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Mitsubishi Electric Lossnay series Technical Manual Page 55

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CHAPTER 4
Characteristics
For the supply air diffuser (and return grille) in Fig. 2, PWL
must be corrected for the sound transmission loss from the
duct work (TL) such that:
PWL' = PWL – TL
Item (i) in formula (II) page 48 is the direct sound (
and (ii) is the echo sound (
The number sources of sound 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
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 frequency band, and
calculated values are combined by formula (III) for an
accurate value. (When A-range overall value is required,
subtract A-range correction value from calculated values of
formula (II), and then combine them by formula (III).)
(2) Reducing Lossnay Unit Operating Sound
1) When the airflow of the unit from above the ceiling is the
sound source.
(See page 48: Fig. 1
(A) Do not install the unit using the following specifications if
disturbing sound could be emitted from large units.
(Refer to Fig. 4)
a) Decrease in diameters in the ductwork:
(Ex. 10"dia(ø 250) → 6"dia(ø 150), 8"dia(ø 200) → 4"dia(ø 100))
b) Curves in aluminum flexible ducts, etc.
(Especially if immediately installed after unit outlet)
c)
Opening in ceiling panels
d) Hanging the unit on materials that cannot support
the wight.
(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 high).
b) Adding of soundproofing materials to areas below
the source of the sound.
( T h e e n t i r e s u r fa c e m u s t b e c o v e r e d w i t h
soundproofing sheets. Note that in some cases,
covering the area around the unit may not be
possible due to generated heat.)
U-46
,
).
) ............................. (III)
/10
SPL
/10
+ 10
1
2
,
, Fig. 2
to
,
)
1
3
Transmission Loss in Ceiling Material (dB) Example
Material
Plaster Board
(
( ) indicates
thickness
Average
,
),
125
250
500
1,000
2,000
4,000
Fig. 4. Large Unit Installation (Example)
a)
Fig. 5. Countermeasure (Example)
Plaster Board
Lauan Plywood
) (
)(
1/4 inch
3/8 inch
7 mm
9 mm
20
22
10
12
11
15
19
21
26
28
34
35
42
39
d)
c)
b)
a)
b)
)
1/2 inch
12 mm
23
20
21
23
26
24

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