Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Decibel A, B and C

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

Sponsored Links

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)

Sound - Decibel A, B and C Criteria

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:

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

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.

dbA decibel calculationAdjustments to dB(A)

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

ContextAdjustment
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

Sponsored Links

Related Topics

Related Documents

Sponsored Links

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro .Add the Engineering ToolBox extension to your SketchUp from the SketchUp Pro Sketchup Extension Warehouse!

Translate
About the Engineering ToolBox!

Privacy

We don't collect information from our users. Only emails and answers are saved in our archive. Cookies are only used in the browser to improve user experience.

Some of our calculators and applications let you save application data to your local computer. These applications will - due to browser restrictions - send data between your browser and our server. We don't save this data.

Google use cookies for serving our ads and handling visitor statistics. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected.

AddThis use cookies for handling links to social media. Please read AddThis Privacy for more information.

Citation

This page can be cited as

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

Modify access date.

. .

close

3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Scientific Online Calculator

Scientific Calculator

3 30

Sponsored Links
.