Measuring and Predicting Rainfall Noise-reduction Effects of
Shizuka-Ace ®
by Hiroshi Ikeda *, Takashi Yoshida *2 and Kazuo Mimura *2

We measured the noise of rain striking the metal roof of a martial arts gymnasium. This gymnasium is located in Tsuruoka-city Yamagata-prefecture in Japan.
The points measured are two buildings: one is the main building of this gymnasium, and the otheris an archery range annexed to the main building.
The roof of the main building is made of metal, which is steel laminated with our productShizuka-Ace ® to increase its damping property.
The roof of the archery gymnasium is made of steel, but is not treated with a damping material(i.e., conventional metal roof). Through this measurement, we clearly confirmed the effectivenessof noise-reduction; the level of the noise at the roof treated with the damping material was around
3 dB lower than that at the conventional metal roof.
In addition to this result, we anticipated that the noise level would increase 5 dB if precipitation
doubled.
From these results we estimated noise levels within a large space using a sound propagation
equation from the plane sound source.

ABSTRACT1.

INTRODUCTION

The noise of rain striking a metal roof is much greater thanthat on a slate or tiled roof, and this may cause problems
for a house-owner and neighbors. Therefore, there wasdemand for a metal roof that reduced this noise.
At Furukawa Electric, we have developed Shizuka-Ace,which is a heat-insulating material with damping properties for a metal roof that reduces the noise of rain and isalso lightweight with good fabricating properties and easeof installation.
Shizuka-Ace is composed of three layers. The first is alayer of damping resin, the second is made of aluminum
sheet, and the third is cross-linked polyethylene foam.
We laminate Shizuka-Ace directly to the inside surface
of a metal roof by making use of the adhesive propertiesof the damping resin. The functions of Shizuka-Ace are
obtained from the sandwiched structure, with the damping
resin between the aluminum sheet and the metal of theroof. As a result, this structure can efficiently absorb
vibrating energy caused by striking rain as internal attenuation.
In addition to this damping property, Shizuka-Ace also
can function as a heat-insulator to prevent dew from being
generated on the surface of a metal roof because of the

Photo 1

View of Chouyo martial arts gymnasium (main building).

Photo 2

View of Chouyo martial arts gymnasium (archerybuilding).

*

Polymer Material Technology Center, Ecology and Energy
Lab.
*2 Engineering Service Gr., Production Dept., Foamed Products
Div.

81

2.9 mSheet metalroofing

6.5 m

Damping layer, 135 µm

12.5 mAluminum foil, 150 µm

Figure 1

Thermal insulation, 4.0 mm(Cross-linked polyethylene foam)

Cross-section of Chouyo martial arts gymnasium.

Damping material, 4.285 mm
20 m

Figure 4
Japanese archery
20 W×17 L×6 H

Schematic illustration of constrained damping materials (Shizuka-Ace).

17 m

87 m

2.

Stage

Martial arts gym
42.7 W×35 L×21.9 H

71 m

Japanesearchery

Shooting area
Kendo gym

Judo gym

Figure 2

Plan view of Chouyo martial arts gymnasium (1stfloor).

Martial arts gym

Sheet metal roofing, 0.4 t
Damping material,
4.0 t

Wood-wool cement,
25.0 tFigure 3

Hall

Japanese archery

Wood-wool cement,
25.0 tAsphalt under-roofing,
0.8 t

Schematics of construction of roof structures.

heat-insulating property of cross-linked polyethylene foam.
So, Shizuka-Ace can function as both a damping andheat-insulating material for a metal roof.
We evaluated the properties of Shizuka-Ace in anexperiment with a small sample using a water-drop vibrating source or artificial rainfall equipment; however, wehave few measurements of its effectiveness when applied
to actual buildings.
Fortunately, we had an opportunity to take measurements at a martial arts gymnasium, which is located in
Tsuruoka-city, Yamagata-prefecture, Japan and in which
Shizuka-Ace is applied. In this gymnasium, we simultaneously measured sound pressure levels at inside spaces of
two buildings to compare the effects of damping treatment
on a metal roof. One is the main building whose roof istreated with Shizuka-Ace, and the other is a building forJapanese archery annexed to the main buildings whoseroof is not treated (i.e., conventional installation).

82

Measurement

2.1 Specification of Measured Roof
We measured noise inside Chouyo martial arts gymnasium caused by rain striking the metal roof. Figures 1 and 2
are first floor plans of Chouyo martial arts gymnasium.
Measurements were taken at two buildings. One is in
the main building where the roof of the main martial arts
room is treated with Shizuka-Ace. The other is a room inthe building for Japanese archery, where it is annexed tothe main building and the roof is not treated with a damping material (i.e., conventional installation).
The dimensions of the measured space are as follows,• Dimensions of main martial arts:
42.7 W/ 35.0 L/ 21.9 H (23,730 m2)• Dimensions of Japanese archery gymnasium:
20.0 W/ 17.0 L/ 6.0 H (2,040 m2)
We divided the main martial arts room into small threespaces with mobile partitions for the convenience of measurements.
We show schematics of the construction of the roof
structures in Figure 3, from which it can be seen that both
structures are almost same except for the use of ShizukaAce; however, the building for Japanese archery has an
air-space between the roof and the ceiling.
The specification of Shizuka-Ace, which is one of ourheat-insulating products, is cross-linked polyethylene
foam (4 mmt, 0.10 kg/m2), aluminum sheet (0.15 mmt,
0.43 kg/m2) for the constrained layer, acric resin (0.135
mmt, 0.15 kg/m2) for viscoelastic damping treatment, andpolyethylene-coated paper for separator. A schematic
illustration of Shizuka-Ace is shown in Figure 4.
To show the damping property, we show the frequency
dependency of the loss factor of Shizuka-Ace in Figure 5.
These data are obtained from a test using a small steelbeam (0.4 mmt, 300 mm x 300 mm) by mechanical
impedance method.
2.2 Measuring Method
We set three noise-meters along vertical positions underthe measured parts of the roof as shown in Figure 6.The
first noise-meter was set 0.3 m from the surface of theroof, the second was set at the middle of the space 5.0 m
from the surface of the roof, and the third was set at astanding position 20 m from the surface of the roof. In theJapanese archery gymnasium we set one noise-meter 0.3

Furukawa Review, No. 21

20021.E+01

Damped beam

Loss factor

1.E+00

1.E-01

1.E-02

Steel beam
1.E-03

1.E-04

10

100

1000

Frequency (Hz)Figure 5

Experimental results of loss factors for beam platesof damping material and steel roof.

Point 3 (Under the A-type roofing, 0.3 m)
Point 2 (Under the A-type roofing, 0.5 m)

Noise analyzer

Point 4
(Under the B-type roofing, 0.3 m)

Photo 3
Data recorderFigure 6

Schematics of measurement position and set-up formeasurements.

m from the surface of the roof .
We used the same type of noise-meter produced byRion Co. (Rion NL-05) and recorded the noise using adata recorder produced by Sony (PC208Ax). We also
monitored precipitation using a rain collector produced by
Davis. Then we measured rain noise simultaneously.
After gathering these data, we analyzed the noise byfrequency using Rion Co. (Rion SA-30). We analyzed thesound pressure level and the A-weighted sound pressurelevel (we call this the noise level) according to the onethird-octave band.
Photos 3 and 4 were taken when the measurements
were performed.
To check acoustic properties of the measured locations,
we generated white noise from a speaker positioned ateach location and calculated the reverberation time of thespace from the acoustic property measurements as the
all-path value after switching off the noise generator.

3.

Measurement points in the main building.

Point 1
(Under the A-type roofing, 20.0 m)

RESULTS OF MEASUREMENT

3.1 Frequency Dependency
When the precipitation reached 4.9 mm/hr, 7.7 mm/hr,
10.4 mm/hr, 15.7 mm/hr, and 17.1 mm/hr, we analyzed
the noise measured at both locations --one treated with adamping material and one not treated, using a frequency
analyzer. Then we evaluated the noise level at the frequency in accordance with the center frequency of the
one-third-octave band. We show the noise level measured

Photo 4

Measurement points in the Japanese archery building.

for the precipitation of 15.7 mm/hr in Figure 7. We can see
a clear difference between the two types of roof with the
exception of 800 Hz, especially at the medium and highbands of noise frequency. This result agreed with the frequency loss factor shown in Figure 5. In addition, we plotthese data on the NC-Curve in Figure 8. From the plot, thenoise level at the middle of the measured space and thestanding position under the roof treated with damping
material is at a low level of NC-50, and a level would not
disturb our conversations in this space during heavy rain.
3.2 Precipitation Dependency
We plot each noise level data for both types of roof in
Figure 9 with an approximated curve and the equation on
the plotted curve.

833.3 Reverberation TimeWe show the reverberation time for each measured pointin the martial art room and the Japanese archery gymnasium in Table 1 as all-path values. These values were calculated from acoustic property measurements as averages of values measured three times.

P1: 20 m
P2: 5 mP3: 0.3 m
P4: 0.3 m

50

B-type roof

40
A-type roof

PREDICTION OF ROOM ACOUSTICS

4.1 Theory of Room Acoustics
If the measured room is assumed to be a perfect diffuse
field, the energy density of noise generated by raindrops
in the room is the same at all parts of the room. However,the noise level of the measured point, which is near thesound source of the roof, is actually higher than of all theothers.
As the sound comprises direct sound from the soundsource and reflected sound from the surrounding wall
mixed in the actual room, the sound pressure level differs
at different parts of the room. We can assume that thenoise is radiated from the roof equally and the plane comprises infinite point sources on the roof.
As shown in Figure 10, in general, under a plane source
having a rectangular shape (a × b), the sound pressure30
20

1/3 octave-band frequency (Hz)Figure 7

Results of typical one -third octave band frequency
spectra.
A-weighted sound pressure level (Precipitation: 15.7mm/hr)

P1: 20 m
P2: 5 mP3: 0.3 m
P4: 0.3 m

65

A-weighted S.P.L. (dB(A))

5000

4000

3160

2800

2000

1600

1250

800

1000

630

400

500

315

250

200

160

125

10

AP

70

100

A-weighted S.P.L. (dB(A))

60

4.

y = 7.8 Ln (x) + 33.9
R2 = 0.974

60
y = 7.0 Ln (x) + 34.8
R2 = 0.936

y = 8.3 Ln (x) + 35.9
R2 = 0.925

5550

y = 6.7 Ln(x) + 34.5
R2 = 0.921

45
40
0

10

100

Precipitation (mm/h)
90

Figure 9

P1: 20 m
80

P2: 5 mA-weighted S.P.L. (dB(A))

P3: 0.3 m
70

Relationship between fitting curve and overall Aweighted sound pressure level at each position as afunction of precipitation.

P4: 0.3 m

1

NC-65

60

NC-60

4

NC-55

50

dx

b

NC-50

dy

NC-45

40

NC-40

yi
2

NC-35

30

NC-30

3

a

NC-25

20

xi

NC-20

10
63

125

250

500

1000

2000

r

4000

e

Octave-band frequency (Hz)Figure 8

Determination of Noise Criterion at each position.
(Precipitation: 15.7 mm/hr)

Table 1
Measurement points

A-type roof

B-type roof

84

riFigure 10 Calculation model for evaluating sound propagationfrom a finite large plane radiating source.

Results of reverberation time at each measurement point.

Reverberation time

Absorption coeff.

Room constantP1 : 20m

2.98

0.274

2410

P2 : 5m

2.51

0.325

3079

P3 : 0.3m

2.39

0.341

3314

P4 : 0.3m

1.83

0.158

211

Room volume (m3)Room area (m2)

32730

6392

2040

1124

Furukawa Review, No. 21

2002the predicted values measured just under the roof, midway to the roof, and standing position, tended to decrease
gradually by 3~4 dB.

P4Relative S.P.L. (dB)0

P1: 20 m
P2: 5 mP3: 0.3 m
P4: 0.3 m

-5
P3

-10

5.

-15
P1
-20
0

5

10

15

20

Precipitation (mm/h)

Figure 11 Comparison between predicted and measured relative sound pressure levels as a function of precipitation.

level of the observed point located r vertical distance fromthe sound source is given by the following equation.
L r = L w - 8 + 10log 1 0 U
Where,
Lr: Sound pressure level at the observed position(dB)
Lw: Power level of the sound source per unit area(dB)
U: Approximated equation of distance attenuation of
the plane source having a rectangular shape
U= π /2 ln(A+ √ 1+A2 )(B+ √ 1+B2 )A+B+ √ 1+A2+B2+kAB

A = a / r , B = b/ r
k= 0 . 7 2 7
It is possible to predict the sound pressure level if the
distance r from the plane source is given.
The sound power level of the sound source per unit
area Lw given in the above formula can be calculated fromthe following procedure.
We calculate the total sound power level of the planesource from the measured sound pressure level in the diffuse field and the sound absorption level calculated by thereverberation method. Then, we convert the total soundpower level into the power level of the sound source perunit area by dividing it by the area of the plane source.
4.2

DISCUSSION

P2

Comparison of Predicted Value with Measured
ValueFirst, we converted both the sound pressure level at eachmeasured point and the predicted level into the A-weighted noise level and we standardized each level to be 0 dB
in the case of the noise level of 15.7 mm/hr precipitation at
the Japanese archery gymnasium. We show the comparison as the relative sound pressure level through theabove standardization in Figure 11.
From this standardization, the predicted relative soundpressure level agreed with the measured one. (This is
attributed to the fact that measurement was done at one
point only.) In the martial arts main room, we found thatIn this measurement, we clearly confirmed that the effectiveness of the roof treated with a damping material was 3
dB lower than that of the roof not treated with damping
material. Naturally, the effect varies whether or not there
is a ceiling and depends on sound absorption. However,as we measured the noise level at just 0.3 m from the roof
in both buildings, so we can easily isolate the effects of
Shizuka-Ace.
Of course, increasing precipitation increases the overall
A-weighted noise level, regardless of the types of roof. We
estimate an increased level of 5 dB if precipitation is doubled. In addition, when comparing each coefficient ofnoise-increasing rate, the value is larger for the roof nottreated with damping material than the roof treated with
damping material. Therefore, the noise increase for theroof not treated with damping material is greater than thatof the roof treated with damping material.
When we evaluated the sound pressure level for 15.7mm/hr of precipitation, in the building for Japanesearchery (roof treated with damping material) the NC value
reached NC-55, but in the martial arts building the NC
value was NC-52. This means that the sound power levelfrom the roof was halved. The sound pressure level in themartial arts building just under the roof, the mid-way to theroof, and standing position each differed by 2 dB. If theroof of the martial arts building had not been treated with
damping material, the NC value would have been over
NC-50 mid-way to the roof and also at a standing position.
We think that treating the roof with damping material
keeps the NC value under NC-50. Therefore, we judge
that noise level can be kept at a level at which our conversation would not be disturbed during heavy rain in themartial arts building at both positions: mid-way to the roof
and at a standing position.
As shown the predicted value in Figure 10, in terms of
relative sound pressure levels, these fit in the measured
values, and the difference between the predicted value
and the measured value is within 4 dB; however, we cansee that the predicted values are overestimated by distance attenuation. Therefore, it is necessary to study further how to calculate the total sound power level from theroof, sound diffusion property, and reverberation property
in the room.

6.

SUMMARY

We confirmed that the noise level for the roof treated with
damping material was 3 dB lower than that of the roof nottreated with damping material, and that as precipitation
increased, the noise level became higher.
Then overall A-weighted sound pressure level increased85

almost in proportion with precipitation.
We anticipated that the noise level would increase 5 dB
if precipitation doubled.
Comparing the estimated noise level as relative soundpressure level with the measured data, it showed good
agreement for the tendency of precipitation and the difference between the estimated level and measured data waswithin 4 dB.

7.

FUTURE WORKTo develop damping materials and reduce cost, it is necessary to study the correlation between noise level andloss factor of damping material. When we design a building, as well as acoustic properties, it is possible to overestimate the effect of noise reduction if we adopt the same
equation as that we introduced here, therefore, it is also
necessary to introduce a more precise equation that considers the acoustical condition inside the building.

REFERENCES
1) Leo L. Beranek: Noise and vibration control, 1988, Institute ofnoise control engineering2) Architectural Institute of Japan: Guidelines for Practical Noise
Suppression, Second Edition, (Gihodo Publishing) (in Japanese)3) Steel Materials Club: Technical References for Damping
Materials Applications (in Japanese)

Manuscript received on January 8, 2002.

86

Furukawa Review, No. 21

2002