Handbook for Sound Engineers

1346 Chapter 35

Thus the rated sensitivity is

(35-6)

where,

is direct sound pressure,

r is distance to the loudspeaker on the main axis,

r 0 is 1 m distance.

Because of its reference to power, this expression is
also designated as rated power sensitivity. According to
DIN 45570 T1, the logarithmic quantity of this expres-
sion is called characteristic sound level LK, but also
sensitivity/dB. It is defined by

(35-7)

An important parameter for approximating the
sound-field conditions in rooms is the front to random
factor J. It characterizes the relationship between the
acoustic power that would be radiated into the room by
an omnidirectional loudspeaker having the same free-
field sensitivity as the real loudspeaker to be assessed,
and the acoustic power of the real loudspeaker:

(35-8)

where,

is sound pressure ( is measured in the main front

direction),

S is the globe surface around the loudspeaker,

-is the room angle,

for * see Eq. 35-17.

A measuring procedure for ascertaining the front to

random factor was established in the IEC publication

268-5 (1972):^9

(35-9)

where,

is direct sound pressure,

is reverberant sound pressure,

r is distance to the loudspeaker,

rH is the critical distance in the diffuse sound field, see

Eqs. 7-10.^10

The directivity factor Q(-) is often used for this

term, but it is a function of the angle - see Eq. 35-20.

The logarithmic quantity of the front to random

factor is the front to random index

(35-10)

It corresponds to the difference between the free-

field and the diffuse-field sensitivity levels:

(35-11)

where,

Gd is the sensitivity level in the direct field,

Gr is the sensitivity level in the diffuse field.

It is also expressed by the sound levels measured at 1 m

distance in the direct field of the loudspeaker (Ld ) and in

the diffuse field (Lr) of a room having the reverberation

time RT 60 and the volume V,

(35-12)

where,

Ld is the direct sound level,

Lr is the diffuse sound level,

RT 60 is the reverberation time in seconds,

V is the volume of the room in m^3.

An equal input power Pel is taken for granted.

Because of the dimensions of the radiators, the

wavelength of the radiated sound in the lower-

frequency range is long compared to the radiating

surface. Because of this difference there results only an

insignificant directivity. With rising frequency the rela-

tionship changes and directivity increases.

For sound reinforcement purposes it has been proven

in practice that slight increases of approximately

3 dB/octave of the front to random index of the loud-

speaker system is appropriate, because most of the

Ps u ̃^2 eZn.=

EK

p ̃d

u ̃

---- -Znr

r 0

= ----

p ̃d r

PS r 0

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

p ̃d

LK 20

EK

EK 0

= log--------- dB

= 20 logEK dB+94 dB.

J

p ̃ 02 Sd

S

³

p ̃

2

• Sd
  S

³

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

S

*

2

Sd

S

³

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

p ̃ p ̃ 0

J

p ̃d r

p ̃rrH

------------

©¹

§·

2

=

p ̃d

p ̃r

C= 10 logJ dB

CGd–= Gr

CLd–10Lr

RT 60

V

= ++log------------ dB 25 dB