.3045, 1958: — "For some purposes, especially to convey to the non-specialized the relative loudness of different sounds, it is preferable to express loudness values in the units of an alternative scale. This scale, in which the reading is expressed in [N], is so constructed that the apparent ratio of the [PNdb] of two sounds, as judged by normal observers, is given directly by the ratio of the [N] values of the sounds." 90 80 . 70 60 50 40- ["Straight and Level," page 831; New York and the FAA, page 847] 30. BREVITIES Sir Patrick Dollan has retired from the board of BEA after nearly14 years' service. He will continue to hold office as chairman of the Scottish Advisory Council. It is reported that Qantas and CPAL are discussing the possibility ofpooling services between Australia and Canada. A further meeting, in Vancouver, is likely to take place later this month. The prices of the Convair 880 and 600 are due to be increased byamounts ranging from 4 to 4i per cent as from July 1. Convair say that this has been made necessary by "rising labour and material costs." Delta's initial order for ten Convair 880s, all of which are to bedelivered in 1960, has been supplemented by a further order for three aircraft to be delivered in August and September 1961. The price quoted by Protea Airways as the cost of each of the fiveDo 28s that it has ordered is £16,000. The company's applications to operate a network of domestic services in the Union of South Africaare being strongly opposed by SAA and by eight other operators. The Spanish authorities have apologized to Continental Air Transport,the Southend independent which had two Vikings detained (one at Barcelona and the other at Palma) last week because of difficulties overdiplomatic clearance. Spain admits that there had been an error. International Aeradio, who now operate in 30 countries, announcethree new contracts: (1) for a new Flight Information Centre for the Aden Government; (2) three air traffic control officers for duty at Nicosia;and (3) two ATC officers for Accra, and an operations officer for duty with the Ghana Government Civil Aviation Department. "Skycoach" is the name to be given by BOAC, British United Airwaysand their British colonial partners EAA and SAA to the new third-class cabotage services to Africa. They will be operated by Viscounts as wellas by Britannias, and the frequency will be fortnightly to East Africa and to the Central African Federation. TAA's perfect safety record was broken on July 11 when a FokkerFriendship, with 25 passengers, including 16 schoolchildren, and a crew of four, disappeared after circling Mackay Airport for 45 minutesunable to land because of fog. Bodies and debris were later recovered from the sea. It is reported that attempts are being made to salvage thewreckage. TAA has not killed a passenger (nor is it believed to have seriously injured one) since its formation 14 years ago. New Guinea, which has been largely opened up by the aeroplane, has recently seen the introduction by Papuan Air Transport, Patair, of a Piaggio P. 166. It is based at Jackson Field, Port Moresby, and accord- ing to the Australian DCA, Donald Anderson, it is proving "a good and ruggsd performer." He expects to see more in service there A BOAC Comet 4 over-ran the end of the runway on landing atMontevideo on June 7. No one was injured and no damage was found. July 18 is the date tentatively arranged for civil aviation delegationsfrom the USA and the USSR to meet in New York to discuss a bi-lateral air agreement. This would, if successful, be followed by direct NewYork - Moscow air services. On June 9 the Civil Aviation (Licensing) Bill, 1960, received the RoyalAssent. It becomes an Act. Now awaited are the Regulations that are to be issued by the Minister of Aviation, as provided for in the Act, inrespect of such matters as the way in which applications for licences shall be mads; how appeals shall be dealt with; the assessment of fees; theinformation that should be required of applicants, etc. The An-24 feederliner (sketch: page 847) is intended, according toY. Loginov, head of Aeroflot, to replace piston aircraft on domestic routes. It will carry 32 passengers in the standard version and 40 in a high-density configuration at a cruising speed of up to 326 m.p.h. Range is said to be 900-1,240 miles, and take-off distance 500m. Mr Loginovalso referred to the Tu-124 which is said to resemble closely the Tu-104 but with double-flow Soloviev turbojets and a take-off (presumablydistance-to-unstick) of about 2,500ft.
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