Units, conversion factors, and physical constants
This section contains information on units (immediately below), physical constants, conversion factors, and miscellaneous data related to air and sound.
To the list of Physics of sound subsections.
Fundamental units
Quantity 
Symbol 
Units 
Dimensions 
Time 
t 
Seconds 
[s] 
Length 

Meters 
[m] 
Mass 
M 
Kilograms 
[kg] 
Charge 
q 
Coulombs 
[q] 
Other physical quantities
Quantity 
Symbol 
Units 
Dimensions 
Weight 

Kgweight 
[kgms^{2}] 
Force 
F 
Newtons (N) 
[kgms^{2}] 
Velocity 
v 

[m s^{1}] 
Acceleration 
a 

[ms^{2}] 
Energy/Work 
E 
Joule (J) 
[kgm^{2}s^{2}] 
Power 

Watt (W) 
[kgm^{2}s^{3}] 
Density 
ρ 

[kgm^{3}] 
Pressure 
P 
Pascal (Pa) 
[kg m^{1}s^{2}] 
Sound intensity 
I 
Wm^{2} 
[kg s^{3}] 
Temperature 
T 
^{0}K 
none 
Volume 
V 

[m^{3}] 
Moles 
n 
mol 
none 
Electrical quantities
Magnetic field 
B 
Tesla = Weberm^{2} 
[kgs^{1}q^{1}] 
Electric field 
E 
Voltsm^{1} 
[kgms^{2}q^{1}] 
Electric current 
I 
Amps 
[qs^{1}] 
Updated 3/18/98 based on values provided by NIST
Constant 
Symbol 
Value 
Units 
Dimensions 
Avogadro's number 
N 
6.0221367x 10^{23} 
mol^{1} 
none 
Gas constant 
R=Nk 
8.31451 
J^{0}K^{1}mol^{1} 
[kgm^{2}s^{2}] 
Boltzmann's constant 
k 
1.380658 x 10^{23} 
J^{0}K^{1} 
[kgm^{2}s^{2}] 
Standard molar volume 
V_{0} 
.0224141 
m^{3}mol^{1} 
[m^{3}] 
Planck's constant 
h 
6.62606876x10^{34} 
Js 
[kgm^{2}s^{1}] 
Length
1m = 3.2808 ft
1 km = .62137 mi.
1 Angstrom ≡ 10^{10} meters = 3.937 x 10^{9} inch
Volume
1 m^{3} = 1000 liters = 35.315 ft^{3}
Mass
1 kg = 2.20462 lbs. avoirdupois
Velocity
1 ms^{1} = 3.6 km/hour = 2.2369 MPH
Force
1 Newton = 10^{5} Dynes = .10197 kgweight = .22481 lbwt
1 kgwt = 2.20462 lb.wt = 70.93 poundals=9.80665 Newtons
Acceleration due to gravity @ sea level, 45deg lat
1 g = 9.80621 ms^{2} = 32.1725 fts^{2}
Energy
1 Joule ≡ 1 watts = .10197 kgmeter = 10^{7} ergs
Temperature
Zero ^{0}C = 273.15 ^{0}K
72 ^{0}F = 22.2 ^{0}C = 295.4 ^{0}K
Pressure
1 atmosphere = 1.0332x10^{4} Kgweightm^{2} = 2116.2 lbs.ft2 = 14.7 psi = 76 cm Hg
= 1.0132x10^{5} Pa = 197.1 dB SPL (Swing from total vacuum to 2atm)
1 Pa 1 Nm^{2} = 93.98 dB SPL = .000145 psi
.00002 Pa RMS ≡ 0 dB SPL RMS
1 Bar ≡ 10^{6} Dynecm^{2} = 10^{5} Pa = 193.98 dB SPL
Warning! Sometimes authors define 1 Bar ≡ 1 Dynecm^{2} = 73.98 dB SPL
Sound intensity
Reference level ≡ 10^{12} Wm^{2}
1 Wm^{2 }= 120 dB above reference, produces a pressure level of 20.7 Pa
Atmosphere=78% N_{2}, 21% O_{2} by volume
N_{2} 1.2506 kgm^{3} at 1 atm, 0^{0} C
Dry air @ 20^{0} C, 1 atm^{ }= 1.205 kg m^{3}
N atomic weight 14.0067, atomic radius .74 Ang
Speed of sound 331.36 ms^{1} at 1 Atm, 0^{0}C.
For other temperatures T in ^{o}C, multiply by sqrt(1+T/273.16)
Characteristic impedance of air Z_{o}=428 rayls at 1 Atm 0^{0}C, 413.3 at 20^{0}C
Molecular constants
Average molecular velocity, air 463 ms^{1} at 20^{0} C
N_{2} at 20^{0} C, 1 atm:
Mean free path, 8.21, 9.29 x10^{8} m
Collision frequency 5070 x 10^{6 }s^{1}
Molecular diameter 3.3 x 10^{10} m
Molecular density, 2.687 x 10^{25} m^{3}