Mitsubishi Electric Lossnay LGH-15RX3 Technical Manual page 52

G
For the supply air diffuser (and return grille) in Fig. 2, PWL
must be corrected for the noise alternation provided by the
duct work (TL) such that:
PWL' = PWL – TL
G
Item i in formula (II) is the direct sound (
the echo sound (
G
The number of sound sources in the room (main unit,
supply air diffuser, return grille etc.) is obtained by
calculating formula (II), and combining the number with
formula (III).
SPL
SPL = 10 log (10
G
The average sound absorption rate in the room and the
ceiling transmission loss differ according to the frequency,
so formula (II) is calculated for each frequwncy band, and is
combined with formula (III) for an accurate value.
(2) Avoiding noise disturbance from
Lossnay unit
1) When unit air passage behind ceiling is sound source
(Fig. 1 , , Fig. 2
(A) Avoid the following types of construction when disturbing
noise may be emitted from large units. (Refer to Fig. 4)
a) Sudden contraction of duct diameter
ø 250 → ø 150, ø 200 → ø 100)
(Ex.
b) Sudden curves in aluminum flexible ducts, etc.
(Especially right after unit outlet)
c) Opening in ceiling plates
d) Suspension on weak material
(B) The following countermeasures should be taken.
(Refer to Fig. 5)
a) Use ceiling material with high soundproofing
properties (high transmission loss). (Care is required
for low frequency components as the difference in
material is great).
b) Addition of soundproofing material to areas below
sound source.
(The entire surface must be covered when using
soundproofing sheets. Note, that in some cases,
covering of the area around the unit may not be
possible due to the heat generated from the unit.)
,
, ).
) ............................ (III)
/10 SPL /10
+ 10
1 2
to , )
1 3
Transmission loss in ceiling material (dB) Example
Material ( )
indicates
thickness (mm)
Average
), and ii is
125
250
500
1,000
2,000
4,000
Fig. 4
Fig. 5
CHAPTER 4 G Characteristics
Plaster Plaster
board (7) board (9)
20 22
10 12
11 15
19 21
26 28
34 35
42 39
a) d) c) b)
a) b)
Lauan
plywood (12)
23
20
21
23
26
24
—
49