PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Rain noise
Brian DONOHUE1; John PEARSE1
1

University of Canterbury, New ZealandDepartment of Mechanical Engineering

ABSTRACTIn this paper we discuss the requirements of the generic international standard (ISO, BS EN ISO
10140) for testing of sound transmission through sample roofs exposed to simulated rainfall and of
lessons learned during a recent test program. The test data forms the basis for calculating in -situ soundlevels in rooms beneath the roof and we discuss the differences in sound produced by simulated rain tothat of natural rain. The differences in impact velocity and raindrop distribution between simulated
and natural rain are key factors that are not addressed by the Standard. In addition, an optional
normalization test using a pane of glass is included, for the explicit comparison of products tested andas quality control for test laboratories, and its results have been incorrectly shown in somemanufacturer’s publicity material as the basis for calculating room sound levels. The Standard does
not specify whether the normalization test should be carried out as a skylight or as glazing but the two
tests have different requirements. Being optional and intended for inter-lab comparison suggests thatthe normalization data should not be released to clients as it is misleading and thus should be excluded
from reporting.
Keywords: rain noise, roof systems

1. INTRODUCTION
Depending upon the listener’s contextual situation, noise generated by rainfall can be soothing or
annoying. Lengthy You Tube videos and mp3 audio are available (1) for playing rain noise to support
relaxation, yet in other circumstances the same sound masks communications and becomes a nuisance.
It is in this latter context that we report on the incidence of rainfall on metal roofs supported bystructural insulated panels (SIPs) forming the roof of an interior space in a building. These panels arecomposite structures typically consisting of a soft elastic core bonded to stiff facing panels such as
oriented strand board, magnesium oxide board and plywood. The intrinsic mass is low and theconsequent poor acoustic insulation properties may lead to high levels of rain induced noise in thebuilding’s interior rooms. The rooms in question could be classrooms or open learning spaces where
good conditions for communication for teaching are paramount (2).
Figure 1 shows the typical form of response curve for the sound transmission loss characterist ic offoam cored SIPs. The dips at frequencies around 630 Hz and 3150 Hz control the STC rating for thepanel. The response in the range of 630 Hz is a bounce mode of the masses of the facing panels on thespringy foam core. Adding mass layers (as in the red curve in Figure 1) to improve the transmission
loss rating also stiffens the panel but the upper and lower modal frequencies are relatively unchanged.
The second mode frequency is reduced as there is an inverse linear relationship between frequency a nd
added mass as the core spring stiffness is unchanged. The effect on the NC rating due to rain noise, as
determined for a room where SIPs are used in a roofing application, can be seen in Figure 2 , plotted forrain noise. Clearly, the resonance mode in the 630 Hz region is limiting the rating for the room.
Rain noise is the result of vibration of the surface on which the rain falls and as it propagates
through the surface and structure its character is changed by the structure’s mass, material damping
and resulting energy loss. Currently SIPs offer little in terms of acoustic attenuation and must betreated by adding surface mass layers such as plasterboard, medium density fibreboard (mdf) or
1

5050

brian.donohue@canterbury.ac.nz; john.pearse@canterbury.ac.nz

plywood and/or damping materials (membranes, paints). Alternative methods of damping andsurface treatment reported by Wawrzynowicz et. al. (3) showed mixed results.

Figure 1 - Typical sound transmission loss curves for SIPs – blue broken line is bare SIP, red line is SIP with
one layer direct fixed plasterboard.

Figure 2 - Sound level in a room with a SIP roof exposed to rain noise. The broken line represents a SIP withadditional face treatments and the red line shows the effect of adding insulation (with a suspended ceiling).

2. RAIN
Rain is a form of impact loading, generating noise by the excitation of vibration of roof panels by
the dynamic force exerted by the falling droplets. The size of raindrops varies in natural rain and isrelated to the intensity or rainfall rate. Rain is classified as light, moderate, heavy or intense. In thelaboratory, simulated rain as defined in Standards (4) is classified as moderate, intense, heavy or
cloudburst, and is generated in the laboratory as a means to make observations under reproducible andstandard conditions. It does not correlate well with natural rain but the spectral character of the noise
is consistent, whilst the sound level is at variance. The impacting raindrops excite the natural modesof vibration of the exposed roof panel and the resulting motion is radiated as sound. Lower frequency
modes require higher input energy to excite and so may not be present in low intensity rain. The
modal frequencies of the roof structure are determined by the mass, boundary conditions (screw or nail5051

fixing and their spacing), the spacing and material of the purlins, and system damping (overlap joints,
membranes, material). For a given installation an increase in rainfall rate consequently leads tohigher noise being generated. The construction and installation of the sample for testing must be as
close as possible to the actual site installation to provide any confidence in the accuracy of predictionsof sound levels from the test results. The curtain wall that supports the sample should have a similar
construction to the exterior walls of the building design and the junction between the test sample andthe wall should be as similar as possible. This becomes a test on the structure rather than simply theroof section. Altering boundary conditions where the roof sample joins the supporting curtain wall
alters the response characteristics of the roof sample and so the sound produced will be different thanunder actual site conditions making predictions difficult and erroneous.
Simulated rain is different from natural rain as it seeks to standardize a testing method. Simulated
rain must comprise 50% of the volume flow of droplets of the same size and have a specified impact
velocity where in natural rain the drop size distribution is related to rainfall rate and ther efore each
event will have a different impact velocity distribution (6).
Rain noise testing is carried out to the international standard BS EN 10140 -Part 1, Appendix-K (4).
Parts 3 and 5 of the older standard ISO 10140 are referenced in the test methodology (5,7), and detailsof the drip tray for generating water droplets is detailed in amendment 1 to part 5 (8). This includes
a table where the hole size and number of holes per unit area is given, but surprisingly there is no
specification of the hole entry and exit conditions. The holes are 1 mm diameter so small enough forsurface tension and edge effects to play a significant role in capillary flow and drop formation (9).
The laboratory test for a roof system uses a drip tray whose area is on ly a fraction of the test sample
area and three test positions are required – not overlapping and offset from the centre to avoid
symmetry. The size of the drip tray given in the Standard is 1.25 m x 1.3 m (1.625m 2 ) that has
approximately 60 holes/m 2 – and the standard states “a random distribution is preferred.” The figurein the Standard shows division of the tray into 100 rectangles each with a single hole – the pattern
appears to show each hole in one of four positions within the rectangle and so not t ruly random. We
determined that no hole should be closer than 10 mm from a wall of the tray so that the area over theholes is 1.56 m2 and the number of holes/m 2 is 64. One possible sample hole pattern is shown in
Figure 3:

Figure 3 – Sample of random hole positions for the drip tray.
The required rainfall rate is 40 mm/h, that is, collected over a period of one hour, the depth of waterover the area of the trip tray will be 40 mm. Thus the volume flow required is (1.56 x 0.04) or 0.0624
m3 /h. The flow through each hole is 1/100 of this, i.e. 6.24E-4 m3 /hole. The volume flow is:5052‫ ݍ‬ൌ ‫ܣܭ‬ሺʹ݄݃ሻ଴ǤହEquation 1

where ൌ   is the product of the flow coefficients for entry, discharge and velocity conditions
respectively, A is the cross-sectional area of a hole, m2, and  is the depth of water in the drip tray, m.
Thus,݄ ൌ ሺ‫ݍ‬Ȁ‫ܣܭ‬ሻଶ Ȁʹ݃Equation 2

and inserting values gives  ൌ ͲǤͲͲ͵ͺ   ͵Ǥͺ mm. A value of K = 0.8 has been assumed here
but this could easily be as low as 0.5 if the edges of the holes are sharp (9). It should be noted that thedepth of water required is very shallow compared to the dimensions of the tray and levelling must be
precise in order to obtain uniform drip formation across its area. Increasing the depth of water in the
tray will increase the flowrate and lower the drip size, or lead to stream flow if too deep.
Water droplets form at the exits from the holes and detach when their internal pressures generated
from their mass and gravitational force exceeds the capacity of surface tension to keep them intact.
Assuming a spherical droplet, the force due to surface tension is given by the expression:‫ܨ‬௧ ൌ ʹߨ‫ߛݎ‬Equation 3

where γ is the surface tension, r is the radius of the water droplet, m. This force is balanced by thegravitational force ǡǡ due to the mass of water in the droplet and so:‫ ݎ‬ൌ ݉݃ȀʹߨߛEquation 4

Inserting values gives d = 0.0047 m or 4.7 mm. Thus, assuming the correct head of water, drips
will naturally form at the exit of the holes that are approximately the correct size.

3. SOUND GENERATIONSound is generated by the impacting droplets exciting vibration of the roof surface. Some sound isradiated into the environment and some passes through the structure into the space below. The latteris the portion we are mainly interested in, but in-situ flanking issues may also require examination of
the former. Predicting the level of sound propagating through roofs has been researched for many
years and remains a task to be solved as the level of confidence in results from the process isquestionable (10). Approximate solutions have been suggested but no reliable method has evolved.
There are several reasons for this but the main ones are the inability to model a flexible orthotropic
composite panel system in a simple manner, defining the characteristics of the impact force on a
flexibly mounted surface, and the effects of pooled or run-off water that is on the roof surface. The
response of the roof system is dependent upon the materials used, the method of construction and theboundary conditions - and these are far from simple. In addition there is the difference between
natural rain and simulated rain and yet there appears to be an expectation that a laboratory test will
give the expected noise characteristics for the roof system directly.
Testing is still the best way to achieve any certainty in forecasting the sound levels in rooms below
a specified roof but it is expected that different laboratories will get different results for the same
product. This is because the results are dependent upon the degree of care in preparing and installingthe sample and the degree to which the boundary conditions will match those resulting from site
construction – in addition to the variable nature of the drip tray. Following testing, the laboratory
provides the value of the averaged sound intensity measured for rainfall over three test positions.
This is a small percentage of the total area of the sample and so the reported intensity must be scaled up
as if rain fell on the whole sample. The data must be further modified to suit the dimensions andacoustic character of the room where a prediction is sought, and then further modified to allow for
natural rain in lieu of simulated rain – since simulated rain has a lower terminal velocity and different
spatial and temporal characteristics.
The standards include an optional reference test using a glass pane for quality control and in order
to compare results from different laboratories. The way in which this pane is actually mounted and5053

tested however is not specified and yet a table of reference values for sound intensity is given and thetest results are compared to this and ‘correction’ factors derived. These factors are then applied torain noise test results for sample roofs tested in the same lab. The mou nting conditions are different,
the exposed area is different and the angle of the surfaces may be different, so we are unsure of what
the rationale behind applying the ‘corrections’ is. In two instances we know of, this exercise returned
lower values of intensity (L Inorm) than the measured values (L I ) and the clients have published thelower values as a basis for room calculations. In one case the laboratory report only showed the LInorm
result. Clearly, since the glass pane test is optional it cannot be the basis for room calculations - norfor intercomparisons of product tests between laboratories as the mounting conditions are not fully
specified. Appendix I of [5] explicitly states that the glass pane is for reference use.
The education sector in many countries now specify sound levels to be achieved in learning spaces
and so if remedial costs are to be avoided (after site testing) then predictions of sound levels andtesting need to be more reliable and reproducible.

4. EXAMPLE SIP
A sample roof construction from 265 mm thick SIP was tested under simulated rainfall of 40 mm/h.
A 0.55 mm metal tray roof mounted on cavity battens on a waterproof membrane was laid over the SIP
and a suspended ceiling installed below, carrying 50mm thick mineral fibre tiles. Standard details
were used for all construction. For predictions a learning space of 200 m 2 was used with a volume of
800m3 and reverberation times of 0.6 s (100 to 500 Hz) and 0.4 s (600 to 5000Hz). The results areshown in Table 2.
Table 2 - Application of test results for metal tray roof over a structural insulated panel roof
From the test samplerd

1/3
Octave band
frequency,
Hz100

Li, dB

Li(s), dB

Lw, dB

Lp(in-situ)

40.50

50.6

73.6

57.2

125

40.90

51.0

74.0

57.7

160

41.70

51.8

74.8

58.5

200

44.40

54.5

77.5

61.2

250

44.70

54.8

77.8

61.5

315

44.70

54.8

77.8

61.5

400

46.10

56.2

79.2

62.9

500

45.40

55.5

78.5

62.2

630

40.00

50.1

73.1

56.8

800

27.40

37.5

60.5

44.2

ൌ ሺሻ൅ͳͲሺ ሻ

1000

18.60

28.7

51.7

35.4

1250

14.00

24.1

47.1

30.8

ሺǦሻൌ൅ͳͲሺͶ ͸ͲȀͲǤͳ͸ሻ൅
ͳǤ͹

1600

12.20

22.3

45.3

29.0

2000

9.80

19.9

42.9

26.6

2500

4.90

15.0

38.0

21.7

3150

1.50

11.6

34.6

18.3

4000

2.10

12.2

35.2

18.9

5000

3.80

13.9

36.9

20.6




 is from test SPL or measured
directly.

 ሺሻ ൌ ൅ͳͲȗሺ Ȁ ሻ, where is the roof sample area, m 2
 is the reference area for rainfall= (1 m2).
 is the sound power radiated by theroof system into the room (area S R)
and is given by:
and thenwhere V is the room volume (m 3 ), andT 60 is the reverberation time (s). The
addition of 1.7 dB is for natural rain(4)*NOTE: * The ‘correction’ for natural rain consists of -3.6dB for the difference in drop size

5054

distribution compared with simulated rain and +5.3dB for the terminal velocity impact of natural rain(6) – but note that this is based on rain in Beijing and may be different in another location.
The in-situ calculated data is plotted in Figure 4 against in-situ data calculated simply from thereported intensity from a laboratory test without scaling for the difference between sample size andarea of sample actually exposed to rain. The latter is further not modified by the correction for natural
as opposed to artificial rain.

Figure 4 – Variance from NC45 curve for rain noise on a SIP showing: a) test data corrected for sample size
and natural as opposed to artificial rain, and b) uncorrected data – based simply on intensity from the test
report.
Figure 4 shows a significant difference in reported data with one result almost compliant with thespecified NC level for the room and the other results some10 to 12 dB above it. It is incumbent upon
the acoustician to ensure clarity in use of laboratory data and on manufacturers to publish the right data
– not necessarily the lowest. Hopkins (11) sets out a methodology – but cautions on using the simple
assumption of linearity in response of the roof system and errors that are inevitable i n scaling from atest sample to a full size in-situ roof system.

5. CONCLUSIONSThe suite of standards for testing roof system exposure to simulated rain is not sufficiently clear or
precise enough to avoid differences between laboratories nor between tests on different products in thesame laboratory. More control is required on the entry and exit conditions for the drip tray holes toensure consistent performance. Less noise is produced by a stream of water onto a roof surface
carrying a film of water than is the case for dripping. Simply measuring the flowrate as a form ofcalibration/validation is not sufficient.
Laboratory reports need to make it clear that the reported intensity is for the sample being rain
impacted over only a small portion of the total (unless a skylight test) , and report on the total sample
size since it is required in order to correct SPL for excitation over the whole sample. Furthermore
there seems to be no value in modifying the test data by correction factors derived from a sample glass
pane test and issuing that data to clients. The standard does not delineate in sufficient detail how thereference pane is to be mounted and tested in order to achieve uniformity between lab s – and,furthermore, is optional.
Prediction of rain noise levels remains a black art but it is hoped that more testing will be promptedby new guidelines and specifications, such as those in the education and health sectors, the results ofwhich will encourage development of more accurate models and lead to more reliable predictions.

5055

ACKNOWLEDGEMENTS
The use of data in this paper was generated during a test program for Method Building Systems
Limited and its use is gratefully acknowledged. MBSL is a New Zealand manufacturer and supplierof Formance SIPs.

REFERENCES
1. https://www.youtube.com/watch?v=IdGUunu7pVI for example (accessed 20th Sept 2018).
2. Minstry of Education, Designing Quality Learning Spaces: Acoustics. Vers2.0, Sept 2016, Min. of
Education, New Zealand.
3. Wawrzynowicz, A.; Krzaczek, M.; Tejchman, J., 2014. Experiments and FE Analyses on airborne soundproperties of composite structural insulated panels. Arch. of Acoustics, vol 39(3), 351-364.
4. BS EN ISO 10140-1:2016 Acoustics - Laboratory measurement of sound insulation of building
elements. Part 1: Application rules for specific products, Appendix K Roofs, roof/ceiling systems, roof
windows and skylights - rainfall sound.
5. ISO 10140-5 2010, Amm-1 2014. Acoustics – laboratory measurement of sound insulation of building
elements Part 5: requirements for test facilities and equipment Amendment 1: Rainfall sound.
6. Yan Xiang; Lu Shuai; Li Junjie 2016 Experimental studies on the rain noise of lightweight roofs: naturalrains vs artificial rains. Applied Acoustics 106(2016) 63-76.
7. ISO 10140-3:2010 Acoustics – laboratory measurement of sound insulation of building elements Part 3:
Measurement of sound impact insulation.
8. ISO 10140-5 2010, Amm-1 2014. Acoustics – laboratory measurement of sound insulation of building
elements Part 5: requirements for test facilities and equipment Amendment 1: Rainfall sound.
9. Ghassemieh, E.; Versteeg, H.K.; Acar, M. 2006. The effect of nozzle geometry on the flow
characteristics of small water jets. Proc. IMechE Vol 220 Part C: J. Mechancial Engineering Science,
1739-1754.
10. Griffin, D. Accuracy of prediction methods for predicting rain noise. Internoise 2016, Hamburg,
Germany.
11. Hopkins, C. Rain noise from glazed and lightweight roofing. UK Building Research Establishment
IP2/06.

5056