G. G. Parfitt 85 
through which vibrational energy would escape to the ship's outer structure. 
These terminations would modify the resonance frequencies and damping of the 
beam and, if they represented tight couplings to further resonanting systems, 
could introduce further resonances. However, it does not seem likely that these 
changes would invalidate the general ideas enunciated. Accordingly, it seems 
reasonable that almost any measure of the beam's motion or of the force upon 
it can be employed when one wishes to estimate the change in the final noise 
radiated due to the isolating measures considered. I would admit, however, that 
although response ratio as used in the paper provides accurate values for the 
effects of inserting an isolating system, it is probably safer, owing to certain 
slightly misleading characteristics mentioned in the paper, to calculate the 
end-to-end transmissibility (e.g., force on beam over force applied by machine) 
for each case in reviewing the over-all performance of the system. 
MR.S. BYARD emphasized the importance of the mechanical impedance 
concept in assessing the performance of resilient mountings. He asked the lec- 
turer to comment further on the use of nonlinear springs to reduce the natural 
frequency of a mounting system for a given deflection under static load. 
Mr. Byard pointed out that Dr. Parfitt had suggested that the over-all weight 
of a compound mounting might be kept down by arranging that the intermediate 
mass comprise in part at least some of the auxiliary components associated 
with the main engine. He thought it would be dificult to ensure that such an in- 
termediate load behaved as a true mass andnot as a complex resonant structure. 
Regarding the question of force "pick-ups," Mr. Byard said that there was no 
real difficulty in constructing pick-ups which effectively measure force and, in 
the practical measurement of the mechanical impedance of a structure, sucha 
pick-up is used in conjunction with a velocity pick-up. 
DR. PARFITT: Mr. Byard is very right to questionthe practicability of using 
auxiliary equipment for the loading masses considered in the paper. The main 
requirement here is that the loading be masslike in impedance over the fre- 
quency range where foundation resonances are important. Inasmuch as founda- 
tion beams may be relatively long, and hence of low frequency, whereas items 
of equipment may be relatively compact, there is a possibility of achieving this 
limited requirement. However, every individual case must certainly be treated 
with great care and on its own merits. 
I have not had any personal experience with nonlinear spring mounts for 
reducing resonance frequency. I understand that they have been used successfully 
in large engineering installations, though I suspect that for mobile ship-borne 
and service use they may be a little critical in adjustment and maintenance. 
Dr. F.M.V. FLINT said that the question of the advantages of the equivalent 
circuit approach to the analysis of structural vibrations merited further dis- 
cussion. He pointed out that with underwater radiated noise, it was not so much 
the disturbance of the base ofthe mounting which was of interest; for the ultimate 
current-carrying network (in the electrical analog) was the entire surrounding 
medium and one wished to know the energy dissipated in the resistive element 
