Dr. L.Y. Chan

1
Basic AcousticThe Hong Kong Polytechnic
University
2
Sound

• For the human listener, sound in the frequency
  domain is defined as the energy between wave
  frequency 2 20,000Hz.
• Lowest frequency, pitch like quality 20Hz
• Upper frequency audible 12,000Hz
• Frequency band with band width 11,980Hz
• Sound is a physical disturbance in a medium (a
  gas, liquid or solid) that is capable of being
  detected by human ear. Noise is unwanted sound.

3
Frequency frequency band
A frequency band defines a range of frequency
between the specific or implied limits. The
center frequency f0 (of a frequency band) is the
frequency corresponding to the mid-pt between the
upper and power frequency limits. In acoustics, a
logarithmic scale is commonly used to cover the
wide range of frequency. e.g. The frequency band
707 to 1414Hz has a center frequency of 1000Hz
not 1060Hz the bandwidth is 707Hz
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Frequency frequency band
The term octave band center frequency (or even
center frequency f0) is frequently used to
specify completely the band. The preferred octave
band center frequencies are 31, 63, 125, 500,
1000, 2000, 4000, 8000, 16,000Hz. Frequency that
are octave apart sound similar and one of the
frequencies is double the other. The octave bands
may be further divided into equal distant parts.
In Europe, a three part division is normally used
and each part is called one-third band.
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Loudness
The loudness of a multi-frequency (normal) sound,
is not linearly related to its intensity. Human
being perceive sounds that have the same
intensity but different frequencies as different
loudness, since the ear is a non-uniform
transducer where sensitivity varies from ear to
ear. These impulses are transmitted to the brain
where analysis takes places, introducing another
variable, our personal ear.
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Equal loudness contour
Human sensation towards loudness of sound is less
sensitive at low and high end. In such condition,
for any objective measurement to represent human
sensation, it is designed to give heavy weighting
at both low and high frequency range, hence the
establishment of weighting networks is sound
level meters.
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Equal loudness contour
Fletcher-Munson equal loudness contours. (Source
Eward B. Magrab, Environmental Noise Control, New
York Wiley, 1975. Reprinted by permission.)
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Octave bands
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Preferred Band Limits and CenterFrequencies for
1/1 and 1/3 Octave Bands
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Intensity
The intensity I of sound can be determined with
precision (quite accurately) and is related to
the amount of sound energy received per second
from the source of sound. Human can perceive a
vast range of intensity from sound 10-12 Wm-2 to
sound 100 Wm-2. The intensity level in decibel
(dB) 10 x log of the intensity ratio 10 x
log10(I/Io) I - measured intensity in Wm-2 Io -
reference intensity, usually 10-12 Wm-2
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Intensity
One important result of working in intensity
level is that if the intensity of the source in
doubled, then because of the mathematical
compression, there is only an increase of 3dB in
the intensity level. In practice, it is more easy
to measure the pressure variation than intensity,
since the intensity is directly proportional to
the square of the pressure, the sound pressure
level, SP in dB is defined as
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Intensity
SPL 20 x log10(P/Po) nearest whole no. P -
measured pressure in Nm-2 Po - reference
pressure, usually 2 x 10-5 Nm-2 Band pressure
level The term band pressure level in dB
combines both the energy and frequency band
concepts. The BPL is the measured or calculated
SPL for a specific range of frequencies.
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Decibel Levels
The response of human ear to sound pressure range
from 2 x 10-5 N/m2 to 200 N/m2 covering fourteen
decades. It is therefore necessary to represent
this wide range of acoustic quantities such as
sound pressure, sound intensity and sound power
as a logarithm of the ratio of the specific
quantify to a reference level.
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Decibel Levels
Sound pressure level, dB SPL 10 log (P/Pref2)
20 log (P/Pref) where Pref 2x10-5 N/m2
Sound intensity level, dB SIL 10 log
(I/Iref) where Iref 10-12 Watt/m2 Sound
power level, dB SWL 10 log (W/Wref) where
Wref 10-12
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Decibel Levels
Note that Iref Pref2 / (PoCo)ref where Pref
2x10-5 N/m2 (Theshold of hearing)
(PoCo)ref 400 rayl ? Iref
(4x10-10)/400 10-12 Watt/m2 Also Wref Iref x
Aref where Iref 10-12 Watt/m2 Aref
1m2 ? Wref 10-12 Watt
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Decibel Levels
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Adding, Subtracting Averaging Decibel Levels
Adding If
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Adding, Subtracting Averaging Decibel Levels
Example Determine the total Sound Power Level due
to contributions
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Adding, Subtracting Averaging Decibel Levels
An alternative but less accurate means to add
decibel is to employ the decibel chart as below
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Adding, Subtracting Averaging Decibel Levels
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Adding, Subtracting Averaging Decibel Levels
Example What sound pressure level results from
combining the following three levels (all re
20??Pa) 68 dB, 79 dB, and 75 dB? Solution We
begin by selecting the two lowest levels 68 dB
and 75 dB. The difference between the values is
75-68 7.00. Using Figure 7-4, draw a vertical
line from 7.00 on the abscissa to intersect the
curve. A horizontal line from the intersection to
the ordinate yields about 0.8 dB.
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Adding, Subtracting Averaging Decibel Levels
Thus, the combination of 68 dB and 75 dB results
a level of 75.8 dB. This, and the remainder of
the computation, is shown diagramatically
below. Rounding off to the nearest whole
number yields an answer of 81 dB re 20 ??Pa.
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Adding, Subtracting Averaging Decibel Levels
Averaging
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Adding, Subtracting Averaging Decibel Levels
Example If variation is 5 dB or less, i.e.
Lmax-Lmin?5
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Adding, Subtracting Averaging Decibel Levels
If variation is between 5 dB 10 dB, i.e. 5Lmax-Lmin?10 Example In the above example
Lmax-Lmin 100 - 90 10 dB Lav ? 1/4(96
100 90 97) 1 96.8 dB
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Weighted Levels, Band Levels Spectrum Levels
Noise level associate with different noise
sources are usually measeured as a function of
frequency. The frequency spectrum of a noise is a
plot of the sound pressure level at each
frequency over the range of interest.
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Weighted Levels, Band Levels Spectrum Levels
Most noise sources radiate sound energy over a
wide frequency range, This whether a noise
contain mainly low frequency or high frequency
content. The weighting networks are labelled A,
B, C.
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Weighted Levels, Band Levels Spectrum Levels
The SPLs AR different frequencies are measured.
Weighting function (A, B, C, or F) is then
applied to the measured SPLs. For A-weighting,
attenuations at low frequencies are higher this
corresponds to the insensitivity of human ear to
low frequency sound. Thus, if the difference
between the A-weighted SPL (in dB A) and the
C-weighted SPL (in dBC) is large, the noise is
primarily composed of low frequencies low than 1
kHz.
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Weighted Levels, Band Levels Spectrum Levels
Figure 7-6 Response characteristics of the three
basic weighting network
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Weighted Levels, Band Levels Spectrum Levels
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Weighted Levels, Band Levels Spectrum Levels
Example A power station worker spends all his day
around a turbine where the octave band noise
levels are as follows
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Weighted Levels, Band Levels Spectrum Levels
Determine the A-weighted band sound pressure
level and the overall A-weighted sound pressure
level Overall A-weighted SPL 10 log
106.9108.2108.2108.1108.0108.3108.3107.4
89.6 dBA
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Community Noise Scale

• For Steady Noise
• Fluctuation within 4 dB
• A-weighted overall sound pressure level is the
  most favourable scale adopted LA
• For Fluctuating Noise
• Fluctuation beyond 4 dB
• Noise rating scales mostly employed-
• 1. Equivalent continuous sound level Leq
• 2. Percentage exceeded sound level LN

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Equivalent Continuous Sound Level, Leq
Equivalent continuous sound level Leq is defined
as a constant sound level which results in the
reception of the same as the actual varying level
over the period of time interested.
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Equivalent Continuous Sound Level, Leq
For discrete time intervals
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Percentile Exceeded Sound Level, LN
Percentile Exceeded sound level is the sound
level that exceeded n of the total
T L10 - Peak Level L50 - Mean
Level L90 - Background Level
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Percentile Exceeded Sound Level, LN
LN value can be measured during a simple sound
level meter by noting the sound level at regular
intervals, say 5 to 10 seconds over a reasonably
long period of time. The results can be used to
plot the cumulative distribution curve from which
LN can be obtained.
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Percentile Exceeded Sound Level, LN
Example A student performed a traffic noise
assessment and recorded the following-
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Percentile Exceeded Sound Level, LN
Example
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Percentile Exceeded Sound Level, LN
Example 10 seconds / reading x 100 readings
1000 s (16 min.)
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Percentile Exceeded Sound Level, LN
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Percentile Exceeded Sound Level, LN
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Percentile Exceeded Sound Level, LN
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Percentile Exceeded Sound Level, LN
Fig. 7-23 Cumulative distribution curve.
(Couttesy of B K Instruments, Inc., Cleveland.)
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Percentile Exceeded Sound Level, LN
Fig. 7-24 Probability distribution plot.
(Couttesy of B K Instruments, Inc., Cleveland.)
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Noise Exposure
The Factories and Industrial Undertakings
(Amendment) Regulation 1982 of Hong Kong
indicates a maximum permissible noise exposure of
an equivalent continuous sound level of 90 dBA
for an 8-hr working duration.
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Noise Exposure
If exposure time is normalized to 8 hr, then t
8 L Leq Noise Dose - total
fractional exposure f ? fi
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Noise Exposure
Example An aircraft engineer spends his working
day in the following environment Determine
the total daily fractional exposure and the
normalized Leq (8hr) value.
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Noise Exposure
Example ?Total daily fractional exposure
1.98 0.12 0.016 2.12 Normalised Leq (8 hr)
value Leq 10 log f 90 10 log 2.12
90 93 dBA
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Noise Exposure

NOISH Occupational Noise Exposure Limits for
continuous or intermittent noise exposure.
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Hearing Impairment

• 1. Eardrum rupture from intense explosive noise
• 2. Neural damage involving injuring to hair cells
• excessive shearing forces-mechanical damage
• overdrives-metabolic failure
• HTL - Hearing Threshold level

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Threshold Shift

• Temporary Threshold Shift (TTS) - removal of the
  noise over stimulationwill result in a gradual
  return to baseline hearing thresholds
• sym ringing in the ear muffing of sound
  discomfort of the ears
• occurs 1-2 hr exposure, recover begins 1-2 hr
  after exposure, within 24 hr.
• Permanent Threshold Shift (PTS)
• NIPTS
• Noise-induced Permanent Threshold Shift

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Threshold Shift

• Permanent Shift in HTL of the ear due to exposure
  to noise
• Sym deft-sensori-neural type of hearing
  loss-irreversible
• Blasts or explosions
• Longer exposure to lower, but still damaging
  noise
• Factors affecting threshold shift
• Important variables in the development of
  temporary and permanent hearing threshold changes
  include the following.

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Threshold Shift
1. Sound level Sound levels must exceed 60 to 80
dBA before the typical person will experience
TTS. 2. Frequency distribution of sound Sounds
having most of their energy in the speech
frequencies are more potent in causing a
threshold shift than sounds having most of their
energy below the speech frequencies. 3. Duration
of sound The longer the sound lasts, the greater
the amount of threshold shift. 4. Temporal
distribution of sound exposure The number and
length of quiet periods between periods of sound
influences the potentiality of threshold shift.
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Threshold Shift
5. Individual differences in tolerance of sound
may vary greatly among individuals. 6. Type of
sound - steady-state, intermittent, impulse, or
impact The tolerance to peak sound pressure is
greatly reduced by increasing the duration of the
sound. Presbycusis - loss of hearing due to
age HL ? drugs, disease, blows on head etc.
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Threshold Shift
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Threshold Shift
Fig 7-17 An audiogram illustrating hearing loss
at the high frequency notch.
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Introduction to Noise Control Techniques
In order to perform noise control, one has to
understand the energy transmission pattern. The
energy flow diagram is illustrated
below- Source ? Transmission ?
Receiver Hence, noise control can be categorized
to- Source Control Transmission Path Control
and Receiver Control
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Introduction to Noise Control Techniques

• Control of Noise at source
• by substitution of quieter machine
• by modification of existing equipment and plant
• by use and sitting of equipment
• by proper maintenance

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Introduction to Noise Control Techniques

• Transmission Path Control
• by increasing the distance between noise source
  and the listener
• by introducing Noise Reduction Screen and
  enclosure
• (1) machinery enclosure
• (2) acoustic shed
• (3) acoustic screen

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Introduction to Noise Control Techniques

• Receiver Control
• Ear Protector
• (1) Permanent re-use ear plugs
• (2) Mineral down and waxed cotton wool plugs
• (3) Compressible plastic foam ear plugs
• (4) Semi-insert protectors (canal caps)
• (5) Ear muffs