Logarithmic unit for sound pressure level — 0 dB = hearing threshold, 120 dB = pain threshold. On set: keep dialogue peaks near −18 dB.
The decibel (dB) is the logarithmic unit of sound pressure level — named after Alexander Graham Bell, who originally developed the scale for telephone lines. Zero decibels marks the human hearing threshold (20 µPa sound pressure), while 120 dB is at the pain threshold. The logarithmic nature of the decibel accurately reflects human loudness perception: a doubling of perceived loudness corresponds to approximately a 10 dB increase in level, not a doubling of physical sound pressure. On a film set, the decibel is the central reference value for the sound team — every boom operator, every sound mixer thinks in dB, whether at the mixing console or when setting the level of a wireless receiver.
Reference Levels on Set
The most important guideline for film sound is −18 dBFS (decibels Full Scale) in the digital recorder — this is the reference level to which dialogue peaks are set. At −18 dBFS, there is enough headroom for sudden loudness jumps (a scream, a slamming door) without the converter clipping. At the mixing console, dBu is often used: 0 dBu corresponds to approximately 0.775 volts, and professional mixing consoles operate at line level around +4 dBu. The boom operator typically aims for −20 to −12 dB on the wireless receiver display; anything below −30 dB is noisy in the post-production noise floor, and anything above −6 dB risks clipping from unexpected sound pressure peaks.
dB(A), dB(C), and Frequency Weighting
Not every decibel is the same: dB(A) weights low frequencies less and simulates the frequency-dependent sensitivity of the human ear at moderate levels. dB(C) is flatter, takes bass more into account, and is the relevant measurement for loud set environments — for example, when the sound mixer measures whether the noise from an HMI ballast reaches 65 dB(C) at the shooting location, thus making the dialogue unusable. The difference between dB(A) and dB(C) can quickly be 15 dB for bass-heavy noise sources (generator, ventilation, traffic). The set sound mixer always measures both and then decides whether a low-cut filter on the boom microphone is sufficient or if the source must be turned off.
Post-Production: Dialogue Normalization
In post-production, dialogue is normalized to defined loudness values: −24 LUFS (Loudness Units relative to Full Scale) is the EBU R128 standard for TV in Europe, and −27 LUFS for cinema according to SMPTE RP 200. LUFS is not the same as dB, but closely related — it measures perceived loudness over time, whereas dB measures the instantaneous level. A film that was cleanly set at −18 dBFS on set typically ends up with an integrated loudness of −27 LUFS after mixing. Those who understand these figures speak the language of sound post-production and avoid the most expensive mistake on set: "The mixer will fix that later" — some things, he won't.