the biological reference level for sound measurements. When the term decibel is used to indicate the intensity of noise level, a reference quantity is implied; this reference value is usually expressed as a sound pressure of 20 micropascals or 20 micronewtons per square meter. The reference level is referred to as 0 decibels, the baseline of the scale of noise levels; this baseline is considered the softest sound that can be heard by a person with very good hearing in an extremely quiet location. Other equivalent reference levels still being used include 0.0002 microbar and 0.0002 dyne per square centimeter.sound pressure level.