Handbook for Sound Engineers

Fundamentals and Units of Measurement 1651

S is the conductance in siemens.

Electric Inductance (L). Electric inductance is the
property that opposes any change in the existing
current. Inductance is only present when the current is
changing. The henry (H) is the unit of inductance and is
the inductance of a circuit in which an electromotive
force of 1 V is developed by a current change of 1 A/s.

(48-13)

Frequency (f). Frequency is the number of recurrences
of a periodic phenomenon in a unit of time. The hertz
(Hz) is the unit of frequency and is equal to one cycle
per second, 1 Hz = 1 cps. Frequency is often measured
in hertz (Hz), kilohertz (kHz), and megahertz (MHz).

Sound Intensity (W/m^2 ). Sound intensity is the rate of
flow of sound energy through a unit area normal to the
direction of flow. For a sinusoidally varying sound
wave the intensity I is related to the sound pressure p
and the density ß of the medium by

(48-14)

where,
c is the velocity of sound.

The watt per square meter (W/m^2 ) is the unit of

sound intensity.

Magnetic Flux (I). Magnetic flux is a measure of the

total size of a magnetic field. The weber (Wb) is the

unit of magnetic flux, and is the amount of flux that

produces an electromotive force of 1 V in a one-turn

conductor as it reduces uniformly to zero in 1 s.

(48-15)

Magnetic Flux Density (ß). The magnetic flux density

is the flux passing through the unit area of a magnetic

field in the direction at right angles to the magnetic

force. The vector product of the magnetic flux density

and the current in a conductor gives the force per unit

length of the conductor.

The tesla (T) is the unit of magnetic flux density and

is defined as a density of 1 Wb/m^2.

(48-16)

Luminous Flux ()v). Luminous flux is the rate of flow

of radiant energy as evaluated by the luminous sensa-

tion that it produces. The lumen (lm) is the unit of lumi-

nous flux, which is the amount of luminous flux emitted

by a uniform point source whose intensity is 1 steradian.

(48-17)

where,

cd is the luminous intensity in candelas,

sr is the solid angle in steradians.

Luminous flux Density (Ev). The luminous flux density

is the luminous flux incident on a given surface per unit

area. It is sometimes called illumination or intensity of

illumination. At any point on a surface, the illumination

is given by

(48-18)

The lux (lx) is the unit of luminous flux density,

which is the density of radiant flux of lm/m^2 ,

(48-19)

Displacement. Displacement is a change in position or

the distance moved by a given particle of a system from

its position of rest, when acted on by a disturbing force.

Speed/Velocity. Speed is the rate of increase of distance

traveling by a body. Average speed is found by the

equation

(48-20)

where,

H Vs

A

----- -=

I p

2

Ec

----- -=

Wb= W s

= 108 lines of flux

kg m

2

s^2 A

------------------.=

T Wb

m^2

---------=

Vs

m^2

=-----------

kg

s

2

A

---------.=

lm cd sr

m

©¹------^2

= §·

=0.0795774 candlepower

Ev

d)v

dA

----------=

lx lm

m^2

=------

cd

sr

m^2

= ------

=0.0929030 fc

S l

t

-=