Why two 80 dB sounds make 83 dB, not 160

Add the energy, not the decibels

A decibel level is 10 × log₁₀ of an energy ratio. To combine independent sources you convert each level back to energy, add, and convert back: L = 10 × log₁₀(10^L1/10 + 10^L2/10 + …). Two equal sources double the energy, which is 10 × log₁₀(2) = 3 dB more. Ten equal sources add 10 dB¹.

Sources	Total	Change
1 × 80 dB	80 dB	0 dB
2 × 80 dB	83 dB	+3 dB
3 × 80 dB	84.8 dB	+4.8 dB
4 × 80 dB	86 dB	+6 dB
5 × 80 dB	87 dB	+7 dB
10 × 80 dB	90 dB	+10 dB
100 × 80 dB	100 dB	+20 dB

A quieter source barely counts

When levels differ, the louder source dominates. Add a 70 dB source to an 80 dB one and the total is 80.4 dB. That is why turning off the second-loudest machine in a workshop changes little, while quietening the loudest one changes a lot.

Difference between two sources	Add to the louder one
0 dB	+3 dB
1 dB	+2.5 dB
2 dB	+2.1 dB
3 dB	+1.8 dB
4 dB	+1.5 dB
6 dB	+1 dB
8 dB	+0.6 dB
10 dB	+0.4 dB
15 dB	+0.1 dB

Try it

Levels in dB, separated by commas

83.0 dB

The same rule works over time

Exposure is energy too. An hour at 100 dBA contains as much sound energy as eight hours at 91 dBA, so for an 8-hour day the average level (L_eq,8h) of one loud hour and seven silent ones is 91 dBA. That is why NIOSH halves the allowed time for every 3 dB increase². OSHA uses a 5 dB rule instead, which is a policy choice rather than physics³; the noise dose calculator shows both.

Loudness is different again

Doubling the energy (3 dB) is a just-noticeable change in most situations. To sound twice as loud, a sound needs roughly 10 dB more, which is ten times the energy⁴. Two identical speakers playing together are therefore only a little louder than one.

Questions

What is 60 dB plus 60 dB?

About 63 dB. Two equal, independent sources double the sound energy, which adds 3 dB.

How many decibels is twice as loud?

Roughly 10 dB more sounds about twice as loud, even though it is ten times the energy.

Can sound cancel out?

Identical, perfectly aligned waves can (that is how noise-cancelling headphones work at low frequencies), but everyday independent sources add by energy as described here.

Sources

Decibels add logarithmically. Two equal, unrelated (incoherent) sources together are 3 dB louder than one (two 90 dB machines make 93 dB, not 180 dB). Ten equal sources are 10 dB louder, because ten times the power is +10 dB. A source 10 dB quieter than another adds almost nothing.
- Noise - Basic Information (OSH Answers), Canadian Centre for Occupational Health and Safety (CCOHS).If one machine emits a sound level of 90 dB, and a second identical machine is placed beside the first, the combined sound level is 93 dB, not 180 dB.When the difference between two noise levels is 10 dB(A) or more, the amount to be added to the higher noise level is zero.Checked 2026-11-02.
- Sound advice: Control of noise at work in music and entertainment (HSG260), Health and Safety Executive (HSE).Two instruments with the same noise level of 85 dB together produce 88 dB.Checked 2026-11-02.
- What is a decibel? (Music Acoustics), School of Physics, UNSW Sydney.To continue the example, if the second had 10 times the power of the first, the difference in dB would be 10 log (P 2 /P 1 ) = 10 log 10 = 10 dB.Checked 2026-11-02.
- Music acoustics FAQ: Does adding two identical sounds give an increase of 3 dB or 6dB?, School of Physics, UNSW Sydney.So the sound intensity is four times as great in some places, zero in others, and intermediate values in most places. Integrate this over all space and the total power is only doubled.Checked 2026-11-02.
NIOSH's recommended exposure limit (REL) is 85 dBA as an 8-hour time-weighted average with a 3 dB exchange rate: every 3 dB louder halves the allowed time (88 dBA 4 h, 91 dBA 2 h, 94 dBA 1 h, 97 dBA 30 min, 100 dBA 15 min). No exposure should exceed 140 dBA.
- Criteria for a Recommended Standard: Occupational Noise Exposure, Revised Criteria 1998 (DHHS (NIOSH) Publication No. 98-126), NIOSH / CDC (CDC Stacks).The REL is 85 decibels, A-weighted, as an 8-hr time-weighted average (85 dBA as an 8-hr TWA). Exposures at and above this level are considered hazardous.In accordance with Section 1.1.l, the REL for an 8-hr work shift is a TWA of 85 dBA using a 3-decibel (dB) exchange rate.88 4 – – 114 – – 3591 2 – – 117 – – 1894 1 – – 120 – – 997 – 30 – 123 – – 4100 – 15 – 126 – – 285 8 – – 111 – 1 1180 25 24 – 106 – 3 45Exposure to continuous, varying, intermittent, or impulsive noise shall not exceed 140 dBA.The generally accepted ceiling limit of 140 dB peak SPL for impulsive noise is based on a report by Kryter et al. [1966].With a 40-year lifetime exposure at the 85-dBA REL, the excess risk of developing occupational NIHL is 8%—considerably lower than the 25% excess risk at the 90-dBA permissible exposure limit (PEL)Checked 2026-11-02.
- About Noise and Hearing Loss Prevention, NIOSH / CDC.NIOSH established a recommended exposure limit (REL) of 85 A-weighted decibels (dBA) averaged over an eight-hour workday.For each 3 dBA increase in noise level, NIOSH recommends reducing the exposure duration by half.Checked 2026-11-02.
OSHA's enforceable permissible exposure limit (PEL) in 29 CFR 1910.95 is 90 dBA for 8 hours with a 5 dB exchange rate (Table G-16: 95 dBA 4 h, 100 dBA 2 h, 105 dBA 1 h, 110 dBA 30 min, 115 dBA 15 min or less). A hearing conservation programme is required at the 85 dBA action level (a 50% dose). Impulsive or impact noise should not exceed 140 dB peak.
- 29 CFR 1910.95 Occupational noise exposure (Code of Federal Regulations, 2024 edition), U.S. Government Publishing Office (govinfo.gov).Duration per day, hours Sound level dBA slow response 8 90 6 92 4 95 3 97 2 100102 1 105 1/2 110 1/4 or less 115Exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level.For purposes of paragraphs (c) through (n) of this section, an 8-hour time-weighted average of 85 decibels or a dose of fifty percent shall also be referred to as the action level.Checked 2026-11-02.
- Criteria for a Recommended Standard: Occupational Noise Exposure, Revised Criteria 1998 (DHHS (NIOSH) Publication No. 98-126), NIOSH / CDC (CDC Stacks).The 5-dB exchange rate is still used by OSHA and MSHA, but the 3-dB exchange rate has been increasingly supported by national and international consensus.Initially, OSHA adopted the Walsh-Healey exposure limit of 90 dBA as an 8-hr TWA with a 5-dB exchange rate as its permissible exposure limit (PEL)Checked 2026-11-02.
Each 3 dB increase doubles sound intensity (energy), which is only just noticeable. A 10 dB increase is ten times the intensity and is perceived as roughly twice as loud; 20 dB is a hundred times the intensity.
- Sound advice: Control of noise at work in music and entertainment (HSG260), Health and Safety Executive (HSE).Each 3 dB added doubles the sound energy (but this is only just noticeable to a listener).When 10 dB is added, the energy is increased ten-fold, while adding 20 dB is a hundred-fold increase.The sound intensity doubles with every three dB increase. Thus sounds at 88 dB are actually twice as intense as they are at 85 dB and 115 dB is 1000 times as intense as 85 dB.Checked 2026-11-02.
- What is a decibel? (Music Acoustics), School of Physics, UNSW Sydney.Experimentally it was found that, above 40 phons, a 10 dB increase in sound level corresponds approximately to a perceived doubling of loudness.Checked 2026-11-02.
- Loudness, HyperPhysics, Department of Physics and Astronomy, Georgia State University.A widely used "rule of thumb" for the loudness of a particular sound is that the sound must be increased in intensity by a factor of ten for the sound to be perceived as twice as loud.Although this rule is widely used, it must be emphasized that it is an approximate general statement based upon a great deal of investigation of average human hearing but it is not to be taken as a hard and fast rule.Checked 2026-11-02.

General information, not medical advice. For any concern about your hearing or a child’s, see an audiologist or doctor.