3dB is a doubling in the pressure differential in the sound wave. The human hearing system is equipped with logarithmic compression, which increases dynamic range. The effect of this is that each doubling in pressure, and therefore energy, is perceived as a step in a linear scale. A 3dB increase in not heard as a doubling in volume. Pitch (sound frequency) works there same way. An octave shift is a doubling (or halfing) in frequency, but each octave is heard as a step in a linear scale.