 
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 fullscale. 

dB SPL  A measure of sound pressure level. 
A few easytoremember 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 16bit systems is 96 dB. For 20bit
digital audio it is 120 dB. For 24 bit digital audio it is 144 dB.
Fullscale 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 fullscale 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 Aweighted sound level represents the human hearing and hearing damage in
the possible best way. Without any other information the Aweighted sound level
is the best information available for measuring noise problems. See the
discussion of Aweighted measurement below and also
see Speech Level.
AWeighting 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 
7076dB 
6571dB 
5864dB 
5561dB 
5258dB 
5056dB 
Source: KlarkTeknik Audio System Designer
