# Mitsubishi Electric Lossnay series Technical Manual Page 54

For north america.

4.4 Indoor Sounds
(1) Indoor Sounds Principles
1) Power Levels
The Power level of the sound source (PWL) must be
understood when considering the effects of sound.
See formula below to obtain PWL from the measured sound
pressure data in an anechoic chamber.
PWL = SPLo + 20 log (
PWL : Sound source power level (dB)
SPLo : Measured sound pressure in anechoic
chamber (dB)
ro
: Distance from the unit to measuring point (ft,m)
2) Principal Model
Consider the room shown in Figs. 1 and 2.
Fig. 1 shows an example of an integrated unit (similar to a
cassette-type Lossnay unit) and supply air diffuser (with
return grille).
Fig. 2 shows an example of a separated unit (similar to a
ceiling-embedded type Lossnay unit) and supply air diffuser
(with return grille).
is the direct sound from the supply air diffuser (return
grille), and
is the echo sound.
sound emitted from the unit and duct that can be detected
through the finished ceiling.
3) Position of Sound Source and Sound Value
SPL [dB] = PWL + 10 log
SPL : Sound pressure level at reception point [dB]
PWL : Power level of sound source [dB]
Q
: Directivity factor (Refer to Fig. 3)
r
: Distance from sound source [ft,m]
R
: Room constant [R = α S/(1 – α )]
α
: Average sound absorption ratio in room
(Normally, 0.1 to 0.2)
S
: Total surface area in room [ft
r
o) + 11 [dB] . . . . . . . . . . . . . . . . . . .(I)
(
to
) is the direct
1
3
is the echo sound of
{
}
Q
4
+
.........................(II)
2
4πr
R
(i)
(ii)
2
2
,m
]
CHAPTER 4
Fig. 1.
Supply air diffuser
(return grille)
Fig. 2.
Unit
.
Fig. 3. (Position of Sound Source and Directivity Factor Q)
c
Position of Sound Source
a
b
Center of ceiling
c
d
Characteristics
Unit
r
o
Supply air diffuser
(return grille)
b
a
Center of room
Edge
Corner
d
Q
1
2
4
8
U-45  