Sound Pressure Level and dB

The decibel is a logarithmic ratio; sound pressure level connects voltage, distance and perceived loudness.

Related

LUFSTrue PeakHeadroomMonitoring

The problem

The same knob setting sounds different at noon and midnight, and two tracks at equal meter level can differ. Separate SPL, level and perception.

Physics or room

Sound pressure level is twenty times the log of pressure over 20 µPa. Plus 6 dB doubles pressure; plus 10 dB roughly doubles perceived loudness at mid frequencies. In a free field level drops about 6 dB per doubling of distance; rooms add reflections and modes. dBFS is digital full scale, dB SPL is air pressure.

Practice and judgement

With a C-weighted, slow SPL meter, play mono pink noise at the listening position and set each speaker to about 83 dB SPL. Keep 6 dB of headroom. Match loudness before comparing tone, or the louder version always sounds clearer.

What to listen for

Play the same mix at 70, 80 and 90 dB. At low level, bass and treble fade first and mids dominate; at high level, bass and sibilance grow. On a dynamic orchestral piece, check that quiet passages are not lost in room noise.

Common misunderstandings

Decibels are always ratios, not absolutes. Doubling power is 3 dB, not double loudness. 0 dB SPL is near the hearing threshold, not silence. Phone meters are rough.

Sources

• Everest, F. Alton，Master Handbook of Acoustics（McGraw-Hill）— 声压级与心理声学章节核对，未引用
• Ballou, Glen，Handbook for Sound Engineers（Focal Press）— 监听与测量章节核对，未引用

Used for fact-checking only; all prose is original.