Noise is becoming of increasing importance and interest in modern life, and we appear to be approaching the time when much that we endure now will be regarded as intolerable. Public protests arise spasmodically on every hand and noise has been the subject of a certain amount of attention in the medical and popular press. With many persons the chief deterrent to air travel is understood to be the discomfort caused by the noise experienced. Whilst we may not wish to investigate the physiological reason for such discomfort, it is of interest to notice that in a recent paper, Laird (Acous. Soc. Am. J., I, p. 256, 1930), has summarised experimental data on the effect of sound on living beings.

No fully representative commission of medical, legal, acoustical and engineering interests has yet been set up in England, but in 1929 the Aeronautical Research Committee appointed a Sub–Committee in connection with the reduction of aircraft noise, and traffic noises have been considered in conferences by the Ministry of Transport.

In this paper a study has been made of certain cases of airflow in which various means are employed to control the behaviour of the air so as to prevent breakdown in the flow and the resulting turbulence. It is mainly an attempt to analyse the evidence which exists on such phenomena as slots, rings and boundary layer control by means of blowing through backwardly–directed slots in the surface, and to determine, if possible, the extent to which their apparent similarity corresponds, if at all, to an identity of physical principle.

A great deal of experimental work has been done at various times on such devices, and in this paper some of the published results are discussed and an attempt made to correlate them. In addition, some further experiments have been made to fill up gaps in the data available, or to extend their scope. They include other examples of control of airflow at sharp corners (Part I.); some of the cases considered differ widely from others, but all exhibit the reduction in eddying which results from assisting air to negotiate sharp corners or bluff obstacles with the least disturbance possible.