How loud is 40 dB?
Updated
Modeled estimates for general information only — not measurements, not a guarantee, and not for legal, medical or real-estate decisions.
How loud is 40 dB? About as loud as a quiet urban nighttime outdoors (about 40 dB on Caltrans’ chart of typical outdoor sounds), or the background of a theater or large conference room indoors. It is 10 dB below a quiet urban day, which is heard as about half as loud. On the Quiet LA map, 40 dB is a Night value: a modeled average between 10 pm and 7 am.
In Los Angeles, the median Night level (10 pm to 7 am, LAeq) at the county’s modeled building-façade points is 47.5 dB (modeled, not a measurement; map data release of 14 October 2026).
What does 40 dB sound like?
- Outdoors, about a quiet urban nighttime (about 40 dB). The chart puts a quiet suburban nighttime a little below it and a quiet rural nighttime below that.
- Indoors, about the background level of a theater or a large conference room (about 40 dB); the chart puts a library lower, near 30 dB.
A change of 3 dB is barely noticeable, 5 dB is readily noticeable and 10 dB is heard as about twice or half as loud (Caltrans). So 50 dB, a quiet urban day on the chart, is heard as about twice as loud as 40 dB. The chart is read by eye, so treat each comparison as about.
What does 40 dB mean on the Quiet LA map?
The map’s Night value is an average sound level (LAeq) between 10 pm and 7 am, outdoors, 4 m up and 2 m out from a building wall or 1.5 m above open ground. A Night value of 40 dB is that modeled average, not the level of any one sound and not what you hear inside. At a single address the modeled value is typically about 6 dB off, higher or lower.
Do not read the lowest color on the map as 40 dB. Land with no freeway or major road within 3 km has no computed value and shows the lowest color; that is an assumption, not a computed value. Other blank areas are not modeled yet, and blank never means quiet. See Limits.
Where do Night values near 40 dB come from?
Night values come from roads and, where the rail model reaches, trains. Aircraft are never part of the Night value. In the tables of the six freeway guides, the median Night level at building façades is at or above 40 dB in every distance band, out to 1 km, the farthest band in those tables. That is a statement about the tables, which stop at 1 km from a freeway, and not about any area. See the 405, 101, I-10, 110, I-5 and 710 guides, train and rail noise and wind and noise for what moves a Night value.
Quiet LA publishes county shares for Night levels of 45, 50 and 55 dB on its area pages, not for 40. Method: How it’s made and Sources & methods.
Is 40 dB loud?
It is the low end of the chart, but whether a sound is loud depends on who is listening and when, and the map does not rate any level as acceptable or not. Quiet LA gives no medical advice. For the published health benchmark numbers and what they are not, see what 45, 55 and 65 dB mean.
How does 40 dB compare with the other levels?
| Level | Compares with (Caltrans chart, approximate) | Map view | Guide |
|---|---|---|---|
| 40 dB | Outdoors, about quiet urban nighttime (about 40 dB). Indoors, about a theater or large conference room with only background noise. | Night | How loud is 40 dB? |
| 45 dB | Outdoors, between quiet urban nighttime (about 40 dB) and quiet urban daytime (about 50 dB). Indoors, a little above a theater or large conference room with only background noise (about 40 dB). | Night | How loud is 45 dB? |
| 50 dB | Outdoors, about quiet urban daytime (about 50 dB). Indoors, about a dishwasher running in the next room (about 50 dB). | Night | How loud is 50 dB? |
| 55 dB | Outdoors, between quiet urban daytime (about 50 dB) and heavy traffic at 300 feet (about 60 dB). Indoors, about a large business office (between 50 and 60 dB). | 24 h (CNEL) or Night | How loud is 55 dB? |
| 60 dB | Outdoors, about heavy traffic at 300 feet (about 60 dB). Indoors, a little below normal speech at 3 feet (about 60 to 65 dB). | 24 h (CNEL) | How loud is 60 dB? |
| 65 dB | Outdoors, about a commercial area, a little above heavy traffic at 300 feet (about 60 dB). Indoors, about normal speech at 3 feet. | 24 h (CNEL) | How loud is 65 dB? |
| 70 dB | Outdoors, about a gas lawn mower at 100 feet (about 70 dB). Indoors, about a vacuum cleaner at 10 feet. | 24 h (CNEL) | How loud is 70 dB? |
Next level up: how loud is 45 dB.
Open this on the map
Open the map, then open the legend at the right edge to find the color of 40 dB, and select a building to read its modeled Night value.
- Los Angeles County at night (the Night view; the legend lists the color of 40 dB)
- The 405 in the 24-hour view (road levels fall with distance)
- Downtown Los Angeles at night
Common questions
How loud is 40 decibels?
On Caltrans’ chart, about a quiet urban nighttime outdoors, or the background of a theater or large conference room indoors. It is half as loud as a quiet urban day by the chart’s 10 dB rule.
Is 40 dB on the map the same as hearing 40 dB?
No. The map’s Night value is a modeled average between 10 pm and 7 am, an estimate and not a measurement, typically about 6 dB off at one spot.
What is 40 dB equivalent to?
About a quiet urban nighttime outdoors, and about the background of a theater or large conference room indoors. By the chart’s 10 dB rule it is half as loud as a quiet urban day (about 50 dB).
What is an example of 40 dBA?
dBA is the A-weighted decibel, the unit of Caltrans’ chart. At about 40 dBA the chart shows a quiet urban nighttime outdoors and the background of a theater or large conference room indoors. The chart is a guide to scale, not a recording, and a sound of 40 dBA depends on how far away it is.
Is 40 dB loud?
It is the low end of the chart: 10 dB below a quiet urban day. Whether a sound is loud depends on who is listening and when; the map does not rate any level as acceptable or not, and Quiet LA gives no medical advice.
Does the lowest color on the map mean 40 dB?
No. The lowest color is used where no freeway or major road is within 3 km; that is an assumption, not a computed value.
Sources
- Caltrans: Technical Noise Supplement to the Traffic Noise Analysis Protocol (September 2013) Table 2-5 (typical noise levels, read by eye) and Table 2-10 (perceived change). Checked 21 and 23 October 2026.