RPM

rpm Sound scheme

The rpm sound scheme is designed for diesel-electric locomotives. In the Diesel Electric, the diesel engine drives a generator which produces electric power which drives the electric motors which propel the train. As the driver opens the power controller, more power is fed to the electric motors and hence more power is required from the diesel engine: the engine rpm increases in order to produce more power.

Thus the sound produced is not related to the locomotive speed, but instead to the power demand, also there is a lag effect between opening or closing the power controller and the engine noise changing as the engine rpm increases or decreases to the relevant speed.

In the Rail3D rpm scheme, the engine rpm is modelled internally - driven by the power control variable and with a lag to model the engines intertia. This rpm value is then used to select the required wave file from the sound set.

Thus if there are five wave files in the set: Wave0, Wave1, Wave2, Wave3 and Wave4, the wave files are selected as follows:

However simply changing waves like this produces some unpleasant discontinuities at the points where the wave is changed, so in the rpm sound scheme, Rail3D uses two sound channels and cross-fades the sound between them.

So at rpm=0% the first sound - Wave0 (usally an idling sound) - will play at full volume in the first sound channel. As rpm increases a second sound - Wave1 - is started in the second sound channel but at low volume, and then as the rpm increases the first channel is faded out whilst the second channel is faded in. This results in a smooth transition between the sounds. Once the first channel has been faded right down, it can then be reused for the third wave - Wave2 - and the sound is faded back as the rpm increases further. And so on.

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