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KEBISINGAN DAN VEGETASI
Kebisingan bukan saja bervariasi menurut tekanan suranya, tetapi
juga sangat berhubungan dengan frekwensinya. Pepohonan dapat
meredam kebisingan dengan cara mengabsorpsi gelombang suara oleh
daun, cabang dan ranting. Penanaman vegetasi pepohonan dalam bentuk
shelter belt, dengan penutupan yang rapat dan berlapis-lapis, dapat
meredam kebisingan yang cukup besar hingga 95% dari sumbernya.
Dengan demikian kriteria bentuk hutan kota yang dicirikan dengan jarak
tanam rapat, mempunyai pelapisan tajuk yang berlapis-lapis, dinilai efektif
dalam meredam kebisingan
www.fhwa.dot.gov/.../fspubs/07232816/page06.htm
MEREDAM KEBISINGAN DENGAN BAMBU
Suara bising memang tidak mengenakan jika didengar terus
menerus di telinga. Paparan kebisingan yang terlalu lama bisa
mengakibatkan berkurangnya pendengaran bahkan tuli pada seseorang.
Di sepanjang jalan dapat ditanami dengan Dracaena surculosa
Lindl atau bambu sebagai peredam kebisingan.
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Pola peredaman suara secara fisika merupakan gejala peredaman
bunyi dimana bambu memiliki koefisien serap bunyi frekuensi tinggi. Ketika
gelombang dengan kecepatan tertentu melalui medium udara lalu
terhalang oleh penghalang (bambu) maka ada tiga kemungkinan yang
terjadi pada gelombang tersebut yaitu diteruskan, dipantulkan dan diserap.
Jika suatu bahan memiliki impedansi yang baik terhadap gelombang yang
datang maka gelombang tersebut dapat teredam secara baik.
Sound attenuation through absorption by vegetation
The sound absorption coefficient (α) of trees was theoretically
derived and expressed as α = G・f^<1/2>, where f is frequency and G is a
constant ; since the value of G cannot be calculated directly, absorption
coefficients of four kinds of trees were experimentally measured by the
reverberation-chamber method. It was found that sound energy was
absorbed mainly by the leaves of trees and not their trunks. The values of
G were determined by comparing theoretical and experimental results, and
were found to be between 0.001 and 0.002. The attenuation in vegetation
was theoretically expressed by G. To check the accuracy of this,
experiments were carried out using two kinds of trees. The sound energy
absorbed by tree leaves was measured. The theoretical values agreed
approximately with experimental results, indicating that the attenuation
through absorption can be predicted.
BARIER UNTUK MEREDAM KEBISINGAN
Barriers such as walls or screens will act to create an acoustic
shadow by obstructing the free flow of sound energy. The reduction in
sound level within this shadow zone is dependent on frequency. At high
frequencies the effect of the barrier is most pronounced, whereas at low
frequencies much diffraction occurs at the edges so the shadow effect is
diminished.
There are a number of quite computationally intensive methods for
calculating this reduction, however a simple method that suits our purposes
is as follows:
where
R = the reduction (dB) over the inverse square law,
f = the frequency (Hz),
d = the path difference between diffracted and direct sounds, and
c = the speed of sound (usually taken as 343m/s).
This prediction is based on the assumption that any transmission
through the wall or screen itself is negligible.
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Barrier Placement
The exact placement of a barrier between the sound source and a
receiver position will determine its effectiveness. To work best, the barrier
must make the sound travel as far as possible from the direct route in order
to traverse it, and change direction by the greatest angle. For a barrier of a
given height, this can be achieved by placing it either directly next to the
source or directly next to the receiver. Halfway between the two is the worst
position. As all barriers will allow some small amount of direct transmission
through them, it is best to place the barrier closest to the source so that any
transmitted sound still has to travel the maximum distance to the receiver.
Thus:
1.
The best position for a barrier is nearest the noise source.
2.
The second best position would be nearest the receiver.
3.
The least effective position is midway between the two.
Vegetation as a Barrier
It appears that there is a widely held belief that vegetation is an
effective controller of sound. However, research by Fricke (1984) and
others shows that vegetation is only effective at higher frequencies (above
2000Hz). The impedance of the ground is the dominant factor at lower
frequencies (125-500Hz) with vegetation having little effect on sound
propagation at mid-frequencies (500-2000Hz). It is suggested that
scattering, rather than absorption, is the more important phenomenon at
mid-frequencies. At higher frequencies, however, absorption takes over as
the dominant phenomenon.
www.rochestermn.gov/.../appendix/APPENDIXD.asp
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The literature generally suggests that the principal effect of
plantings is psychological. By removing the noise source from view,
plantings can reduce human annoyance to noise. The fact that people
cannot see a highway can reduce their awareness of it, even though the
noise remains. Another effect to consider is increased wind noise
generated by the leaves of large trees, effectively masking other mid-high
frequency sounds.
Large areas of vegetation can be effective, but only in belts greater
than 50m thick.
www.fhwa.dot.gov/.../fspubs/07232816/page06.htm
Meredam Kebisingan Lalulintas
Traffic Noise Attenuation as a Function of Ground and Vegetation by
Rudoff Hendricks (CALTRANS, office of Materials Engineering and Testing,
5900 Folsom Blvd., Sacramento, CA. 95819)(July 1995).[TD100:CA 95-23]
This report concluded that vegetation is usually not an effective
highway noise mitigation measure. In this research project, the term
"vegetative barriers" refers to shrubs and trees planted in relatively narrow
and dense strips along highways for the primary purpose of landscaping.
As used in this report, vegetative barriers do not include the specially
designed "green" or "living" noise barriers that incorporate vegetation and
structural materials for the specific purpose of noise abatement.
Objectives
Measure traffic noise attenuation rates as a function of distance
from source, height above ground, and absorptive characteristics of six
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ground types, ranging from reflective paved surfaces to soft, plowed dirt
and ground covers. Measure traffic noise attenuation provided by four
species and three heights or thicknesses of vegetation belts alongside
highways, such as ivy covered fences, dense oleander and other shrubbery.
Establish improved traffic noise attenuation rates and shielding values to be
used as inputs for Caltrans noise prediction methods, based on findings in
this study. Develop guidelines for use of evergreen vegetation belts
(barriers) in Caltrans noise abatement procedures, if effectiveness were
proven in this study.
www.fhwa.dot.gov/.../fspubs/07232816/page06.htm
Conclusions
A continuous strip of oleander or equivalent shrubs, at least 2.4 m
(8 ft) high and 4.5 to 6 m (15 to 20 ft) wide, planted along the edge of a
highway shoulder, provides noise attenuation of 1-3 dBA at distances of up
to 15 m (50 ft) from the rear edge of vegetation. A single line of pine trees
planted about 7.5 m (25 ft) from the edge of a highway shoulder, 12 m (40
ft) tall, 9 m (30 ft) in diameter, spaced 3-6 m (10-20 ft) apart, low branches
intertwined and touching ground, provides noise attenuation of 0-1 dBA at
distances of up to 18 m (60 ft) from the rear edge of vegetation.
A combination of oleander, planted 11 m (35 ft) from the edge of a
highway shoulder, 2.7 m (9 ft) high and 3 - 4.5 m (10 - 15 ft) wide, and
redwood trees, equally spaced at 9 m (30 ft) in the oleander strip, 15 m (50
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ft) tall and 6 m (20 ft) in diameter provides noise attenuation of 0 to 1 dBA
at distances of up to 21 m (70 ft) from the rear edge of the oleander strip.
Vegetative barriers (as defined in this study) are not an effective
highway noise mitigation measure to use on a routine basis. In some
borderline cases (to mitigate or not mitigate), thick oleander (at least 4.5-6
m, or 15 - 20 ft wide and 3 m, or 10 ft high) may have some value by
avoiding conventional noise barrier construction, if adequate right-of-way is
available.
Trimming or removal of shrubs and trees along highways by
maintenance or construction does not cause perceptible noise level
increases to nearby residences. However, the sudden visibility of highway
traffic previously shielded visually by the vegetation, and the possibility of a
shift in sound frequencies, may bring on a renewed awareness of the
presence of noise sources. This may result in additional noise complaints.
www.fhwa.dot.gov/.../fspubs/07232816/page06.htm
ABSORPTION OF LOW-FREQUENCY SOUND ENERGY BY
VEGETATION: A LABORATORY INVESTIGATION
Ryan J. Lee; ENGINEER RESEARCH AND DEVELOPMENT CENTER
CHAMPAIGN IL CONSTRUCTION ENGINEERING RESEARCH LAB
Abstract
Many field research projects have been conducted to study the effects of
natural foliage on the propagation and attenuation of sound. This research
takes natural foliage into a controlled laboratory setting to test its lowfrequency acoustic characteristics. Absorption of low-frequency
components of unwanted noise is of interest to the Army, but has been an
unsolved problem due in part to the cumbersome and expensive testing
facilities needed to study long wavelengths. In this research, low-frequency
absorption and reflection coefficients were found reliably and consistently.
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Due to study of the steady state conditions, the methods presented here
could constitute a more consistent method than ever before. The
procedures described in this paper can serve as a handbook for future
research; recommendations are included.
Jalur Hijau Jalan Raya
A study was carried out to determine the effect of roadside
vegetation on the reduction of road traffic noise levels under varying traffic
conditions. Roadside vegetation which have the potential to act as noise
barriers were selected for this study. The road traffic noise was measured
together with the parameters that control the vegetation. Several noise level
descriptors were recorded together with the A-weighted continuous noise
level.
The results show that higher frequency noise (above 4 kHz) is
heavily attenuated by the vegetation barriers with virtually no
attenuation for low frequency noise (below 100 Hz). The width of the
vegetation barrier is linearly proportional to the amount of sound
absorption. Without the vegetation barrier, the observed maximum
and minimum noise levels were 72 dB(A) and 64 dB(A) respectively.
On average, vegetation barriers were able to reduce the noise by 4
dB(A) which corresponds to an approximately 40% acoustic energy
reduction. Thus, with careful planning and growing of roadside
vegetation, the effect of road noise can be reduced.
Median jalan dapat dioptimalkan sebagai lahan RTH dengan melakukan penanaman
vegetasi di semua lahan median jalan. Median jalan yang diperkeras dengan elemen
non tanaman tidak akan memberikan sumbangan terhadap optimalisasi ruang
terbuka hijau kota (RTHK), malah akan menyumbang kenaikan suhu udara terhadap
lingkungan sekitar.
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Barier Kebisingan
Ada beberapa cara untuk mengurangi pengaruh kebisingan:
mengurangi kebisingan pada sumbernya, membuat penghalang pada
media penghantar, dan memasang penutup telinga.
Peredaman kebisingan dapat dilakukan dengan menanam tanaman
berupa rumput, semak dan pepohonan.Jenis tumbuhan yang efektif untuk
meredam suara ialah yang mempunyai tajuk yang tebal dengan daun yang
rindang. Dengan menanam tanaman dengan berbagai strata yang cukup
rapat dan tinggi akan dapat mengurangi kebisingan. Dedaunan tanaman
dapat menyerap kebisingan sampai 95%. Tanaman selain dapat meredam
kebisingan, pada saat tertiup angin dapat menghasilkan suara.
Sebagai salah satu alternatif pemecahan, maka tanaman Jati emas
plus (Tectona grandis L) adalah jawabannya. tanaman ini mempunyai daun
yang lebar dan kehijauan sehingga dapat memberikan rasa sejuk dan
indah, serasi di perkotaan dengan gedung-gedung yang megah dan jalanjalan yang lebar. Bunganya yang mulai mekar pada bulan Oktober sampai
Mei akan menambah keasrian dengan keharuman dan keindahan
bunganya yang berbentuk malai yang lebar. Tanaman ini mampu menahan
air tanah melalui sistem perakarannya dan tingkat respirasi yang rendah.
Dengan bulu-bulu dan ukuran daun yang sangat lebar, mampu
mengabsorpsi polutan terutama debu dan zat pencemar udara
lainnya.Serta dari segi ekonomis sangat menguntungkan karena hasil kayu
dapat dipanen dalam waktu yang singkat.
Peredam kebisingan
Hutan kota mampu mereduksi kebisingan, tergantung dari jenis
spesies, tinggi tanaman, kerapatan dan jarak tumbuh, dan faktor iklim yaitu
suhu, kecepatan angin, dan kelembaban. Penelitian di hutan kota Sabilal
Muhtadin Banjarmasin (luas ± 2,5 ha) menunjukkan bahwa hutan kota
mampu menurunkan kebisingan. dengan luas areal penghijauan.
Penurunan kebisingan dari titik 1 (di luar areal hutan kota) dengan
kebisingan dititik ukur 2 (di dalam hutan kota) sebesar 7,51 dB atau
12,74 %, penurunan kebisingan titik ukur 1 dan titik ukur 3 adalah sebesar
10,58 dB atau 17,95 %, dan penurunan kebisingan dari titik ukur 2 ke titik
ukur 3 sebesar 3,07 dB atau 5,96 %, berarti penurunan rata rata kebisingan
di luar hutan kota dengan kebisingan di dalam hutan kota sebesar 12,07 %
(Zulfahani dkk. 2005).
Hutan kota mempunyai fungsi majemuk dapat meredam kebisingan
di kota dan merupakan komunitas vegetasi berupa pohon dan asosiasinya
yang tumbuh di lahan kota atau sekitar kota, berbentuk jalur, menyebar
atau bergerombol (menumpuk) dengan struktur meniru (menyerupai) hutan
alam, membentuk habitat yang memungkinkan kehidupan bagi satwa dan
menimbulkan lingkungan sehat, nyaman, dan estetis. Kendala
pembangunan hutan di kota yaitu lahan di kota semakin hari semakin
berkurang dan semakin sangat mahal, persepsi masyarakat masih belum
sama. Telah dilakukan penelitian yang bertujuan untuk mencari hubungan
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bentuk dan struktur hutan kota terhadap kebisingan kota dengan mengukur
kebisingan setiap jam pada pukul 06.00-20.00 di tiga titik (di tepi, di tengah
dan di luar belakang) pada lima jenis hutan kota (kombinasi bentuk regresi
linier multidimensi dan analisis deskriminan. Hasil penelitian menunjukkan
bahwa hutan kota menurunkan kebisingan sekitar sebesar 18, 94% pada
siang hari dipermulaan musim hujan. Hutan kota berstrata dua menurunkan
kebisingan pada yang berbentuk jalur = 5,54%, menyebar = 21,87%,
bergerombol = 16,34%. Hutan kota berbentuk menyebar strata banyak
menurunkan kebisingan = 19,37% dan bergerombol strata banyak
menurunkan kebisingan= 30,41%.
Peredam Kebisingan
Pohon dapat meredam suara dengan cara mengabsorpsi
gelombang suara oleh daun, cabang dan ranting. Jenis tumbuhan yang
paling efektif untuk meredam suara ialah yang mempunyai tajuk yang tebal
dengan daun yang rindang. Dengan menanam berbagai jenis tanaman
dengan berbagai strata yang cukup rapat dan tinggi akan dapat
mengurangi kebisingan, khususnya dari kebisingan yang sumbernya
berasal dari bawah. Vegetasi pepohonan yang rapat dapat menyerap
kebisingan sampai 95%.