Transcript Materi 4b Noise measurement warna

Lanjutan…materi 3
Noise Measurement
Audible Range
• Audible range : 20 Hz – 20 KHz.
• Telinga manusia sangat sensitif pada frekuensi
3000 Hz – 4000 Hz dan kurang sensitif pada
bunyi-bunyi frekuensi rendah.
• Pada frekuensi sensitif, dalam keadaan
tertentu manusia bahkan mampu mendengar
bunyi pada kekuatan -5 dB.
• Respons telinga berbeda untuk setiap
frekuensi yang muncul  mekanisme dengar
Phon Scale
• Mekanisme dengar  grafik yang tidak linear
• Test-test yang dilakukan menghasilkan
serangkaian kurva : equal loudness level contours
atau disebut juga Fletcher-Munson equal
loudness contours.
• Kurva ini telah distandarkan secara internasional
sebagai kurva referensi untuk menunjukkan
respons telinga normal pada tingkat kekuatan
bunyi tertentu pada frekuensi tertentu.
Phon Scale: Phon scale dapat diartikan sebagai tingkat kekerasan bunyi pada frekuensi
tertentu yang sama dengan tingkat dB pada frekuensi 1000 Hz pada kontur atau kurva tersebut.
• Bunyi dengan tingkat 60 dB
dirasa lebih keras, yaitu
dalam tingkat 60 phon, bila
berada pada frekuensi 1000
Hz, namun dirasa lebih
lemah, yaitu hanya 30 phon
ketika frekuensinya 50 Hz.
Sound Weighting
• Respons telinga yang
berbeda-beda terhadap
bunyi pada frekuensi
tertentu akhirnya
mengelompokkan bunyibunyi dalam bobot tertentu,
sesuai kesan atau sensasi
yang diterima oleh telinga
 sound weighting
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Sound level meters incorporate
frequency weighting to simulate this
effect.
Dept. of Mech. Engineering
University of Kentucky
The A-weighted Sound Level
A-weighting
response
(Ear’s response at
approximately 40 dB)
19 dB
100 Hz
1000 Hz
The A-weighting is a filter that approximates the ear’s response at low levels.
It filters and integrates the actual spectrum to yield a single value, e.g., 95
dB(A) overall sound level.
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Dept. of Mech. Engineering
University of Kentucky
Example of A-Weighting
Sound Pressure Level
110
Unweighted (114.4 dB)
100
A-weighted (109.6 dBA)
SPL (dB)
90
80
70
60
50
0
500
1000
1500
2000
Frequency (Hz)
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Dept. of Mech. Engineering
University of Kentucky
The A-weighting Curve Approximation
0 dB
A-weighting
response
19 dB
1000 Hz
100 Hz
An approximate function that can be used to calculate the response of
the A-weighting curve is:
 0 .2129 x  1 . 9867 x  4 . 2 x  21 . 51 x  122 . 06 x  168 .5
5
x  log
10
f
4
3
2
( f in Hz)
This function is accurate to within 0.6 dB maximum error at any f.
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Dept. of Mech. Engineering
University of Kentucky
Historical Notes
• Octave and one-third octave filters predate by many years the use of the
FFT (introduced in the early 1970’s) to obtain the frequency spectrum of
complex sounds.
• Many standards were written around octave and one-third octave filter
frequency analysis and have yet to be fully updated.
• The A-weighting system is, strictly speaking, only valid at low sound levels
(around 40 dB), but it is commonly used to rate sounds having levels over
100 dB.
• The A-weighting curve is often mistakenly used to rate noise for
“annoyance” or other subjective (psychological) measures of sound
quality. It is, in fact, only a measure of the physical response of the ear
mechanism.
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Dept. of Mech. Engineering
University of Kentucky
Working with Decibels
• In as much as decibels are logarithmic quantities, we cannot
manipulate them arithmetically.
• Instead, we must convert decibels to their “base” values, perform the
arithmetic operation, and then convert back to decibels
• Addition of sound levels. Sound levels are added by adding their
mean-square sound (i.e., rms2) pressures:
 p
L P  20 log 10  rms
 p
 ref

  10 log


2
p rms
10
2
p ref
p Total  p1  p 2  p 3    p N
2
2
2
p i  p ref 10
2
10
2
2
2
L i 10
Dept. of Mech. Engineering
University of Kentucky
Examples
1.
Two sources each have a sound pressure level of 95 dB at a distance
of 5 m. What is the combined level?
L1  L 2  95 dB
p  p
2
1
2
ref
95
10
10
 3 . 16  10 p ref  p 1
9
2
2
p Tot  6 . 32  10 p ref
2
9
2
2
L Tot  10 log
p Tot
10
2
p ref
 10 log
10
6 . 32  10
9
L Tot  98 . 0 dB
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Dept. of Mech. Engineering
University of Kentucky
Examples (2)
2.
A machine has a sound pressure level of 95 dB in the presence of
background noise. A background noise measurement made with the
machine off yields a level of 92 dB. What is the true level of the
machine?
 p
p
2
Tot
p
2
Background
2
ref
95
10
 p
10
2
ref
 3 . 16  10 p ref
9
92
10
2
 1 . 58  10 p ref
9
10
2
p Machine  3 . 16  1 . 58   10 p ref  1 . 58  10 p ref
2
9
2
9
2
L Machine  10 log
p Machine
10
2
p ref
 10 log
2
1 . 58  10 
9
10
L Machine  92 . 0 dB
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Dept. of Mech. Engineering
University of Kentucky
Examples (3)
3.
A machine has a sound pressure level of 86 dB at a distance of 3 m. If 6
identical machines are arranged in a circle about the measurement point,
what is the total level?
For each machine:

p  p
2
i
2
ref
p Tot  6  3 . 98  10 p ref
2
L Tot
8
2
86
10
10
 3 . 98  10 p ref
8
2

2
 3 . 98  10 8 p ref
 10 log 10 
2

p
ref


  10 log


10
6 
L Tot  86  7 . 8  93 . 8 dB
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Dept. of Mech. Engineering
University of Kentucky
Truth behind noise
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