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Understanding dB

dB is an abbreviation for "decibel". One decibel is one tenth of a Bel, named for Alexander Graham Bell.  The measurement quoted in dB describes the ratio (10 log power difference, 20 log voltage difference, etc.) between the quantity of two levels, the level being measured and a reference.  To describe an absolute value, the reference point must be known. There are a number of different reference points defined.  Here are a few:
dBV represents the level compared to 1 Volt RMS. 0dBV = 1V. There is no reference to impedance.
dBu represents the level compared to 0.775 Volts RMS with an unloaded, open circuit, source (u = unloaded).
dBm represents the power level compared to 1 mWatt. This is a level compared to 0.775 Volts RMS across a 600 Ohm load impedance.  Note that this is a measurement of power, not a measurement of voltage.
dbFS - relative to digital full-scale.
dB SPL - A measure of sound pressure level.

A few easy-to-remember facts that may help:
If you're dealing with voltage measurments, convert from dBV to dBu: 1dBV equals +2.2dBu.
+4dBu equals 1.23 Volts RMS.
The reference level of -10dBV is the equivalent to a level of -7.8dBu.
+4dBu and -10dBV systems have a level difference of 11.8 dB and not 14 dB. This is almost a voltage ratio of 4:1 (Don't forget the difference between dBu and dbV  !!)

dBFS - dB Full Scale

0 dBFS represents the highest possible level in digital gear. All other measurements expressed in terms of dBFS will always be less than 0 dB (negative numbers).
0 dBFS indicates the digital number with all digits ="1", the highest possible sample.
The lowest possible sample is (for instance for 16 bit audio):
0000 0000 0000 0001, which equals -96 dBFS.  Therefore the dynamic range for 16-bit systems is 96 dB.  For 20-bit digital audio it is 120 dB.  For 24 bit digital audio it is 144 dB.

Full-scale input level is the analog input voltage level that will cause the A/D converter to just equal full scale with no clipping on either positive or negative peaks.

Output full scale is defined as the analog output voltage produced while playing a 997 Hz digital full-scale sine wave, assuming the THD+N is less than -40 dB relative to the signal level.

The dynamic range of a digital system is the ratio of the full scale signal level to the RMS noise floor.

 

Sound Pressure Level

The definition of dB SPL is the 20 log of the ratio between the measured sound pressure level and the reference point.  This reference point is defined as 0.000002 Newtons per square meter, the threshold of hearing.  However, the threshold of hearing (and sensitivity to level) changes by frequency and for soft and loud sounds, as discovered by Fletcher and Munson in 1933, shown in the graph below:

Note that human hearing is relatively insensitive to low bass (below 100 Hz), and also compresses at higher sound levels.


Here are some typical sounds, and their levels.

Sounds dB SPL
Rocket Launching 180
Jet Engine 140
Thunderclap, Air Raid Siren 1 Meter 130
Jet takeoff (200 ft) 120
Rock Concert, Discotheque 110
Firecrackers, Subway Train 100
Heavy Truck (15 Meter), City Traffic 90
Alarm Clock (1 Meter), Hair Dryer 80
Noisy Restaurant, Business Office 70
Air Conditioning Unit, Conversational Speech 60
Light Traffic (50 Meter), Average Home 50
Living Room, Quiet Office 40
Library, Soft Whisper (5 Meter) 30
Broadcasting Studio, Rustling Leaves 20
Hearing Threshold 0

The A-weighted sound level represents the human hearing and hearing damage in the possible best way. Without any other information the A-weighted sound level is the best information available for measuring noise problems.  See the discussion of A-weighted measurement below and also see Speech Level.


A-Weighting dB(A), Relationship between Frequency and Level

A standard for noise measurement that takes into consideration the human ear's sensitivity to certain frequencies.  This is expressed as part of noise specifications and can be denoted by adding the letter 'A' to the spec - i.e. 15dBA.
(Sweetwater Archive)


10Hz 12,5Hz 16Hz 20Hz 25Hz 31,5Hz 40Hz 50Hz
-70,4dB -63,4dB -56,7dB -50,5dB -44,7dB -39,4dB -34,6dB -30,2dB
 
63Hz 80Hz 100Hz 125Hz 160Hz 200Hz 250Hz 315Hz
-26,2dB -22,5dB -19,1dB -16,1dB -13,4dB -10,9dB -8,6dB -6,6dB
 
400Hz 500Hz 630Hz 800Hz 1kHz 1,25kHz 1,6kHz 2kHz
-4,8dB -3,2dB -1,9dB -0,8dB 0dB +0,6dB +1,0dB +1,2dB
 
2,5kHz 3,15kHz 4kHz 5kHz 6,3kHz 8kHz 10kHz 12,5kHz
+1,3dB +1,2dB +1,0dB +0,5dB -0,1dB -1,1dB -2,5dB -4,3dB
 
16kHz 20kHz
-6,6dB -9,3dB


Speech Level

As a reference, here are the SPLs for two persons talking (not shouting) at various differences (level at the receiver's ear).:
    0,25m      0,5m         1m       1,5m         2m         3m
70-76dB 65-71dB 58-64dB 55-61dB 52-58dB 50-56dB


Source: Klark-Teknik  Audio System Designer

 

 

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This page last updated 7/31/2007