Mitsubishi Electric Lossnay LGH-15RX3 Technical Manual page 51

CHAPTER 4 G Characteristics
4.4 Indoor noise
(1) Principle of indoor noise
1) Power levels
The Power level (PWL) of the sound source must be
understood when considering noise effects.
The following formula is used to obtain PWL from the
measured sound pressure data (values noted in catalog) in
an anechoic chamber.
PWL = SPLo + 20 logro + 11 [dB] . . . . . . . . . . . . . . . . . . . . (I)
PWL : Sound source power level (dB)
SPLo : Measured sound pressure in anechoic
chamber (dB)
ro : Measurement distance (m)
2) Principal model
Consider the room shown in Figs. 1 and 2.
G
Fig. 1 shows an example of the integrated main unit and
supply air diffuser (and return grille). This is equivalent to
the cassette-type Lossnay.
Fig. 2 shows an example of a separated main unit and
supply air diffuser (and return grille). This is equivalent to
the ceiling embedded-type Lossnay.
G
is the direct sound from the supply air diffuser (return
grille) and is the echo sound.
sound that is emitted from the main unit and duct and
passes through the finished ceiling and leaks.
echo sound of .
3) Setting of noise
G
The following formula is used to obtain the noise value at a
position in the room.
SPL [dB] = PWL + 10 log
SPL : Sound pressure level at reception point [dB]
PWL : Sound source power level [dB]
Q : Directivity factor (Refer to Fig. 3)
r : Distance from sound source [m]
: Room constant [R = αS/(1 – α)]
R
α
: Average sound absorption ratio in room
(Normally, 0.1 to 0.2)
S : Total surface area in room [m
48
( to ) is the direct
1 3
{ }
Q 4
+ ........................(II)
2
4πr R
(i) (ii)
2
]
Fig. 1
Supply air diffuser
(return grille)
Fig. 2
Main unit
is the
Fig. 3 (Sound source position and directivity factor Q)
Sound source position
a
b
c
d
Main unit
r
o
Supply air diffuser
(return grille)
b
c
a
Centre of room
Centre of ceiling
Edge
Corner
d
Q
1
2
4
8