What 45, 55, 65 dB mean

Modeled estimates for general information only — not measurements, not a guarantee, and not for legal, medical or real-estate decisions.

Decibels (dB) use a logarithmic scale. A change of 3 dB is barely noticeable, a change of 5 dB is clearly noticeable, and a change of about 10 dB is heard as twice or half as loud.

On Caltrans’ chart of typical outdoor sounds, about 45 dB falls between a quiet urban night and a quiet urban day, about 55 dB falls between a quiet urban day and heavy traffic at 300 feet, and about 65 dB is a commercial area.

See the levels on the map

What 45, 55 and 65 dB sound like

These comparisons are approximate positions read from Table 2-5 in Caltrans’ Technical Noise Supplement, in A-weighted decibels (dBA).

• About 45 dB. A little above a quiet urban night (about 40 dB) and a little below a quiet urban day (about 50 dB).
• About 55 dB. Between a quiet urban day (about 50 dB) and heavy traffic at 300 feet (about 60 dB).
• About 65 dB. About a commercial area, or normal speech at 3 feet, and a little above heavy traffic at 300 feet.

For scale, the same chart puts a gas lawn mower at 100 feet near 70 dB and a diesel truck at 50 feet at 50 mph in the 80s.

The map’s numbers are averages over a period, not the level of one sound. A modeled 55 dB is related to these examples but is not the same as hearing a 55 dB sound.

How the decibel scale works

• Decibels are logarithmic. Two equal sound sources add only 3 dB: two sources of 70 dBA produce 73 dBA.
• A-weighting adjusts the measurement to follow how the human ear hears different pitches, which is why levels are written dBA.
• 0 dB is roughly the threshold of the best human hearing. It is not silence.
• Caltrans lists 3 dB as a barely perceptible change, 5 dB as readily perceptible and 10 dB as twice or half as loud.

Day, Evening, Night and 24 h CNEL on the map

Day (7 am to 7 pm), Evening (7 to 10 pm) and Night (10 pm to 7 am) are each an average sound level, LAeq, in dB. The map takes them outdoors at a point 4 m up and 2 m out from a building wall, or 1.5 m above open ground.

The 24 h value is CNEL, the Community Noise Equivalent Level. It averages the whole day with evening counted 5 dB higher and night 10 dB higher than day, to reflect sensitivity to noise at those hours. The 24 h view adds aircraft, as an estimate derived from official airport contours; Day, Evening and Night never include it.

See how to look up the noise level at an address.

Why a few dB between two addresses is not a difference you can rely on

At a single address the modeled estimate is typically about 6 dB off, higher or lower. A difference of a few dB between two addresses is within that error. Use the map to compare broad situations, such as being on a freeway, on a side street or under a flight path.

See Limits and How it’s made for how well the model matches measurements.

WHO Europe 2018 guideline values, as published numbers

The World Health Organization’s Regional Office for Europe published environmental noise guidelines in 2018. They recommend that policy-makers reduce average exposure below these long-term values.

• Road traffic noise: below 53 dB Lden, and below 45 dB Lnight for night-time noise.
• Railway noise: below 54 dB Lden, and below 44 dB Lnight.
• Aircraft noise: below 45 dB Lden, and below 40 dB Lnight.

These are WHO’s published guideline numbers for populations. They are benchmarks, not safe or unsafe lines for any home.

Lden and Lnight are long-term averages with their own weighting. The map’s 24 h CNEL and Night values are related but different, so Quiet LA does not tell you whether an address is above or below a WHO value. Quiet LA gives no medical advice. See Limits.