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A note on the relative importance of discrete frequency and broad-band noise generating mechanisms in axial fans
J. Sound Vib. (1969) 9 (2), 192-196
A NOTE
ON
FREQUENCY
THE
RELATIVE IMPORTANCE
AND
BROAD-BAND
OF DISCRETE
NOISE GENERATING
M E C H A N I S M S IN A X I A L F A N S P. E. D O A K AND P. G. VAIDYA
Institute of Sound and Vibration Research, University of Southampton, Southampton. England (Received 7 August 1968) Investigations on a model axial compressor have shown that the discrete frequency noise of a single rotor row can be reduced to a level insignificantrelative to the broad-band, vortex-shedding, noise level from the rotor blades. This can be accomplished by effectively removing all disturbances from the flow into the rotor row (wakes from upstream supports, etc.) and similarly, by arranging to have no downstream obstacles on which the rotor wakes might exert a fluctuating force. The level of the residual broad-band, vortex-shedding noise from the model compressor is significantlybelow the average level predicted by standard empirical formulas. 1. INTRODUCTION Mechanisms of noise generation in axial flow fans have been classified by Sharland [1]. Broad-band noise is caused by random fluctuations (turbulence, for example) in the intake air flow, and also, more or less independently of these, by random fluctuations in lift on the fan blades associated with turbulence in the wakes of the blades (" vortex-shedding" noise). Experiments [1, 2, 3] have shown that, generally speaking, there are two types of discrete frequency noise. The first is the familiar rotational noise first investigated by Gutin [4]. In this type of mechanism the radiated sound can be associated with the (assumed) steady forces exerted by the rotating blades on the fluid. Recent work has shown that the assumption of steady forces is neither necessary theoretically nor particularly applicable in practice to fans, such as helicopter rotors, of low solidity [5, 6]. The second type of discrete frequency noise can be called "interaction noise". Here, it is necessary to distinguish two types of interaction: wake interaction and potential field interaction, respectively. A discussion of wake interaction has been given by Moravec [3]. In this type of interaction the wake from an upstream object (such as a support strut, stator vane or rotor blade) impinges on a downstream blade which consequently experiences a periodic fluctuating lift, and this, in turn, produces discrete frequency noise radiation. Here, of course, only the mean velocity distribution in the wake is important. Turbulent fluctuations in the wake will cause some broad-band noise to be produced. In the second type of interaction, investigated by Fincher [2], the upstream object (e.g. stator vane) is so close to the downstream blade that there is a local periodic interference between the potential fields of the two objects. This interference produces a greater fluctuating lift than that due to the wake alone. From Fincher's measurements [2], it appears that, for representative fans (including aircraft engine compressors), the potential field interaction dominates when the separation between the trailing edge of the upstream object and the leading edge of the downstream object is less than about 0.2 of a representative chord length, while 192
Plate 1. Photograph of the new axial fan rig, showing the traversing microphone. The rotor is in the straight cylindrical section, the internal geometry of which is the same as shown in Figure 1 for the old rig, except that the front-bearing spider has been entirely removed. The air exit annulus and supports for the entire outer housing can be seen at the bottom of the rig.
(facing p. 193).
NOISE GENERATING MECHANISMS IN AXIAL FANS
193
wake interaction predominates when this separation is greater than about 0.3 of the representative chord length. These remarks, of course, apply only to typically streamlined aerofoil sections. Theoretical considerations of discrete frequency noise generation, by any of the mechanisms mentioned above, show that the Gutin-type rotational noise is very small for fan rotors having many blades. On the other hand, experimental work [1, 7] has indicated that observed levels of discrete frequency noise can be much higher than that to be expected from Gutin's theory. In the case of the original noise measurements on the Southampton University model axial compressor the contributions to overall noise levels by broad-band mechanisms and discrete frequency mechanisms (as classified above) were approximately equal [1]. It was thought that the only important discrete frequency mechanism in the fan rig was that of the wakes from the four support struts of the nose cone spider, upstream of the rotor. Calculations [8] showed that the discrete frequency noise to be expected from this wake interaction was of the order of magnitude of that observed. To confirm this experimentally, and to see if, in effect," all" discrete frequency noise could be eliminated, it was decided to reconstruct the model axial compressor rig, for a single rotor row, in such a way as to have no upstream obstructions (and consequently no wakes) and to have negligible downstream supports. Results from the new rig could then be compared with the original results. 2. EXPERIMENTAL PROCEDURE AND RESULTS 2.1. APPARATUS The experimental work was carried out in two stages on two different rigs. A diagrammatic sketch of the first one is shown in Figure 1, which is reproduced from reference 1. A photograph of the second rig (and the experimental set-up), is shown in Plate 1. A principal objective in the design and construction of the second rig was to provide entry and outflow conditions for the compressor which were, in effect, ideally smooth, with no 15 |
I
Stators
Ist S t a g e
IProvision forl
16
Rotors
12 a d d i t i o n a l I | stages J I Blade tip 1 clearance I ......
Tip dlo B i n .
h J I I
Provision
Hub
dia
4in.
bearlng spider
41 °
'67 °
Bifurcated exit d u c t belt -- 0 . 6 0
i n.'--PI
a.,'~= =. 1.60in,
t.60in.
Figure 1. An illustrative diagram of the axial fan geometry, a front bearing spider with four support struts being present as in the original rig. In the work reported here, both inlet and outlet guide vanes were removed; the only obstructions were the support struts. 14
194
P . E . DOAK AND P. O. VAIDYA
appreciable cross-flow obstructions either upstream or downstream. This requirement necessitated cantilevered construction. In order to minimize the effects at high rotor speeds of small errors in balancing, the rig was designed with a vertical axis. The total length of the shaft is 2 ft 6 in. and it is carried in three main bearings supported in a central pillar. The pillar is mounted on a heavy cylindrical block, which also carries the drive motor. The delivery air passage is also unobstructed by cross-flow supports except at the outlet where the maximum air-velocity is only of the order of 20 ft/sec. This is made possible by diffusing the flow in an annulus of progressively increasing mean diameter. The outer wall of the duct is, in fact, in the form of an inverted horn which surrounds the shaft and bearing structure, and which is supported only at the bottom rim. The gradual diffusion also eliminates any aerodynamic noise which might have been generated at sharp bends or collector boxes. Care was taken to maintain similarity in the other design aspects of both rigs. Thus both the new and old rigs have identical rotor blading, diameters of duct, positioning of the rotors with respect to the intake bells, etc. Care was taken to reduce the mechanical noise as much as possible. However, it was still necessary to "calibrate o u t " the mcchanical noise. The spectrum of the mechanical noise was obtained by running the fan when the blades were taken out. This showed that it would not interfere significantly with the discrete frequency noise measurements. However, it had a small effect on the measured overall noise (in which, as will be seen, the broad-band noise was predominant). Therefore, all overall measurements described below were repeated with the blades taken off and the appropriate decibel corrections determined in this way were used to calculate net overall blade noise levels, exclusive of mechanical noise contributions. 2.2
MEASUREMENTS ON THE ORIGINAL RIG
(i) Net overall sound pressure levels were measured, as explained above, with a Brim and Kj~er microphone and analyser, of the type BK 2107, at various angular positions with a spacing of 15 ° in between them. This was carried out at eight different speeds, which were checked by means of a stroboscope. (ii) The same microphone, coupled to a narrow band analyser, a Muirhead-Pametrada type D 489, of 6 ~ bandwidth, was used to obtain the sound pressure levels at the Made passage frequency and its multiples. This also was carried out at various angular positions at 15 ° intervals and repeated for the same speeds as in (i). For this part at least three sweeps were made, at each frequency and the mcan dB value was taken. All these measurements were made with a ½ in. Briiel and Kj~er condenser microphone mounted on a boom, traversing a circle of radius 3 ft, the centre of which coincided with the centre of the bellmouth. 2 . 3 . MEASUREMENTS ON THE NEW RIG
Exactly the same sequence of measurements was carried out on the second rig. Care was taken to keep the conditions of the experiments as closely identical as possible. Both the experiments were carried out in the semi-anechoic chamber of the Westland Acoustics Laboratory of the Southampton University Institute of Sound and Vibration Research. The shaft rotational speeds chosen were the same. The distance of the microphone was again 3 ft from the opening. The same analyser was used as in part (i) and in both cases the microphone was regularly calibrated with a piston-phone. For the discrete frequency measurements of part (ii), however, the levels were much reduced and hence a Hewlett-Packard analyser of type 302A, which has a bandwidth of 7 Hz, was used. A computer program was devised and used to compute the sound power levels from the sound pressure levels. It was applied, in both cases, to compute net overall, first harmonic and second harmonic levels. The results are shown in Figure 2.
NOISE GENERATING MECHANISMS IN AXIAL FANS l
I
I
I
I
Overall
I00
I
195
1
(both c a s e s ) ~ .'° o x
x
to
o~
o 9(3 ~:
_.
o
/ /
J
/
~ - . .,or 2nd harmonic
J-
o//
/ Fundomental~
8(3 "-o,,., ~''~ / . dl
~_
, ~
II
xl
/
//
x~
6C
I
I
I
x 2ndharmon~//
I
I
I
I
I
;0( )0 7000 8000 9000 I0,000 II,oo012,000 1~,000 Ray/rain
Figure 2. Sound power levels measured for the axial fan with (old rig) and without (cantilevered rig) upstream support struts. The results show that over the indicated speed range, discrete frequency levels were reduced by about 15 dB when the support struts were removed, and no longer contribute at all significantly to overall noise. , Cantilevered rig; - - -, old rig.
3. DISCUSSION AND CONCLUSIONS A comparison of the sound power levels shows that while the overall sound power levels are about the same for both the rigs, the levels of the first two harmonics of the discrete frequency noise have been very significantly reduced in the new rig, the reduction with respect to the old rig levels being of the order of 15 dB. These reductions are not significantly dependent on the rotor speed over the operating range studied. However, a much greater reduction would be required for the discrete frequency levels to drop to the levels predicted by Gutin's theory. One of the possible explanations which could be offered is based on considerations of aerodynamic asymmetry [5]. An asymmetry caused in the incoming flow due to the geometry of the inlet, say, would give rise to a lift distribution on each rotor blade that is not constant to an observer moving with the blade. This situation can result in more discrete frequency noise generation than predicted by Gutin's formula. Low-frequency variations in the incoming turbulence would have a similar effect. Possibly a more important conclusion in practice, however, is the clear indication that the fan in the new rig has no significant discrete frequency aerodynamic noise component. It can be noted here that the broad-band noise levels remain at the levels previously measured and predicted by Sharland [1]. This demonstrates that it is technically possible to design and construct an axial fan which produces, principally, only broad-band, vortex-shedding noise, at a relatively low level. To make such a fan even more quiet, the vortex shedding mechanism would have to be silenced, possibly by removing the rotor blade wakes by blowing from the trailing edges, or by boundary layer and wake control by suction through the blade surfaces. The research emphasis on fan noise for axial fans of sophisticated aerodynamic design can thus be shifted to the reduction of broad-band vortex-shedding noise.
196
P.E. DOAKAND P. G. VAIDYA
Due to the absence o f significant discrete frequency noise and a relatively low level of broad-band noise, the experimental fan is seen to be a quieter fan. For example, the overall noise levels are about 10 dB lower than those indicated by the Peistrup and Wesler correlation (see, for example, reference 9). It has thus been shown that it is possible to design quieter fans in practice by ensuring clean, uniform intake flows. One might also conclude from this fact that the noise levels, both discrete frequency and broad-band, of many practical, commercial fans are of the order of 10 dB higher than they would be if intake turbulence were removed, as well as steady perturbations of the mean flow by upstream support struts and the like. ACKNOWLEDGMENTS The authors wish to express their thanks for the considerable assistance given in the experimental work, particularly in the design of the cantilevered rig, by Mr. I. J. Sharland and Dr. E. J. Walker. REFERENCES l. I.J. SHARLAND1964 J. Sound Vib. 1 (3), 302. Sources of noise in axial flow fans. 2. H.M. FINCrmR 1966 J. Sound Vib. 3 (1), 100. Fan noise--the effects of a single upstream stator. 3. Z. MORAWC 1964 Proceedings of the Eleventh International Congress of Applied Mechanics-Munich (Germany) 1964 (Edited by H. Gortler, Springer-Verlag, Berlin-Heidelberg-New York). Der Einfluss der aerodynamischen Parameter auf den Sirenenl~irm einer axialen Schaufelstufe. 4. L. GOWN 1947 Tech. Memo. natn. advis. Comm. Aeronaut., Wash. No. 1105 (Translated from the original Russian). On the sound field of a rotating propeller. 5. S. E. WRIGHT1968 University of Southampton LS. V.R. Technical Report No. 5. Sound radiation by a lifting rotor generated by asymmetric disc loading. 6. M.V. LowsoN and J. B. OLLERrmAD1968ProceedingsAFOSR-UTIAS Symposiumonaerodynamic noise (Toronto)--University of Toronto, Institute for Aerospace Studies. Theoretical studies of helicopter rotor noise. 7. N. LE S. FILLEUL1966 J. Sound Vib. 3 (2), 147. An investigation of axial flow fan noise. 8. I. J. SrIARLAND1964 Unpublished communication. 9. W. RIZK and D. F. SEYMOUR1965 Proc. lnst. mech. Engrs. 179 (1). Investigations into the failure of gas circulators and circuit components at Hinkley point nuclear power station.
A NOTE
ON
FREQUENCY
THE
RELATIVE IMPORTANCE
AND
BROAD-BAND
OF DISCRETE
NOISE GENERATING
M E C H A N I S M S IN A X I A L F A N S P. E. D O A K AND P. G. VAIDYA
Institute of Sound and Vibration Research, University of Southampton, Southampton. England (Received 7 August 1968) Investigations on a model axial compressor have shown that the discrete frequency noise of a single rotor row can be reduced to a level insignificantrelative to the broad-band, vortex-shedding, noise level from the rotor blades. This can be accomplished by effectively removing all disturbances from the flow into the rotor row (wakes from upstream supports, etc.) and similarly, by arranging to have no downstream obstacles on which the rotor wakes might exert a fluctuating force. The level of the residual broad-band, vortex-shedding noise from the model compressor is significantlybelow the average level predicted by standard empirical formulas. 1. INTRODUCTION Mechanisms of noise generation in axial flow fans have been classified by Sharland [1]. Broad-band noise is caused by random fluctuations (turbulence, for example) in the intake air flow, and also, more or less independently of these, by random fluctuations in lift on the fan blades associated with turbulence in the wakes of the blades (" vortex-shedding" noise). Experiments [1, 2, 3] have shown that, generally speaking, there are two types of discrete frequency noise. The first is the familiar rotational noise first investigated by Gutin [4]. In this type of mechanism the radiated sound can be associated with the (assumed) steady forces exerted by the rotating blades on the fluid. Recent work has shown that the assumption of steady forces is neither necessary theoretically nor particularly applicable in practice to fans, such as helicopter rotors, of low solidity [5, 6]. The second type of discrete frequency noise can be called "interaction noise". Here, it is necessary to distinguish two types of interaction: wake interaction and potential field interaction, respectively. A discussion of wake interaction has been given by Moravec [3]. In this type of interaction the wake from an upstream object (such as a support strut, stator vane or rotor blade) impinges on a downstream blade which consequently experiences a periodic fluctuating lift, and this, in turn, produces discrete frequency noise radiation. Here, of course, only the mean velocity distribution in the wake is important. Turbulent fluctuations in the wake will cause some broad-band noise to be produced. In the second type of interaction, investigated by Fincher [2], the upstream object (e.g. stator vane) is so close to the downstream blade that there is a local periodic interference between the potential fields of the two objects. This interference produces a greater fluctuating lift than that due to the wake alone. From Fincher's measurements [2], it appears that, for representative fans (including aircraft engine compressors), the potential field interaction dominates when the separation between the trailing edge of the upstream object and the leading edge of the downstream object is less than about 0.2 of a representative chord length, while 192
Plate 1. Photograph of the new axial fan rig, showing the traversing microphone. The rotor is in the straight cylindrical section, the internal geometry of which is the same as shown in Figure 1 for the old rig, except that the front-bearing spider has been entirely removed. The air exit annulus and supports for the entire outer housing can be seen at the bottom of the rig.
(facing p. 193).
NOISE GENERATING MECHANISMS IN AXIAL FANS
193
wake interaction predominates when this separation is greater than about 0.3 of the representative chord length. These remarks, of course, apply only to typically streamlined aerofoil sections. Theoretical considerations of discrete frequency noise generation, by any of the mechanisms mentioned above, show that the Gutin-type rotational noise is very small for fan rotors having many blades. On the other hand, experimental work [1, 7] has indicated that observed levels of discrete frequency noise can be much higher than that to be expected from Gutin's theory. In the case of the original noise measurements on the Southampton University model axial compressor the contributions to overall noise levels by broad-band mechanisms and discrete frequency mechanisms (as classified above) were approximately equal [1]. It was thought that the only important discrete frequency mechanism in the fan rig was that of the wakes from the four support struts of the nose cone spider, upstream of the rotor. Calculations [8] showed that the discrete frequency noise to be expected from this wake interaction was of the order of magnitude of that observed. To confirm this experimentally, and to see if, in effect," all" discrete frequency noise could be eliminated, it was decided to reconstruct the model axial compressor rig, for a single rotor row, in such a way as to have no upstream obstructions (and consequently no wakes) and to have negligible downstream supports. Results from the new rig could then be compared with the original results. 2. EXPERIMENTAL PROCEDURE AND RESULTS 2.1. APPARATUS The experimental work was carried out in two stages on two different rigs. A diagrammatic sketch of the first one is shown in Figure 1, which is reproduced from reference 1. A photograph of the second rig (and the experimental set-up), is shown in Plate 1. A principal objective in the design and construction of the second rig was to provide entry and outflow conditions for the compressor which were, in effect, ideally smooth, with no 15 |
I
Stators
Ist S t a g e
IProvision forl
16
Rotors
12 a d d i t i o n a l I | stages J I Blade tip 1 clearance I ......
Tip dlo B i n .
h J I I
Provision
Hub
dia
4in.
bearlng spider
41 °
'67 °
Bifurcated exit d u c t belt -- 0 . 6 0
i n.'--PI
a.,'~= =. 1.60in,
t.60in.
Figure 1. An illustrative diagram of the axial fan geometry, a front bearing spider with four support struts being present as in the original rig. In the work reported here, both inlet and outlet guide vanes were removed; the only obstructions were the support struts. 14
194
P . E . DOAK AND P. O. VAIDYA
appreciable cross-flow obstructions either upstream or downstream. This requirement necessitated cantilevered construction. In order to minimize the effects at high rotor speeds of small errors in balancing, the rig was designed with a vertical axis. The total length of the shaft is 2 ft 6 in. and it is carried in three main bearings supported in a central pillar. The pillar is mounted on a heavy cylindrical block, which also carries the drive motor. The delivery air passage is also unobstructed by cross-flow supports except at the outlet where the maximum air-velocity is only of the order of 20 ft/sec. This is made possible by diffusing the flow in an annulus of progressively increasing mean diameter. The outer wall of the duct is, in fact, in the form of an inverted horn which surrounds the shaft and bearing structure, and which is supported only at the bottom rim. The gradual diffusion also eliminates any aerodynamic noise which might have been generated at sharp bends or collector boxes. Care was taken to maintain similarity in the other design aspects of both rigs. Thus both the new and old rigs have identical rotor blading, diameters of duct, positioning of the rotors with respect to the intake bells, etc. Care was taken to reduce the mechanical noise as much as possible. However, it was still necessary to "calibrate o u t " the mcchanical noise. The spectrum of the mechanical noise was obtained by running the fan when the blades were taken out. This showed that it would not interfere significantly with the discrete frequency noise measurements. However, it had a small effect on the measured overall noise (in which, as will be seen, the broad-band noise was predominant). Therefore, all overall measurements described below were repeated with the blades taken off and the appropriate decibel corrections determined in this way were used to calculate net overall blade noise levels, exclusive of mechanical noise contributions. 2.2
MEASUREMENTS ON THE ORIGINAL RIG
(i) Net overall sound pressure levels were measured, as explained above, with a Brim and Kj~er microphone and analyser, of the type BK 2107, at various angular positions with a spacing of 15 ° in between them. This was carried out at eight different speeds, which were checked by means of a stroboscope. (ii) The same microphone, coupled to a narrow band analyser, a Muirhead-Pametrada type D 489, of 6 ~ bandwidth, was used to obtain the sound pressure levels at the Made passage frequency and its multiples. This also was carried out at various angular positions at 15 ° intervals and repeated for the same speeds as in (i). For this part at least three sweeps were made, at each frequency and the mcan dB value was taken. All these measurements were made with a ½ in. Briiel and Kj~er condenser microphone mounted on a boom, traversing a circle of radius 3 ft, the centre of which coincided with the centre of the bellmouth. 2 . 3 . MEASUREMENTS ON THE NEW RIG
Exactly the same sequence of measurements was carried out on the second rig. Care was taken to keep the conditions of the experiments as closely identical as possible. Both the experiments were carried out in the semi-anechoic chamber of the Westland Acoustics Laboratory of the Southampton University Institute of Sound and Vibration Research. The shaft rotational speeds chosen were the same. The distance of the microphone was again 3 ft from the opening. The same analyser was used as in part (i) and in both cases the microphone was regularly calibrated with a piston-phone. For the discrete frequency measurements of part (ii), however, the levels were much reduced and hence a Hewlett-Packard analyser of type 302A, which has a bandwidth of 7 Hz, was used. A computer program was devised and used to compute the sound power levels from the sound pressure levels. It was applied, in both cases, to compute net overall, first harmonic and second harmonic levels. The results are shown in Figure 2.
NOISE GENERATING MECHANISMS IN AXIAL FANS l
I
I
I
I
Overall
I00
I
195
1
(both c a s e s ) ~ .'° o x
x
to
o~
o 9(3 ~:
_.
o
/ /
J
/
~ - . .,or 2nd harmonic
J-
o//
/ Fundomental~
8(3 "-o,,., ~''~ / . dl
~_
, ~
II
xl
/
//
x~
6C
I
I
I
x 2ndharmon~//
I
I
I
I
I
;0( )0 7000 8000 9000 I0,000 II,oo012,000 1~,000 Ray/rain
Figure 2. Sound power levels measured for the axial fan with (old rig) and without (cantilevered rig) upstream support struts. The results show that over the indicated speed range, discrete frequency levels were reduced by about 15 dB when the support struts were removed, and no longer contribute at all significantly to overall noise. , Cantilevered rig; - - -, old rig.
3. DISCUSSION AND CONCLUSIONS A comparison of the sound power levels shows that while the overall sound power levels are about the same for both the rigs, the levels of the first two harmonics of the discrete frequency noise have been very significantly reduced in the new rig, the reduction with respect to the old rig levels being of the order of 15 dB. These reductions are not significantly dependent on the rotor speed over the operating range studied. However, a much greater reduction would be required for the discrete frequency levels to drop to the levels predicted by Gutin's theory. One of the possible explanations which could be offered is based on considerations of aerodynamic asymmetry [5]. An asymmetry caused in the incoming flow due to the geometry of the inlet, say, would give rise to a lift distribution on each rotor blade that is not constant to an observer moving with the blade. This situation can result in more discrete frequency noise generation than predicted by Gutin's formula. Low-frequency variations in the incoming turbulence would have a similar effect. Possibly a more important conclusion in practice, however, is the clear indication that the fan in the new rig has no significant discrete frequency aerodynamic noise component. It can be noted here that the broad-band noise levels remain at the levels previously measured and predicted by Sharland [1]. This demonstrates that it is technically possible to design and construct an axial fan which produces, principally, only broad-band, vortex-shedding noise, at a relatively low level. To make such a fan even more quiet, the vortex shedding mechanism would have to be silenced, possibly by removing the rotor blade wakes by blowing from the trailing edges, or by boundary layer and wake control by suction through the blade surfaces. The research emphasis on fan noise for axial fans of sophisticated aerodynamic design can thus be shifted to the reduction of broad-band vortex-shedding noise.
196
P.E. DOAKAND P. G. VAIDYA
Due to the absence o f significant discrete frequency noise and a relatively low level of broad-band noise, the experimental fan is seen to be a quieter fan. For example, the overall noise levels are about 10 dB lower than those indicated by the Peistrup and Wesler correlation (see, for example, reference 9). It has thus been shown that it is possible to design quieter fans in practice by ensuring clean, uniform intake flows. One might also conclude from this fact that the noise levels, both discrete frequency and broad-band, of many practical, commercial fans are of the order of 10 dB higher than they would be if intake turbulence were removed, as well as steady perturbations of the mean flow by upstream support struts and the like. ACKNOWLEDGMENTS The authors wish to express their thanks for the considerable assistance given in the experimental work, particularly in the design of the cantilevered rig, by Mr. I. J. Sharland and Dr. E. J. Walker. REFERENCES l. I.J. SHARLAND1964 J. Sound Vib. 1 (3), 302. Sources of noise in axial flow fans. 2. H.M. FINCrmR 1966 J. Sound Vib. 3 (1), 100. Fan noise--the effects of a single upstream stator. 3. Z. MORAWC 1964 Proceedings of the Eleventh International Congress of Applied Mechanics-Munich (Germany) 1964 (Edited by H. Gortler, Springer-Verlag, Berlin-Heidelberg-New York). Der Einfluss der aerodynamischen Parameter auf den Sirenenl~irm einer axialen Schaufelstufe. 4. L. GOWN 1947 Tech. Memo. natn. advis. Comm. Aeronaut., Wash. No. 1105 (Translated from the original Russian). On the sound field of a rotating propeller. 5. S. E. WRIGHT1968 University of Southampton LS. V.R. Technical Report No. 5. Sound radiation by a lifting rotor generated by asymmetric disc loading. 6. M.V. LowsoN and J. B. OLLERrmAD1968ProceedingsAFOSR-UTIAS Symposiumonaerodynamic noise (Toronto)--University of Toronto, Institute for Aerospace Studies. Theoretical studies of helicopter rotor noise. 7. N. LE S. FILLEUL1966 J. Sound Vib. 3 (2), 147. An investigation of axial flow fan noise. 8. I. J. SrIARLAND1964 Unpublished communication. 9. W. RIZK and D. F. SEYMOUR1965 Proc. lnst. mech. Engrs. 179 (1). Investigations into the failure of gas circulators and circuit components at Hinkley point nuclear power station.