E. J. Richards, J. L. Willis, and D. J. M. Williams 281 
Two experimental rigs have been used to do this; these are shown in Figs. 15.1 
and 15.2. Figure 15.1 shows an induced-flow airchannel of massive construction 
incorporating two measurement areas, one subsonic, the other supersonic. The 
supersonic region acts incidentally as a choke to prevent the radiation of any 
injector noise forward into the subsonic working section and results in a low 
level of background noise against which the boundary-layer pressure fluctuations 
are measured. Some difficulties have arisen, however, in eliminating low- 
frequency vibration [1] and, consequently, the results are tentative in this 
frequency region. 
The second facility (Fig. 15.2) is a water rig. Water flows vertically from 
an overhead tank via a series of gauzes and a well-designed contraction and into 
a base tank. The header tank andthe pipe are isolated from any outside vibrations 
of the building. The elimination of background machinery noise by the use of a 
gravity feed has proved to be useful andmeasurements may be made in boundary 
layers of up to one inch in thickness. Again, some low-frequency vibration 
occurred, but much of this has been traced to a vibration of the pipe and has 
been reduced by the addition of a damping material to the outside of the pipe. 
Even so, it is suspected that some acoustical interference at frequencies of 200 
Fig. 15.2. The water rig. 
