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Chairman's report on the workshop on infrasound
Journal of Sound and Vibration (1975), 43(2), 465466
CHAIRMAN’S
REPORT ON THE WORKSHOP ON INFRASOUND
The Chairman (Dr C. Ashley) opened the Workshop by covering briefly some aspects of infrasound. He defined it as air-pressure variation below frequencies of 40 Hz. He pointed out that at very low frequencies the threshold of hearing and the threshold of pain came very close together. Measurements on motor vehicles showed a high level of infrasound, of the order of 100-120 dB, in the frequency range considered. This high level of infrasotmd in vehicles came from two forms of excitation: firstly aerodynamic noise, and secondly road surface undulations. The noise excited by road-surface undulations showed a peak between 10-25 Hz, with resonance at body modes and also at engine-bounce frequency. At this frequency the engine acted rather like a piston in a chamber. Measurements on the vehicle showed good correlation between vibration velocity on panels and infrasound levels inside the vehicle. He felt, but could not substantiate, that the level of infrasound inside the vehicle had a very significant effect on the comfort and long-term well-being of the driver and occupants. Mr M. Austin (British Leyland) confirmed that their measurements had shown high levels of infrasound on certain models and these had been associated with customer complaints. They would continue to make strenuous efforts to reduce infrasound levels inside the vehicle. Mr R. Perry (I.S.V.R.) commented upon measurements they had been making of noise in cabs. He said that the higher-frequency noise from 40 Hz upwards was associated with ring modes, but the lower-frequency noise was associated with general distortion of the cab. He confirmed high levels of infrasound at 10 Hz in cabs, with levels reaching 110-l 15 dB. Dr Yeh (English Electric) raised a problem which had occurred within his company. Two gas turbines of nominally similar specification gave a very different noise output in infrasound terms. He quoted a level at 20 Hz of 27 dB(A) in one case and 37 dB(A) in the other case. For one gas tubine set a lOO-ft exhaust stack was used, whereas the other exhaust stack was 106 ft high. He was asked in the interest of science to increase the height of the 100 ft stack to 106 ft to see whether it made any difference. Mr Martins (Rolls-Royce) said that several gas turbine generating sets produced for the CEGB had high levels of infrasound associated with them. He quoted typical values of 90 dB. Mr J. W. Leverton (Westland Helicopters) referred to research work which had been done at Chelsea and which showed an increase in reaction time under infrasound conditions. The Chairman pointed out that the problem of infrasound was not restricted to motor vehicles and mobile generator sets. Problems also had arisen in large buildings with infrasound generated through ventilation and heating ducts. Infrasound also could be induced by lowspeed compressors running typically at 600 rev/min. He asked members of the workshop to give information on infrasound test facilities and proposed work. Mr H. H. Hubbard (NASA Langley Research Center) said that they had a large pistonphone facility. This was capable of generating up to 110 dB at 10 Hz. They had noticed when operating the system that buildings were particularly likely to be excited at their own natural resonance of 7 Hz. At Virginia there was also a ride simulator with vertical, lateral and roll facilities. They intended to study combined environmental effects, particularly in relation to buses and tractors. He also mentioned possible infrasound generation by floor vibration in aircraft. Mr B. K. Rao (University of Birmingham) suggested that 7 Hz was a particularly tricky frequency because it could affect the Alpha rhythm of the body system. Dr Yeh mentioned a test facility they had built at Leicester using a 2000 lbf vibrator and a 465
466
WORKSHOPREPORT
large-diameter polystyrene piston. They had been unable to obtain more than 100 dB of infrasound at 10 Hz and had found the noise to attenuate very rapidly with distance, becoming 80 dB only 12 in from the piston. Dr Judith Lang (Austria) confirmed problems with infrasound in buildings. They had had complaints in a particular underground office and found the measured noise level to be only 50-60 dB(A). Measurement of the infrasound showed 80-90 dB in the critical range. Mr Hubbard said that a blow-down wind tunnel facility had been built in connection with the Apollo programme. No significant work had been done upon the project because it was felt that infrasound would not affect the capability of the men upon the mission. He did confirm that buildings could be excited in an infrasound mode by vibrations transmitted through the ground over distances up to one mile. Mr S. 0. Benjegard (Sweden) quoted examples of traffic induced infrasound. At one house they found 90 dB, whereas on the “A’‘-weighted scale the internal noise level was only 40-45 dB(A). The house concerned was built upon clay. Dr P. J. Dickinson (I.S.V.R.) referred to the French work of 1962. He said the reports that the investigator had been killed by infrasound at 7 Hz were erroneous and 37 Hz was the most likely frequency. He thought that the research in France had been directed towards production of a death machine. He mentioned sleeping problems and dysentery which had been associated with infrasound in the 30-40 Hz region. When asked how the investigator had been killed by 37 Hz he mentioned conflicting reports suggesting that his stomach had been gelatinized and other reports that he had died of natural causes due to cancer of the brain. He also gave a warning that investigations at high levels of infrasound could cause physical damage. C. ASHLEY
CHAIRMAN’S
REPORT ON THE WORKSHOP ON INFRASOUND
The Chairman (Dr C. Ashley) opened the Workshop by covering briefly some aspects of infrasound. He defined it as air-pressure variation below frequencies of 40 Hz. He pointed out that at very low frequencies the threshold of hearing and the threshold of pain came very close together. Measurements on motor vehicles showed a high level of infrasound, of the order of 100-120 dB, in the frequency range considered. This high level of infrasotmd in vehicles came from two forms of excitation: firstly aerodynamic noise, and secondly road surface undulations. The noise excited by road-surface undulations showed a peak between 10-25 Hz, with resonance at body modes and also at engine-bounce frequency. At this frequency the engine acted rather like a piston in a chamber. Measurements on the vehicle showed good correlation between vibration velocity on panels and infrasound levels inside the vehicle. He felt, but could not substantiate, that the level of infrasound inside the vehicle had a very significant effect on the comfort and long-term well-being of the driver and occupants. Mr M. Austin (British Leyland) confirmed that their measurements had shown high levels of infrasound on certain models and these had been associated with customer complaints. They would continue to make strenuous efforts to reduce infrasound levels inside the vehicle. Mr R. Perry (I.S.V.R.) commented upon measurements they had been making of noise in cabs. He said that the higher-frequency noise from 40 Hz upwards was associated with ring modes, but the lower-frequency noise was associated with general distortion of the cab. He confirmed high levels of infrasound at 10 Hz in cabs, with levels reaching 110-l 15 dB. Dr Yeh (English Electric) raised a problem which had occurred within his company. Two gas turbines of nominally similar specification gave a very different noise output in infrasound terms. He quoted a level at 20 Hz of 27 dB(A) in one case and 37 dB(A) in the other case. For one gas tubine set a lOO-ft exhaust stack was used, whereas the other exhaust stack was 106 ft high. He was asked in the interest of science to increase the height of the 100 ft stack to 106 ft to see whether it made any difference. Mr Martins (Rolls-Royce) said that several gas turbine generating sets produced for the CEGB had high levels of infrasound associated with them. He quoted typical values of 90 dB. Mr J. W. Leverton (Westland Helicopters) referred to research work which had been done at Chelsea and which showed an increase in reaction time under infrasound conditions. The Chairman pointed out that the problem of infrasound was not restricted to motor vehicles and mobile generator sets. Problems also had arisen in large buildings with infrasound generated through ventilation and heating ducts. Infrasound also could be induced by lowspeed compressors running typically at 600 rev/min. He asked members of the workshop to give information on infrasound test facilities and proposed work. Mr H. H. Hubbard (NASA Langley Research Center) said that they had a large pistonphone facility. This was capable of generating up to 110 dB at 10 Hz. They had noticed when operating the system that buildings were particularly likely to be excited at their own natural resonance of 7 Hz. At Virginia there was also a ride simulator with vertical, lateral and roll facilities. They intended to study combined environmental effects, particularly in relation to buses and tractors. He also mentioned possible infrasound generation by floor vibration in aircraft. Mr B. K. Rao (University of Birmingham) suggested that 7 Hz was a particularly tricky frequency because it could affect the Alpha rhythm of the body system. Dr Yeh mentioned a test facility they had built at Leicester using a 2000 lbf vibrator and a 465
466
WORKSHOPREPORT
large-diameter polystyrene piston. They had been unable to obtain more than 100 dB of infrasound at 10 Hz and had found the noise to attenuate very rapidly with distance, becoming 80 dB only 12 in from the piston. Dr Judith Lang (Austria) confirmed problems with infrasound in buildings. They had had complaints in a particular underground office and found the measured noise level to be only 50-60 dB(A). Measurement of the infrasound showed 80-90 dB in the critical range. Mr Hubbard said that a blow-down wind tunnel facility had been built in connection with the Apollo programme. No significant work had been done upon the project because it was felt that infrasound would not affect the capability of the men upon the mission. He did confirm that buildings could be excited in an infrasound mode by vibrations transmitted through the ground over distances up to one mile. Mr S. 0. Benjegard (Sweden) quoted examples of traffic induced infrasound. At one house they found 90 dB, whereas on the “A’‘-weighted scale the internal noise level was only 40-45 dB(A). The house concerned was built upon clay. Dr P. J. Dickinson (I.S.V.R.) referred to the French work of 1962. He said the reports that the investigator had been killed by infrasound at 7 Hz were erroneous and 37 Hz was the most likely frequency. He thought that the research in France had been directed towards production of a death machine. He mentioned sleeping problems and dysentery which had been associated with infrasound in the 30-40 Hz region. When asked how the investigator had been killed by 37 Hz he mentioned conflicting reports suggesting that his stomach had been gelatinized and other reports that he had died of natural causes due to cancer of the brain. He also gave a warning that investigations at high levels of infrasound could cause physical damage. C. ASHLEY