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Early in the development process, BMW began to research what customers prefer as a sporty sound from the car’s engine. Once again a roadster presents unique challenges to the engineers because the acoustics are much different whether the top is up or down.
The process began in the sound lab with people listening to different sound samples.
These samples were engine noises but they had been synthetically created on the computer. The goal was to find the ideal roadster sound. This sound shouldn’t be so loud as to disturb one’s neighbors but it should be louder than a luxury sedan. The sound must be “solid” and correspond to what is expected from a BMW. These aspects were all combined and a sound target was designed. This sound target, which is stored as a computer file, was specified in terms of tone and decibel levels. But it also was somewhat subjective.
On the real car, the sources that contribute to the sound the driver and passenger hear include the engine itself, the air intake and the exhaust. The engine noise is very mechanical sounding and while, for example, a Ferrari’s sound is indeed very mechanical; BMW feels that their cars should be more refined.

“Sound generator” is a misnomer; “selective sound filter” would be more accurate.