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# Decibel A, B and C

## Sound pressure filters that compensates for the hearing sensed by the human ear.

The human ear is more sensitive to sound in the 1 to 4 kHz frequency range than to sound at very low or very high frequencies. Regarding noise - higher sound pressures are therefore more acceptable at lower and higher frequencies than in the mid range.

Knowledge about the human ear is important in acoustic design and sound measurement. To compensate for the human hearing sound meters are normally fitted with filters that adapts the measured sound response to the human sense of sound. Common filters are

• dB(A)
• dB(B)
• dB(C)

Download and print Sound Pressure Level - db(A), dB(B) and dB(C) criteria chart

### dB(A)

The decibel A filter is widely used. dB(A) roughly corresponds to the inverse of the 40 dB (at 1 kHz) equal-loudness curve for the human ear.

With the dB(A) filter the sound level meter is less sensitive to very high and very low frequencies. Measurements made with this scale are expressed as dB(A).

Note! The abbreviation dBA or db(A) is not recognized by SI. According to SI - use

"the A weighted sound pressure level is x dB"

or

"LA is x dB"

where

x = the weighted sound pressure level (dB)

Anyway - dBA (or dB(A)) is commonly used.

### dB(B) and dB(C)

The decibel C filter is practically linear over several octaves and is suitable for subjective measurements at very high sound pressure levels. The decibel B filter is between C and A. The B and C filters are seldom used.

### Comparing dB(A), dB(B) and dB(C)

The A, B and C decibel filters compared:

Decibel A, B and C
Relative Response (dB)Frequency (Hz)
31.2562.51252505001000200040008000
dB(A) -39.4 -26.2 -16.1 -8.6 -3.2 0 1.2 1 -1.1
dB(B) -17 -9 -4 -1 0 0 0 -1 -3
dB(C) -3 -0.8 -0.2 0 0 0 -0.2 -0.8 -3

### Example - Measuring dB(A)

If sound pressure is measured at different octaves the resulting dB(A) sound pressure can be calculated by logarithmic addition

Decibel A Filter
Octave band12345678
Center Frequency (Hz)62.51252505001000200040008000
Measured Sound Pressure Level
(dB
54 60 64 53 48 43 39 32
db(A) Filter
(dB)
26 16 9 4 0 -1 -1 1
Resulting Sound Pressure Level dB(A)
(dB
28 44 55 49 48 44 40 31

The resulting db(A) sound pressure can be calculated by logarithmic adding (adding signals with different strengths) of the sound pressure for each octave.

1. Adding octave band 4 and 5 (check this link)

The difference between octave 4 and 5 is

49 dB(A) - 48 db(A) = 1 dB(A)

=> approximately 2.5 db(A) shall be added to the highest value, resulting in

49 dB(A) + 2.5 db(A) = 51.5 dB(A)

2. The resulting value from octave band 4 and 5 can be added to octave band 3

The difference between octave (4, 5) and 3 is

55 dB(A) - 51.5 dB(A) = 3.5 dB(A)

=> approximately 1.5 db(A) shall be added to the highest value, resulting in

55 dB(A) + 1.5 db(A) = 56.5 dB(A)

3. The resulting sound pressure level in octave 1, 2, 6, 7 and 8 is low compared with octave band (4,5 og 3) and can be neglected.

• The resulting sound pressure level can therefore be estimated to approximately 57.5 dB(A)

### db(A), dB(B) and dB(C) Calculation Spreadsheet

Sign in to your Google Account to copy and modify an example spreadsheet with the dB(A), dB(B) and dB(C) calculation and the graph below.

### Adjustments to dB(A)

Adjustments to the base level of 40 dB(A):

Decibel A - Adjustments to Context
dB(A)
Character of sound Tones or impulsive noise readily detectable -5
Tones or impulsive noise just detectable -2
Time of day Evening -5
Night time -10
Neighborhood Rural and outer suburban areas with little traffic 0
Suburban areas with infrequent traffic 5
Suburban areas with medium density traffic 10
Suburban areas with some commerce or industry 10
Areas with dense traffic and/or commerce or industry 15
City or commercial areas with very dense traffic and/or bordering industrial areas 20
Industrial areas and/or extremely dense traffic 25

## Related Topics

• ### Acoustics

Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.
• ### Noise and Attenuation

Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.

## Related Documents

• ### Comparing Flowmeters

A limited comparison of flowmeter principles - regarding service, rangeability, pressure loss, typical accuracy, upstream pipe diameters, viscosity and relative costs.
• ### Decibel

Logarithmic unit used to describe ratios of signal levels - like power or intensity - to a reference level.
• ### Decibel A, B and C

Sound pressure filters that compensates for the hearing sensed by the human ear.
• ### Decibel A, B and C

Sound pressure filters that compensates for the hearing sensed by the human ear.
• ### Human Effects when Exposed to Low-Frequency Noise or Vibration

Physiological effects from low-frequency noise or vibrations.
• ### Maximum Sound Pressure Levels in Rooms

Maximum recommended sound pressure levels in rooms like kindergartens, auditoriums, libraries, cinemas and more.
• ### Noise - Recommended Exposure Limits (REL)

The NIOSH noise Recommended Exposure Limit - REL - for occupational noise
• ### Noise Criterion vs. Noise Rating and dB(A)

Comparing Noise Criterion (NC, NCB, RNC) to Noise Rating (NR) and dB(A).
• ### Notes, Octaves and Frequencies

Frequencies vs. notes and octaves.
• ### Octave Band Frequencies

The octave and the 1/3 octave band frequencies.
• ### Signals - Adding Decibels

The logarithmic decibel scale is convenient when adding signal values like sound power, pressure and others from two or more sources.
• ### Sound - Doppler Effect

The doppler effect is the change in sound frequency due to the relative motion between a source and a listener.
• ### Sound - Frequency, Wavelength and Octave

An introduction to the nature of sound with frequencies, wave-lengths and octaves.
• ### Sound - Hearing Threshold vs. Age

Shift in hearing threshold for men and women vs. age.
• ### Sound - Leq - Equivalent Level

The EPA Equivalent Sound Level - Leq - quantifies the noise environment to a single value of sound level for any desired duration.
• ### Sound - Ls - Exposure Level

The EPA Sound Exposure Level - Ls - describes the noise from a variable source.
• ### Sound Intensity, Power and Pressure Levels

Introduction to decibel, sound power, intensity and pressure.
• ### Sound Pressure

Sound Pressure is the force of sound on a surface perpendicular to the propagation of sound.

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## Citation

• The Engineering ToolBox (2003). Decibel A, B and C. [online] Available at: https://www.engineeringtoolbox.com/decibel-d_59.html [Accessed Day Month Year].

Modify the access date according your visit.

12.8.9