Decibels Explained: dB vs dBA

Decibels trip people up because they do not add the way normal numbers do. Two sounds at 80 dB do not make 160 dB. A sound 10 dB louder is not a little louder; it sounds about twice as loud. This guide works through six everyday problems so the scale starts to make sense.

The basics in one paragraph

A decibel (dB) is a logarithmic unit: each step represents a multiplication of sound energy, not an addition. That is why a small change in the number can mean a large change in energy. Most noise limits use dBA, which adjusts the reading to match how human ears hear, since we are less sensitive to very low and very high pitches. dBC applies much less adjustment and keeps more of the bass.

Problem 1: Two identical sources

You have one fan at 60 dB. You turn on a second identical fan. How loud is it now?

About 63 dB. Doubling the sound energy adds about 3 dB. On the perception scale that transportation agencies use, a 3 dB change is barely noticeable.

Problem 2: Ten sources

One machine reads 70 dB. What do ten identical machines read?

About 80 dB. Ten times the energy adds 10 dB. A 10 dB increase is perceived as roughly twice as loud. So ten machines do not sound ten times louder; they sound about twice as loud as one.

Problem 3: Distance

A sound reads 90 dB at 4 feet. What does it read at 8 feet, outdoors?

About 84 dB. For a small source in open space, doubling your distance lowers the level by about 6 dB. Halving the distance raises it by about 6 dB. Indoors the drop is smaller because walls and ceilings reflect sound back.

This is why moving back from a speaker or a lawn mower is one of the easiest ways to protect your hearing.

Problem 4: Why bass reads low

A neighbor's subwoofer feels loud, but a phone app set to dBA reads only moderately. Is the app wrong?

Not necessarily. A-weighting reduces the contribution of low frequencies, so bass-heavy sound can read lower in dBA than it feels. Switching to dBC keeps more of the bass and gives a higher reading. That is why some cities set limits in both; Toronto's bylaw, for example, limits amplified sound in both dB(A) and dB(C). See bass from a neighbor's music.

Problem 5: Safe time

You can safely spend 8 hours at 85 dBA under the NIOSH recommendation. How long at 88 dBA? At 100 dBA?

About 4 hours at 88 dBA, and about 15 minutes at 100 dBA. NIOSH uses a 3 dB "exchange rate": every 3 dB louder halves the safe time. OSHA's legal workplace rule uses a 5 dB exchange rate starting at 90 dBA, which is less protective. Our safe decibel levels guide compares the two in full.

Problem 6: Can you trust your phone?

Your free sound meter app says 83 dBA. How accurate is that?

It depends on the app. In testing, NIOSH found that most sound meter apps lacked the accuracy of professional instruments. Its own NIOSH Sound Level Meter app for iPhone met a benchmark of within about 2 dB of a professional meter, and within about 1 dB over a common range when used with a calibrated external microphone. A 2020 study comparing the NIOSH app with personal noise dosimeters found differences under 2 dBA, with the best accuracy above about 75 dBA.

NIOSH also notes that no smartphone app has met the formal standards for professional sound level meters. So treat an app reading as a good estimate: useful for telling 70 dB from 95 dB, not for settling a 1-decibel dispute.

Quick reference

Myths worth dropping

• "Two 80 dB sounds make 160 dB." They make about 83 dB.
• "Twice the distance means half as loud." It is about 6 dB quieter; sounding half as loud takes about 10 dB.
• "Any free app is accurate." Accuracy varies; validated apps are good to about 2 dB.

The short version

Decibels multiply rather than add. +3 dB doubles the energy, +10 dB sounds twice as loud, and doubling your distance outdoors drops the level by about 6 dB. dBA matches human hearing but underplays bass; dBC keeps it. A validated phone app is good to about 2 dB. Put it into practice with our decibel chart and the free safe listening calculator.