The effect of engine speed load and size (swept volume) on noise has been evaluated on a limited range of engines. Total sound intensity (but omitting low frequency components due to air intake noise) varies as (speed)3, as (swept volume) and very little with load.
The noise from a petrol engine was of the same order of magnitude as that of the diesel engines at full load but considerably less at low load.
The following sources of noise have been identified and means of reducing them adequately established in principle.
Narrow band frequency analysis has been shown to be a convenient criterion of the “noisiness” of the form of the cylinder pressure and in conjunction with noise measurement makes it possible to evaluate separately the noisiness of the engine structure.
In the small diesel engines tested in this respect, it should be possible to quieten some (by about 5 db in the critical frequency range) by smoothing the cylinder-pressure-rise. In one, the pressure-rise is already as smooth that no improvement is to be expected without sacrifice of efficiency.
Observed levels of vibration of engine surfaces are shown to agree roughly with values calculated from the measured noise and assumed sizes of the radiating parts. A hypothesis is advanced which accounts for the vibration due to the measured cylinder pressure in the important frequency range from 800 c/s upwards.
The noise emitted is assigned tentatively to 3 different mechanisms in 3 frequency ranges.