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dB, dBA and dBC: what the letters mean

A decibel is a ratio, not a unit of loudness. The letter after it tells you how the measurement treated low and high pitches, and that can change the number by tens of decibels.

A decibel compares a pressure with a reference

Sound is a tiny, fast swing in air pressure. Sound pressure level in decibels is 20 × log10(p / p0), where p0 is 20 micropascals, roughly the quietest sound a healthy young ear can detect at 1 kHz1. So 0 dB is not silence; it is the reference. Rooms quieter than that, like the best anechoic chambers, get negative numbers.

Because the scale is logarithmic, equal steps in decibels are equal ratios. Each 10 dB is ten times the sound energy (intensity), and each 20 dB is ten times the pressure. Your ear hears a 10 dB rise as roughly a doubling of loudness2.

LevelRMS pressureEnergy compared with 0 dB
0 dB20.0 µPa1×
20 dB200 µPa100×
40 dB2.00e+3 µPa10,000×
60 dB20.0 mPa1,000,000×
80 dB200 mPa100,000,000×
94 dB1.00 Pa2,511,886,431.51×
100 dB2.00 Pa10,000,000,000×
120 dB20.0 Pa1,000,000,000,000×
140 dB200 Pa100,000,000,000,000×

Pressures and ratios calculated from the definition; 94 dB is the 1 pascal that many calibrators produce.

dBA: weighted like your ears

Ears are much less sensitive to deep bass and to very high pitches than to the middle of the range. The A-weighting curve, defined in the international sound level meter standard IEC 61672, turns down each frequency by roughly the amount the ear does at moderate levels before the meter adds everything up3. A reading in dBA (or dB(A)) is the level after that filter. Workplace limits, including NIOSH’s 85 dBA and OSHA’s 90 dBA, are written in dBA because A-weighted levels track the risk of hearing damage reasonably well4.

dBC: for bass and for bangs

C-weighting is almost flat: it trims only the extreme low and high ends. It is used for peak levels of impulses and for low-frequency noise, where A-weighting would hide energy that matters. The UK and EU set their peak action values in dB(C): 135 and 137 dB(C), with a 140 dB(C) limit5. dBZ (or “linear”, “unweighted”) applies no weighting at all.

FrequencyA-weightingC-weighting
31.5 Hz−39.4 dB−3 dB
63 Hz−26.2 dB−0.8 dB
125 Hz−16.1 dB−0.2 dB
250 Hz−8.6 dB−0 dB
500 Hz−3.2 dB+0 dB
1 kHz+0 dB−0 dB
2 kHz+1.2 dB−0.2 dB
4 kHz+1 dB−0.8 dB
8 kHz−1.1 dB−3 dB
16 kHz−6.6 dB−8.5 dB

Calculated from the IEC 61672 weighting formulas, which this site’s live meter uses and tests against the standard’s table.

The same sound can therefore have very different numbers. A pure 63 Hz hum reads 26.2 dB lower in dBA than unweighted, while a 2 kHz whistle reads about the same either way. Bass-heavy noise such as engines, generators and subwoofers always looks quieter in dBA than in dBC.

Average, maximum and peak

A level can also describe different moments. An equivalent continuous level (Leq) is the energy average over a period, the number exposure limits use. A maximum (Lmax) is the loudest moment of a meter’s smoothed reading. A peak is the highest instantaneous pressure, the right measure for a gunshot or a firework, and it can be 20 dB or more above the smoothed maximum of the same event. On this site each sound says which it is: “dBA”, “dBA max” or “dB peak”.

Under water

Underwater levels use a 1 micropascal reference instead of 20, which alone makes the numbers 26 dB higher, and water’s much higher acoustic impedance means the same intensity needs far more pressure. Why whale decibels are not comparable with air decibels.

Questions

Is dBA the same as dB?

No. dBA is a decibel level measured through the A-weighting filter, which reduces low and very high frequencies the way the ear does. For a mid-pitched sound the two are close; for deep bass the dBA figure can be much lower.

Why do sound level apps show dBA?

Because hearing-risk limits are written in dBA, and A-weighting makes a reading roughly match how loud a sound seems at moderate levels.

What does dB SPL mean?

Sound pressure level: decibels relative to 20 micropascals in air. Without a letter, a level is usually unweighted SPL.

Sources

  1. In air, 0 dB SPL is a sound pressure of 20 micropascals (20 µPa). Underwater levels are referenced to 1 µPa, so the same pressure reads about 26 dB higher in water notation, and because water and air have very different acoustic impedances, underwater and in-air decibel figures cannot be compared directly.
  2. 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.
  3. A-weighting (IEC 61672-1) approximates the ear's lower sensitivity to low and very high frequencies. Its analytic form uses pole frequencies f1 = 20.6 Hz, f2 = 107.7 Hz, f3 = 737.9 Hz and f4 = 12194 Hz with a +2.00 dB normalisation so the response is 0 dB at 1 kHz. C-weighting uses only the f1 and f4 poles (+0.06 dB normalisation), is nearly flat, and is commonly used for peak levels; Z is unweighted.
  4. 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.
  5. Under the Control of Noise at Work Regulations 2005, the lower exposure action values are 80 dB(A) daily or weekly exposure and 135 dB(C) peak; the upper exposure action values are 85 dB(A) and 137 dB(C); the exposure limit values are 87 dB(A) and 140 dB(C), and the limit values take account of hearing protection.
    • The Control of Noise at Work Regulations 2005, Regulation 4 (as made), legislation.gov.uk (The National Archives).(a) a daily or weekly personal noise exposure of 80 dB (A-weighted); and (b) a peak sound pressure of 135 dB (C-weighted).(a) a daily or weekly personal noise exposure of 85 dB (A-weighted); and (b) a peak sound pressure of 137 dB (C-weighted).(a) a daily or weekly personal noise exposure of 87 dB (A-weighted); and (b) a peak sound pressure of 140 dB (C-weighted).Checked 2026-11-02.
    • Noise at work: The Control of Noise at Work Regulations 2005, Health and Safety Executive (HSE).There is also an exposure limit value of 87 dB(A), taking account of any reduction in exposure provided by hearing protection, above which workers must not be exposed.The level at which employers must provide hearing protection and hearing protection zones is 85 dB(A) (daily or weekly average exposure) and the level at which employers must assess the risk to workers' health and provide them with information and training is 80 dB(A).Checked 2026-11-02.
    • Sound advice: Control of noise at work in music and entertainment (HSG260), Health and Safety Executive (HSE).These noise exposures are identical: 80 dB for 8 hours 83 dB for 4 hours 86 dB for 2 hours 89 dB for 1 hour 92 dB for 30 minutesChecked 2026-11-02.

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