Handbook for Sound Engineers

Fundamentals and Units of Measurement 1653

The lower fixed point (the ice point) is the tempera-
ture of a mixture of pure ice and water exposed to the
air at standard atmospheric pressure.
The upper fixed point (the steam point) is the
temperature of steam from pure water boiling at stan-
dard atmospheric pressure.
In the Celsius scale, named after Anders Celsius
(1701–1744) and originally called Centigrade, the fixed
points are 0°C and 100°C. This scale is used in the SI
system.
The Fahrenheit scale, named after Gabriel Daniel
Fahrenheit in 1714, has the fixed points at 32°F and
212°F.
To interchange between °C and °F, use the following
equations.

(48-25)

The absolute temperature scale operates from abso-
lute zero of temperature. Absolute zero is the point
where a body cannot be further cooled because all the
available thermal energy is extracted.
Absolute zero is 0 kelvin (0 K) or 0q Rankine (0°R).
The Kelvin scale, named after Lord Kelvin (1850), is
the standard in the SI system and is related to °C.

The Rankine scale is related to the Fahrenheit
system.

The velocity of sound is affected by temperature. As
the temperature increases, the velocity increases. The
approximate formula is

(48-26)
where,
T is the temperature in °C.

(48-27)

where,
T is the temperature in °F.

Another simpler equation to determine the velocity of
sound is

(48-28)

Things that can affect the speed of sound are the

sound wave going through a temperature barrier or

going through a stream of air such as from an air condi-

tioner. In either case, the wave is deflected the same

way that light is refracted in glass.

Pressure and altitude do not affect the speed of sound

because at sea level the molecules bombard each other,

slowing down their speed. At upper altitudes they are

farther apart so they do not bombard each other as often

so they reach their destination at the same time.

Thevenin’s Theorem. Thevenin’s Theorem is a method

used for reducing complicated networks to a simple

circuit consisting of a voltage source and a series

impedance. The theorem is applicable to both ac and dc

circuits under steady-state conditions.

The theorem states: the current in a terminating

impedance connected to any network is the same as if

the network were replaced by a generator with a voltage

equal to the open-circuit voltage of the network, and

whose impedance is the impedance seen by the termina-

tion looking back into the network. All generators in the

network are replaced with impedance equal to the

internal impedances of the generators.

Kirchhoff’s Laws. The laws of Kirchhoff can be used

for both dc and ac circuits. When used in ac analysis,

phase must also be taken into consideration.

Kirchhoff’s Voltage Law (KVL). Kirchhoff ’s voltage

law states that the sum of the branch voltages for any

closed loop is zero at any time. Stated another way, for

any closed loop, the sum of the voltage drops equal the

sum of the voltage rises at any time.

In the laws of Kirchhoff, individual electric circuit

elements are connected according to some wiring plan

or schematic. In any closed loop, the voltage drops must

be equal to the voltage rises. For example, in the dc

circuit of Fig. 48-1, V 1 is the voltage source or rise such

as a battery and V 2 , V 3 , V 4 , and V 5 are voltage drops

(possibly across resistors) so

(48-29)

or,

(48-30)

In an ac circuit, phase must be taken into consider-

ation, therefore, the voltage would be

(48-31)

qC qF32– q

5

9

= u---

qF qC^9

5

u---

©¹

=§·+ 32 q

0 qC273.15K=

32 qF=459.67qR

C=331.4 m/s+0.607T SI units

C=1052 ft/su1.106T U.S. units

C=49.00 459.69qqF+

V 1 =V 2 +++V 3 V 4 V 5

V 1 – V 2 – V 3 – V 4 –0V 5 =

V 1 e^ jZt–V 2 e^ jZt–V 3 e^ jZt–V 4 e^ jZt–0V 5 e^ jZt=