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Sound levels around buildings near roadways
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1 - --- .ASOUND LEVELS AROUND BUILDINGS NEAR ROADWAYS by J.D. Quirt
ANALYZED
Reprinted from Canadian Acoustics
Vol. 10, No. 1, January 1982 p. 1 3 - 18
DBR Paper No. 1055
Division of Building Research
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SOUND LEVELS AROUND BUILDINGS NEAR ROADWAYS
J.D. Quirt
National Research Council of Canada
Division of Building Research
Ottawa, Ontario K1A OR6
.ABSTRACT
This paper presents the results of preliminary
measurements of the difference between the incident
sound levels at the front and rear facades of suburban
detached and semi-detached houses adjacent to major
roadways. The measurements also yielded data on sound
transmission through open windows and comparisons
between the sound levels measured in open windows, at
the surface of the building facade, and 2 m from the
facade
.
A preliminary series of measurements, now reported, is part of an ongoing
effort to provide accurate prediction of the indoor sound levels in buildings
affected by major transportation noise sources, in this case, highway traffic.
The study involved three specific aspects of the problem:
1) the effect of reflections on the sound field near the exposed facade
of a building;
2)
the difference in the incident sound levels at exposed and sheltered
facades of detached housing in the first row of buildings near a major
highway;
3) the noise reduction characteristic of open windows.
As anticipated, the results demonstrated that simple, well-established
approaches to these problems provide reasonable predictions.
Figure 1 illustrates the typical microphone positions used in taking
simultaneous measurements at a number of positions inside and outside a building,
including 2 m from the exposed facade, immediately adjacent (within 10 mm) to it,
and inside and at the open windows of rooms on both the exposed and sheltered
sides of the house. Metrosonics dB-301 logger units were used to measure the
A-weighted equivalent sound level for 1-min intervals and store the data for up to
480 such intervals. By synchronizing the starting time of the six (or more)
dB-301 units used at each site the difference in sound levels for any time
interval could be readily obtained. Typically, data were logged for 80 to 100 min
at each building, the indoor microphones being moved every 15 to 20 min to provide
data for at least five positions and permit calculation of average room response.
Reprinted from Canadian Acoustics, Volume
1 0 ,IJwnber
I ,January
1982.w i t h d i s t a n c e from t h e s u r f a c e a r e l a r g e r ) . The c u r v e s shown i n Fig. 2 ( a ) a r e f o r a p u r e t o n e , b u t v i r t u a l l y i d e n t i c a l r e s u l t s a p p l y f o r a 1/3-octave band; w i t h i n c r e a s i n g band width t h e maxima and minima a r e reduced because t h e extrema a r e l o c a t e d a t d i f f e r e n t p o s i t i o n s f o r each frequency.
I n Fig. 2(b) t h e c a l c u l a t e d SPL f o r a l i n e s o u r c e p a r a l l e l t o a r e f l e c t i n g p l a n e i s shown a s a f u n c t i o n of d i s t a n c e from t h e s u r f a c e ( i n m e t r e s ) . For
i n d i v i d u a l t h i r d o c t a v e bands t h e SPL 2 m from t h e f a c a d e may d e v i a t e a p p r e c i a b l y from 3 dB above t h e i n c i d e n t l e v e l ( e s p e c i a l l y a t low f r e q u e n c i e s ) , b u t f o r
A-weighted t r a f f i c n o i s e t h e observed SPL should be v e r y c l o s e t o 3 dB above t h e i n c i d e n t l e v e l f o r d i s t a n c e s of more t h a n about 0.2 m. For d i s t a n c e s up t o ~ 1 0 mm from t h e s u r f a c e t h e SPL i n c r e a s e should be w i t h i n 0.1 dB of t h e 6 dB p r e s s u r e doubling. These p r e d i c t e d i n c r e a s e s of 6 dB and 3 dB a r e a p p r o p r i a t e f o r a r i g i d , p e r f e c t l y r e f l e c t i n g s u r f a c e ; sound a b s o r p t i o n by m a t e r i a l s such a s window g l a s s o r wood s i d i n g could reduce t h e i n c r e a s e by a s much a s h a l f a d e c i b e l .
Although t h e i n c i d e n t SPL seems t o be t h e a p p r o p r i a t e r e f e r e n c e l e v e l f o r t h i s s o r t of s t u d y , t h e r e i s no convenient way t o measure it d i r e c t l y ; d i r e c t i o n a l microphones would d i s t o r t t h e r e l a t i v e c o n t r i b u t i o n s from d i f f e r e n t p a r t s of t h e
l i n e s o u r c e and a b s o r p t i v e t r e a t m e n t of each f a c a d e t o e l i m i n a t e r e f l e c t i o n s over
t h e r e l e v a n t frequency range (60
-
5000 Hz) i s n o t r e a l l y p r a c t i c a l . Measurementsi n a room w i t h an open window can be used, however, t o o b t a i n a r e a s o n a b l e e s t i m a t e . I f t h e room's a b s o r p t i o n ( i n c l u d i n g t h e window opening) i s n o t t o o l a r g e , t h e sound f i e l d i n t h e room can be d e s c r i b e d f a i r l y a c c u r a t e l y a s t h e combination of a r e v e r b e r a n t f i e l d p l u s a d i r e c t f i e l d from t h e sound i n c i d e n t on
t h e window opening. I n t h e window opening t h e measured sound l e v e l (LWINDOW) i s
dominated by t h e i n c i d e n t sound (LING), b u t t h e r e v e r b e r a n t sound f i e l d ( L R O O ~ ~ ) i s n o t i n s i g n i f i c a n t . Using t h e e q u a t i o n
LINC = 10 l o g [ a n t i l o g (LWINDOW / l o )
-
0.5 a n t i l o g (LROOM/lO)]t h e i n c i d e n t SPL can r e a d i l y be c a l c u l a t e d . To t h e e x t e n t t h a t one may i g n o r e d i f f r a c t i o n a t t h e window opening and t r e a t t h e i n c i d e n t and r e v e r b e r a n t f i e l d s a s u n c o r r e l a t e d , t h i s should p r o v i d e a r e a s o n a b l e measure of t h e i n c i d e n t sound l e v e l .
The d a t a o b t a i n e d i n t h e window openings and t h e c e n t r a l a r e a of t h e rooms
were processed i n t h i s way t o o b t a i n t h e i n c i d e n t SPL. These v a l u e s were t h e n
compared w i t h t h e corresponding d a t a f o r microphones immediately a d j a c e n t t o t h e facade (Fig. 3 ) . The mean d i f f e r e n c e of 5.6 dB i s i n q u i t e good agreement w i t h
t h e expected i n c r e a s e of 6 dB. There i s some s c a t t e r , but it i s t o be expected
f o r s e v e r a l r e a s o n s :
1 ) t h e 1 dB r e s o l u t i o n l i m i t of t h e measuring u n i t s and t h e comparable c a l i b r a -
t i o n u n c e r t a i n t y would be expected t o i n t r o d u c e s c a t t e r Q +1 dB;
2 ) s y n c h r o n i z a t i o n of t h e time i n t e r v a l s was imperfect (up t o Q 5 s i n some
c a s e s ) and t h e e f f e c t of b r i e f , loud e v e n t s could f a l l i n nominally d i f f e r e n t t i m e i n t e r v a l s f o r t h e two u n i t s being compared;
3 ) i n t e r f e r e n c e e f f e c t s a s s o c i a t e d w i t h t h e f i n i t e , somewhat i r r e g u l a r
f a c a d e s and d i f f r a c t i o n a t t h e window opening could cause r e a l d e v i a t i o n s from t h e simple model used;
4 ) o c c a s i o n a l extraneous n o i s e s .
150 I 1 I 1 I
-
does provide a f a i r l y c l e a r i n d i c a t i o n t h a t t h e sound f i e l d s a r e b a s i c a l l y c o n s i s t e n t with simple p h y s i c a l 125 - A e x p e c t a t i o n s . The d e v i a t i o n from 6 dB i s c o n s i s t e n t with t h e expected e f f e c t of V)sound absorption by t h e window g l a s s on
:
100-
- which t h e microphones were mounted.X
cc PI
I3
U S i m i l a r agreement i s found when t h e
U
0 7 5 - i n c i d e n t SPL i s compared with t h e l e v e l
L
a 2 m from t h e facade (Fig. 4 ) . The mean
E u m i n c r e a s e i n t h e SPL r e l a t i v e t o t h e I =, 50 - - i n c i d e n t l e v e l i s 2.5 dB, i n reasonable Z
agreement with t h e expected value of 3 dB. Again, t h e r e i s a p p r e c i a b l e s c a t t e r , much
25
-
- of it presumably due t o l i m i t e d measure-ment accuracy and t h e e f f e c t of extraneous n o i s e s .
0
0 2 4 6 0 10 1 2 A s a l r e a d y i n d i c a t e d , t h e procedure
D I F F E R E N C E IN A - W E I G H T E D S P L , d~ t o measure t h e i n c i d e n t sound l e v e l
r e q u i r e d measurement of t h e sound l e v e l i n s i d e rooms with open windows and t h u s FIGURE 3 provided t h e d a t a needed t o a s s e s s sound
transmission through open windows.
Difference between t h e measured Previous s t u d i e s 2 i n d i c a t e d a wide range sound l e v e l a t t h e s u r f a c e of t h e i n t h e n o i s e r e d u c t i o n a s s o c i a t e d with exposed facade and t h e i n c i d e n t open windows; it seemed u s e f u l t o a s s e s s sound l e v e l t h e cause of t h e s e v a r i a t i o n s . Before
examining t h e d a t a , t h e e x p e c t a t i o n s from a simple extension of t h e Transmission Loss (TL) measurement between two r e v e r b e r a n t rooms3 should be considered. In l a b o r a t o r y measurements t h e TL i s determined from t h e d i f f e r e n c e between t h e r e v e r b e r a n t sound l e v e l s i n t h e source and r e c e i v i n g rooms (LSOURCE and LREC r e s p e c t i v e l y ) normalized t o allow f o r t h e a b s o r p t i o n (A) i n t h e r e c e i v i n g room and t h e s u r f a c e a r e a (S) of t h e element t r a n s m i t t i n g t h e sound:
TL = LSOURCE - LREC. + 10 log [S/A]. (2) With t h i s d e f i n i t i o n t h e TL corresponds t o 10 log ( l / ~ ) , where T i s t h e r a t i o
of t r a n s m i t t e d t o i n c i d e n t sound power. For an open window one expects a l l t h e sound power t o p a s s through ( i . e . , T = 1 and TL = 0); experimental r e s u l t s
g e n e r a l l y agree with t h i s f o r frequencies where t h e wavelength i s appreciably l e s s than t h e dimensions of t h e opening and d i f f r a c t i o n can be ignored. For a p p l i c a - t i o n s r e l a t i n g t o e x t e r i o r facades it i s a p p r o p r i a t e t o consider t h e n o i s e
r e d u c t i o n r e l a t i v e t o i n c i d e n t SPL a t t h e t e s t specimen r a t h e r than t h e reverberant l e v e l i n t h e source room (LSOURCE). For t h e r e v e r b e r a n t source room
LING
= LSOURCE-
3 dB, a s shown by waterhousel and i l l u s t r a t e d i n Fig. 2a. Thus, t h e n o i s e reduction r e l a t i v e t o i n c i d e n t SPL when normalized l i k e TL t o allow f o r component a r e a and r e c e i v i n g room absorption (LING-
LREC + 10 log [S/A]) should correspond t o TL-
3 dB; i . e . , t o - 3 dB f o r an open window.The a c t u a l measured n o i s e r e d u c t i o n r e l a t i v e t o t h e i n c i d e n t SPL (when
normalized l i k e l a b o r a t o r y TL d a t a ) i s shown i n Fig. 5. The mean value i s - 3 . 4 dB with a standard d e v i a t i o n of s l i g h t l y l e s s than 1 dB, i n r a t h e r good agreement
-
1 6-
-6
-
3' Q 3 6 9 D I F F E R E N C E I N A - W E I G H T E D S P L , dBFIGURE
4Difference between the measured sound
level at 2 m from the exposed facade
and the incident sound level
ZOO
- 1 2 - 8 - 4 0 4
D I F F E R E N C E I N A - W E I G H T E D S P L , d B
FIGURE
5Noise reduction for open windows
(relative to the incident sound
level), normalized to the case
where absorption (A)
=open area (S)
D I F F E R E N C E I N A - W E I G H T E D S P L , d B D I F F E R E N C E I N A - W E I G H T E D S P L , d B
FIGURE
6Noise reduction for open windows
(relative to the incident sound level)
when normalized to typical room
conditions, as discussed in text
FIGURE 7
Difference between the incident
sound levels measured at the
exposed and sheltered facades
of the houses
with the expected value. Because of the rather large number of measurements
(Q
50 rooms) it is tempting to interpret this deviation from -3 dB as systematic.
This is not unreasonable, as the increased sound transmission associated with
diffraction at the lower frequencies4 would tend to introduce this sort of shift
in the results. The essential features, however, are the quite small scatter in
data and the good agreement with the expected value of -3 dB. It should be noted
that this shift of -3 dB (as well as corrections to allow for the difference
between random incidence and a line source) must be allowed for in applying the
laboratory data for any building element to predict indoor noise from an incident
outdoor sound level.
Having established the sound transmission characteristics of the windows
themselves, it is useful to examine their implications for typical indoor sound
levels relative to the incident level. Figure
6shows the same data re-normalized
to assumed "typical" conditions: room reverberation time of 0.5 s and window
opening of 3 ft2. The latter was chosen because it is the required minimum
opening for natural ventilation in Canada's National Building Code. The data in
Fig. 6 show much more scatter than the data in Fig. 5 owing to the range of room
size. Although the assumed window opening is a reasonable compromise between the
discomforts of noise and heat, different occupants would obviously choose a range
of openings and might furnish the room to give shorter or longer decay times.
These individual variations would further broaden the range of expected noise
reductions beyond that indicated in Fig. 6. It should be stressed, however, that
for a given room and window a much smaller range (which can be readily calculated)
would be expected.
Figure 7 presents the data pertinent to the original motivation for the
study, the difference between the incident SPL at the exposed and sheltered
facades of these houses. Clearly the data are of great concern in deciding on the
noise control measures necessary to provide an acceptable indoor noise environment,
particularly since bedrooms (where noise sensitivity tends to be greatest) are
more likely to be on the sheltered side. For most of the cases studied here the
noise level was 15 to 20 dB lower on the sheltered side. Because these houses
were fairly large and closely spaced, however, and in most cases the buildings in
the second row were smaller and more widely spaced, there are reasons to believe
that the reflected sound power was somewhat less than would be encountered with a
broader sample. Subsequent stages of the measurement program will try to
establish data for a broader range of conditions. It is hoped that they will
provide the basis for a simple empirical procedure for estimating noise at the
sheltered wall, given simple data on size and spacing of nearby buildings. In the
meanwhile, a reduction of 10 to 15 dB appears to give a fairly conservative
estimate of the noise reaching the sheltered facade.
References
1. R.V. Waterhouse, "Interference Patterns in Reverberant Sound Fields,"
J. Acoust. Soc.
Am., 27, 247 (1955).
2.
L. C
.
Sutherland, lt1nd=r Noise Environments Due to Outdoor Noise Sources
,It
Noise Control Engineering, 11, 124 (1978).
3 .
ASTM'
E90-75, "Laboratory ~ezurement
of Airborne Sound Transmission Loss of
Building Partitions.If
4. R.D. Ford and G. Kerry, "The Sound Insulation of Partially Open Double
Glazing," Appl. Acoustics,
-
6, 57 (1973).
-
-This paper is a contribution from the Division of Building Research, National
Research Council of Canada, and is published with the approval of the Director 'of
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