"Loudness" is a feature used on many stereo receivers in home audio. It boosts the high and low frequencies. The reason being, human ears have low sensitivity to very high and very low frequencies at low volume. Ever notice how your home audio system sounds flat or not as good at very low volumes, but comes alive when you turn it up? Then you have experienced the effects of the Fletcher-Munsen Curve. If you have a subwoofer in you Fortwo, you should notice more bass, when the Loudness feature is turned on, without the sub, you may not notice anything. Loudness features aren't needed in automobiles because the ambient noise levels are too high. I leave it off.