A relative unit used to show the ratio between a reference value and a measured value, scaled logarithmically. As a result, decibel values (denoted dB) cannot be directly added or subtracted. While decibels are used for relative sound power levels, voltage levels, and acceleration levels, sound pressure levels are the most common use of decibels in acoustics. For reference, the sound pressure level of a conversation is about 50–60 dB while a rock concert is about 100–110 dB.