Measure the floor, then read the band: under 30 dB(A) you can record anything; 30–40 dB(A) rules out quiet voice work; 40–50 dB(A) leaves only loud sources; above 50 dB(A) you change rooms.
Noise floor: the number that decides if a room is usable
checked 2026-09-16responsible Room Tone Register (a named register, not a person)
The four bands
| Floor, dB(A) | What it permits | What it forbids | Typical room |
|---|---|---|---|
| < 30 | Close-mic'd voice, quiet Foley, classical | Nothing, within reason | Purpose-built studio, isolated booth |
| 30 – 40 | Dialogue at normal projection, most voice-over | Whispered dialogue, quiet acoustic instruments | Good office after hours, treated room with HVAC off |
| 40 – 50 | Loud dialogue, singing, percussive sources | Any quiet-source recording | City apartment, untreated room |
| 50 + | Nothing without heavy processing | Everything that matters | Street-facing room, plant room, workshop |
The bands are deliberately coarse. A two-decibel difference inside a band does not change the decision, and pretending otherwise is how people end up buying foam instead of moving the shoot.
Weighting and time constant
A noise floor quoted without its weighting and time constant is not a measurement. dB(A) is the usual choice because it approximates the ear's sensitivity at low levels; dB(C) is occasionally used when the low end is the complaint, and a 60 Hz hum reads far higher on C than on A. Slow averaging is standard for a floor because fast averaging chases the meter needle rather than the room.
This is the single most common way two people get different numbers for the same room: one is on A and one is on C, or one is on fast. See the note on two meters disagreeing 4.5 dB apart.
What moves the number
- Air handling. Usually the largest single term and the one you can switch off — if the building lets you.
- Refrigeration. Cyclical, so the floor is a range, not a value.
- Lighting ballast. Often tonal rather than broadband, which is worse under dialogue than its level suggests.
- Outside traffic. Broadband, and correlated with time of day.
- The people in the room. Bodies absorb; a full room is measurably quieter than an empty one.
A floor measured with the HVAC off is not the floor you will record in. Measure both states and log them separately; the difference is the decision you actually have to make.
Near kin, and the misread that costs the most
The near kin are dynamic range, self-noise and background level. Dynamic range is the span between the floor and the loudest usable signal, so a low floor is only half of it. Self-noise belongs to the microphone, not the room, and it sets a second floor you cannot treat. Background level is the same measurement taken with the events included.
| Term | What it is | How it differs from a noise floor |
|---|---|---|
| Dynamic range | Span from floor to loudest usable | Depends on the source as well as the room |
| Self-noise | Noise generated by the microphone | Belongs to the equipment, not the room |
| Background level | Level with events included | Changes with what is happening outside |
| Leq | Energy-equivalent level over a window | A floor figure is one case of this, not the definition |
The common misread is quoting an unweighted or unstated number. A floor of '42' with no weighting, no averaging and no duration is not a measurement; it is a reading. Two people can each hold a defensible figure 4.5 dB apart for the same room and both be right, which is why the register refuses to publish a floor without its conditions.
Source log
- 01NIOSH Sound Level Meter app (CDC)supports the method for measuring room noise with a calibrated meter
- 02Decibels, UNSW Music Acousticssupports the logarithmic addition of uncorrelated levels used in the worked example
- 03ISO 3382-2: Measurement of room acoustic parameters, ordinary roomssupports the requirement that a room acoustic measurement state its method and conditions
Each line points at one specific document. No line is a home page.
Leave this page Close this page if your question is what to record rather than whether the room is quiet enough — that is the room tone page.