Noise & Noise Monitoring

NA 3823 – Hearing
Conservation

Nor Haniza Abdul Wahat

1

Noise & Noise Level
Monitoring

2

NOISE

 A periodic signal
 Various means of measuring noise levels:

 Peak
 Averaging

3

 Physical and psychological attributes of
sound:

Complexity Frequency

Amplitude

4

Frequency and Pitch

5

Frequency and Pitch

 For an oscillating current, frequency is the number of complete
cycles per second in alternating current direction

 The standard unit of frequency is hertz (Hz)
 Pitch of a sound is the sensation of a frequencies
 Mel is the perceptual scale used to describe the pitch of a sound
 A high pitch sound corresponds to a high frequency sound wave and

a low pitch sound corresponds to a low frequency sound wave

6

Amplitude and Loudness

7

Amplitude and Loudness

 Amplitude is defined as the intensity or the acoustic energy of a
sound

 The decibel (dB) is the logarithmic measurement used to measure
the intensity of the sound

 Loudness : subjective impression of the intensity of a sound
 Sone : is the unit used for the perceived loudness of a sound

8

Complexity and timbre

 Complexity of a signal reflects its spectral energy
 Timbre is another characteristics that is used to describe sound

quality
 Timbre is mainly determined by the harmonic content and the

dynamic characteristics of a sound

9

 What is a harmonic content of a sound?
 What is meant by the dynamic

characteristics of a sound?

10

The decibel (dB) notation

 dB
 is a logarithmic unit used to describe a ratio of two
sound pressures OR powers

 Non-linear scale
 Sensation increases as the log of the stimulus

11

Examples of a logarithm:

 Two loudspeakers, the first playing a sound with power P1, and
another playing a louder version of the same sound with power P2,
with the distance and frequency is the same

 The difference in decibels between the two is defined to be
10 log (P2/P1) dB where the log is to base 10

 If the second produces twice as much power than the first, the
difference in dB is 10 log (P2/P1) = 10 log 2 = 3 dB.

 If the second had 10 times the power of the first, the difference in dB
would be 10 log (P2/P1) = 10 log 10 = 10 dB.

 If the second had a million times the power of the first, the difference
in dB would be 10 log (P2/P1) = 10 log 1,000,000 = 60 dB.

12

Group Q & A

13

 What does 0 dB means?

•0 dB means that level of sound has equal power or
pressure to the reference power or pressure upon
which the scale is constructed

•0 reference for:
Sound power/energy = 10-12 watts/m2
Sound pressure = 20 µPa

14

Sound Power/Energy & Pressure

 Output of any sound source: in the form of energy. Energy per
second (power) per square meter is called intensity of the sound
wave.

 Energy / second power

 Power / m² intensity

 Energy is distributed over the area of contact in the form of
pressure (amount of force per unit area)

 Relation between intensity, I & pressure, p:

I  p²

15

Sound energy & sound pressure

 Sound energy or sound power and sound pressure are two distinct
and commonly confused characteristics of sound and the term
sound level is commonly substituted for each

 Both share the same unit of measure, the decibel (dB).

 Sound energy (power) is the acoustical energy emitted by the
sound source, and is an absolute value. It is not affected by the
environment.

 The normal reference level is 10-12 W, which is the lowest sound
persons of excellent hearing can discern. Sound energy (power) is
measured as the total sound power emitted by a source in all
directions in watts (joules / second).

16

Sound energy & sound pressure

 Sound pressure is a pressure disturbance in the atmosphere whose
intensity is influenced not only by the strength of the source, but also
by the surroundings and the distance from the source to the receiver

 Sound pressure is what our ears hear, what sound meters measure

 Because sound measuring instruments respond to sound pressure,
the "decibel" is generally associated with sound pressure
level. Sound pressure levels quantify in decibels the intensity of
given sound sources. Sound pressure levels vary substantially with
distance from source, and also diminish as a result of intervening
obstacles and barriers, wind and other factors.

17

Absolute intensity (Ix) and dB (sound
energy)

dB IL Absolute intensity, Ix Ix = the
(re 10-12 watt/m2) reference
10-6 intensity, Io
60 10-7
50 10-8
40 10-9
30 10-10
20 10-11
10 10-12
0

18

Absolute pressure (Px) and dB (sound
pressure)

dB SPL Absolute pressure,
(re 20 µPa) Px

100 2 x 106
80
60 2 x 105
40
20 2 x 104
0
-20 2 x 103

2 x 102 Pressure
2 x 101 created in air by
2 x 100 a sound wave
whose intensity
= 10-12 watt/m2

19

Qs: examples of the application for
absolute intensity and absolute
pressure

20

Combining dB values

 To determine the total sound energy from
different sources

 Combining sound sources always employs
the energy concept because it is always the
energy output that is combined, rather than
the sound pressure

21

Example

 An engineering consulting firm is invited to bid on a
noise abatement project. This firm proposes that
they will reduce the noise energy by a factor of 78:1
and the cost will be $21,000.

 Question:

 In dB, how much reduction will be accomplished and
what will be the cost per dB?

22

Solving the dB problem

 Step 1: The energy equation will be used because
the bid stated reduction in sound energy would be
attempted.

 Step 2: Insert the data into the equation: dB = 10
log 78/1 = 10 log 78 = 18.9 dB

 Step 3:

 Answer – The bid indicates that the noise reduction will
be 18.9 dB which will cost $1055.3 per dB.

23

The weighting scales

 Human ear is not equally sensitive to all frequencies

 Human perception of loudness is highly dependent upon the
distribution of the frequencies of the sounds and also by the overall
sound pressure level of the sound.

 The weighting scales have been created and internationally
standardized to best relate the physical noise measurement to
human perception and response to sound.

 The three weighting scales are:
 The A-
 The B-
 The C-

 These weighting scales differ from each other in the amount each 24
discriminates against sounds in the lower frequency range.

The weighting scales

 ‘A’ weighting scale: based on a smoothed inverse of the 40-phon
curve on the Equal Loudness Contours (ELCs)

 ‘B’ weighting scale: rough approximation of the inverse of the 70-
phon curve

 ‘C’ weighting scale: almost linear

 Sound measures using weighting filters – same measurements for
‘peak’, ‘mid-point’ or ‘troughs’

 Only valid for the phon levels from which they were derived
 E.g. ‘A’ weighting – theoretically appropriate for use only with
relatively low level sound (40 phon)

25

ELC curve

26

The weighting scales

•The A-scale attenuates the most, and the C-scale the least.

27

Different types of noise

 Steady-state noise: persistent noise with the
difference between the max & min intensity of
less than 3 dBA during the observation
period. E.g. – textile machine in operation

 Fluctuating: noise with the difference
between the max and min intensity is more
than 3 dBA. . E.g – road traffic, rock music

28

Different types of noise

 Impulse: sounds which are produced by a
sudden, explosive release of energy such as
a gunshot or the operation of a high pressure
release valve. Peak sound pressures usually
exceed 140 dB.

 Intermittent: contains rest periods of effective
quiet alternating with noisy periods. E.g. –
lawn mowing

29