Handbook for Sound Engineers

Fundamentals and Units of Measurement 1661

Max Program Level. Max program level is the

maximum program level attainable at a specific point

from the available input power. Max program level is

(48-64)

Lsens. Loudspeaker sensitivity (Lsens) is the on-axis SPL
output of the loudspeaker with a specified power input
and at a specified distance. The most common Lsens are
at 4ft, 1W and 1m, and 1W.

Sa. Sa is the total absorption in sabines of all the
surface areas times their absorption.

dB-SPLT. The dB-SPLT is the talker’s or sound source’s
sound pressure level.

dB-SPLD. The dB-SPLD is the desired sound pressure

level.

dB-SPL. The dB-SPL is the sound pressure level in

decibels.

EIN. EIN is the equivalent input noise.

(48-65)

where,

BW is the bandwidth,

Z is the impedance.

Thermal Noise. Thermal noise is the noise produced in

any resistance, including standard resistors. Any resis-

tance that is at a temperature above absolute zero gener-

ates noise due to the thermal agitation of free electrons

in the material. The magnitude of the noise can be

calculated from the resistance, absolute temperature,

and equivalent noise bandwidth of the measuring

system. A completely noise-free amplifier whose input

is connected to its equivalent source resistance will have

noise in its output equal to the product of amplification

and source resistor noise. This noise is said to be the

theoretical minimum.

Fig. 48-8 provides a quick means for determining the

rms value of thermal noise voltage in terms of resis-

tance and circuit bandwidth.

For practical calculations, especially those in which

the resistive component is constant across the band-

width of interest, use

(48-66)

where,

f 1  f 2 is the 3 dB bandwidth,

R is the resistive component of the impedance across

which the noise is developed,

T is the absolute temperature in K.

RT 60. RT 60 is the time required for an interrupted

steady-state signal in a space to decay 60 dB. RT 60 is

normally calculated using one of the following equa-

tions: the classic Sabine method, the Norris Eyring

modification of the Sabine equation, and the Fitzroy

equation. The Fitzroy equation is best used when the

walls in the X, Y, and Z planes have very different

absorption materials on them.

Sabine:

(48-67)

Norris Eyring:

(48-68)

Fitzroy:

program levelmax 10

wattsavail

10

log------------------------

'D 2 – 'Dref

• 
  

+Lsens

=

EIN –10198 dB logBW 10 logZ

6dB 20–– log0.775

= ++

Figure 48-8. Thermal noise graph.

Resistance— 7

100

10

1.0

0.1

0.01

Thermal noise (23oC)

Erms

noise voltage—

MV

102 2 4 6 10^3 2 4 6 10^4 2 4 6 10^5 2 4 6 106

106

5 × 10

5

105

2 × 10

5 × 10^4

4

104

2 × 10

5

2 × 10

5 × 10^3

3

103

500

100

10

values of

$Hz

Erms= 410 u 23– T f 1 – f 2 R

RT 60 0.049**V

Sa

= ----- -

** 0.161 for SI units

RT 60 0.049** V

–1Sln – a

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

** 0.161 for SI unit.