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ACCELERATED MOTION, LAMINAR AND TUBULENT FLOW
BY
GILA ARHIWO MANGA (UAM/APP/P/1491)
AND
OGAH PETER EJEGWOYA (UAM/APP/P/3564)
A PAPER PRESENTATION ON PHY 687: ADVANCED AEROSOL
PHYSICS.
FEDERAL UNIVERSITY OF AGRICULTURE MAKURDI
APRIL, 2013
1
ABSTRACT
The need to ascertain the noise exposure of workers during their n
ormal
working day, has led to the development of the noise dosimeter. This is a small,
light and compact instrument to be worn by the worker. It measures the total
Aweighted sound energy received and expresses it as a proportion of
the
maximum A-
weighted energy that can be received per day. This study was
carried out on Noise Dose Meter and Audiometer. The working principle of a
NDM and the Audiometer, the difference between NDM and SLM where all
vividly explained.
2
1.1 NOISE DOSE METER
A noise dosimeter (American) or noise dose meter (British) is a
specialized
sound level meter
intended specifically to measure the noise
exposure of a person integrated over a period of time; usually to comply with
Health and Safety regulations such as the Occupational Safety and Health
(OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard or
EU
Directive
2003/10/EC.
Basically, there are two different instruments to measure noise exposures:
the sound level meter and the dosimeter. A sound level meter is a device that
measures the intensity of sound at a given moment. Since sound level meters
provide a measure of sound intensity at only one point in time, it is generally
necessary to take a number of measurements at different times during the day to
estimate noise exposure over a workday. If noise levels fluctuate, the amount of
time noise remains at each of the various measured levels must be determined.
To estimate employee noise exposures with a sound level meter it i
s also
generally necessary to take several measurements at different locations within
the workplace. After appropriate sound level meter readings are obtai
ned,
people sometimes draw "maps" of the sound levels within different areas of the
workplace. By using a sound level "map" and information on emplo
yee
locations throughout the day, estimates of individual exposure levels can be
developed. This measurement method is generally referred to as "area" noise
monitoring.
A dosimeter is like a sound level meter except that it stores sound level
measurements and integrates these measurements over time, providing
an
average noise exposure reading for a given period of time, such as an 8-hour
workday. With a dosimeter, a microphone is attached to the employee's clothing
and the exposure measurement is simply read at the end of the desired time
period. A reader may be used to read-out the dosimeter's measurements. Since
the dosimeter is worn by the employee, it measures noise levels in
those
locations in which the employee travels. A sound level meter can als
o be
positioned within the immediate vicinity of the exposed worker to obtain an
individual exposure estimate. Such procedures are generally referred t
o as
"personal" noise monitoring.
Area monitoring can be used to estimate noise exposure when the noise levels
are relatively constant and employees are not mobile. In workplaces
where
employees move about in different areas or where the noise intensity tends to
fluctuate over time, noise exposure is generally more accurately estimated by
the personal monitoring approach.
3
In situations where personal monitoring is appropriate, proper positioning of the
microphone is necessary to obtain accurate measurements. With a dosimeter, the
microphone is generally located on the shoulder and remains in that position for
the entire workday. With a sound level meter, the microphone is stationed near
the employee's head, and the instrument is usually held by an individual who
follows the employee as he or she moves about. Manufacturer's instructions,
contained in dosimeter and sound level meter operating manuals, should be
followed for calibration and maintenance. To ensure accurate results, it is
considered good professional practice to calibrate instruments before and after
each use.
1.2 TYPES OF NOISE DOSE METER
1.21 Personal Noise Dose Meter
Personal sound level meters are in fact integrating sound level meters
designed as dosimeters in order to be worn by the worker during his regular
work. These instruments make it possible to record on almost any increment of
time the equivalent level, the peak level or any statistical parameter. Typically it
will record the LAeq,T (in dB(A)) and Lpeak (in dB) every second. This is
extremely interesting as it makes it possible to analyse the evolution of the noise
exposure during the day and to correlate it to the type of work or the location of
the worker. This type of instrument makes use of the equal energy principle and
offers generally a much broader dynamic range than dosimeters. They are
definitely expected to replace dosimeters in the near future and in fact are
already referred to as dosimeters by some manufacturers and users. Personal
sound level meters or personal sound exposure meters conform to the IEC
61252 standard.
1.3 WORKING PRINCIPLE OF A NOISE DOSE METER
The need to ascertain the noise exposure of workers during their normal
working day, has led to the development of the noise dosimeter. This is a small,
light and compact instrument to be worn by the worker. It measures the total
A-weighted sound energy received and expresses it as a proportion of the
maximum A-weighted energy that can be received per day. This instrument is
particularly useful whenever the exposure varies appreciably during the
working day. The maximum A-weighted energy that is permitted to be received
per day is defined in standards or regulations: it is absolutely necessary that the
dosimeter be calibrated on the basis of the adopted standard (e.g. 85 dB(A) or
90 dB(A) for an 8-hour exposure), including the accepted trading rule, which is
3 dB(A) in accordance with the ISO 1999 - 1990 standard (and for most
European countries) and 5 dB(A) for the OSHA Standard (USA). The 3 dB(A)
trading rule is consistent with the equal energy principle: 96 dB(A) during 2
4
hours providing the same energy as 93 dB(A) during 4 hours or 90 dB(A)
during 8 hours. The 5 dB halving rate assumes that 90 dB(A) during 8 hours is
equivalent to 95 dB(A) for 4 hours or 100 dB(A) for 2 hours. Dosimeters are
actually sound level meters having a DC output signal converted into a series of
impulses which are counted to provide the dose. The technical characteristics of
dosimeters must then be the same as for type II sound level meters.
1.4 USES OF NOISE DOSE METER
If mobile work patterns exist in your workplace and a noise dosimeter fits
the bill, it is important to realize precisely how these instruments must be used
and understand their limitations. Given the logarithmic nature of the decibel
scale, a variance of only 1 or 2dB can often mean serious misinterpretation of
noise levels. This margin of error should be accounted for and the worst-case
scenario measurement taken as the reading, particularly when close to an action
level.
A noise dosimeter consists of a microphone on a cable, which can be
clipped to a collar. The microphone cable is then passed under the clothing to
the unit itself, which is small enough to be located in a pocket or clipped to a
belt. The dosimeter can then be started at the beginning of the shift. If it runs
until the end of the working day, the noise dose can be directly read from the
instrument or downloaded without the need for calculations. Another useful
feature of noise dosimeters is that they will log the noise data so that when
downloaded to a PC, the time history of the noise can be viewed. This gives the
ability to analyze when and where high noise exposures occur. This can be even
more useful when the dosimeter is placed on an employee who is prepared to
make a list of the times and jobs he or she was performing throughout the day.
This will give the employer the ability to see which operations most need noise
control in order to reduce exposure.
A traditional noise dosimeter is fixed to the worker's belt, and then a
microphone on a cable is attached to the collar near to the ear. You should make
allowances for human nature. Employees fitted with dosimeters and their
colleagues will often shout into the microphones, distorting the readings, so it's
best to ignore the first few days' results until the novelty wears off.
One advantage of dosimeters is that if employees wear them for complete
working shifts, the noise dose is measured in full. However, if you need to make
several measurements of different employees in the same day, a dosimeter can
be moved to different employees, as long as the measurements taken for each
employee are representative of their working day. Most modern dosimeters also
5
will project the noise dose forward to the standard eight hours, so no
calculations are needed.
With innovations in digital technology, noise dosimeters are becoming smaller
and smaller. The latest "badge" dosimeters have certain advantages over
traditional dosimeters. Because the dosimeter is small and light enough to be
worn on the shoulder, it means there are no cumbersome microphone cables. If
there are no cables to get in the way, not only is it safer to wear, but also
employees are less resistant to wearing it and much more likely to forget it is
there. This means the quality of the noise data collected will be improved.
Because of the small size of badge-type products, it is also possible to mount
them in more innovative ways, such as on a hard hat, close to the ear, without
interfering with an employee's working process in any way. This allows the
dosimeter to not be mounted on clothing at all, therefore completely removing it
from the employee's mind.
The dose provided by the instrument is of course dependent on the
duration during which the instrument is used. Therefore, it should first be
corrected for an 8 hour period and then converted to the daily noise exposure
(LEX,8) level according to the relevant formula (ISO or OSHA).
It is important to know that some old dosimeters do not take into account levels
below 89 dB(A) or 80 dB(A), as they assume that lower levels do not lead to
hearing impairment. The LEX,8 is then physically not correct. These dosimeters
are obsolete and should be discarded. On certain instruments, a warning marker
is activated if the peak level ever exceeds 140 dB.
It is worth noting that the characteristics of the dosimeters have never been
standardized.
6
2.1 AUDIOMETER
An audiometer is a machine used for evaluating
hearing loss
.
Audiometers are standard equipment at
ENT (ear, nose, throat)
clinics and in
audiology
centers. They usually consist of an embedded hardware unit
connected to a pair of
headphones
and a test subject feedback button, sometimes
controlled by a standard PC. Such systems can also be used with bone vibrators,
to test conductive hearing mechanisms. Audiometer requirements and the test
procedure are specified in
IEC
60645,
ISO
8253, and
ANSI
S3.6 standards. An
alternative to hardware audiometers are software audiometers, which are
available in many different configurations. Screening PC-based audiometers use
a standard computer and can be run by anybody in their home to test their
hearing, although their accuracy is not as high due to lack of a standard for
calibration. Some of these audiometers are even available on a handheld
Windows
driven device. Clinical PC-based audiometers are generally more
expensive than software audiometers, but are much more accurate and efficient.
They are most commonly used in hospitals, audiology centres and research
communities. These audiometers are also used to conduct Industrial
Audiometric Testing. Because these audiometers can typically be
calibrated
to
an accuracy of fractions of a
decibel
, calibration is more accurate than hardware
audiometers. Some audiometers even provide a software developer's kit that
provides researchers with the capability to create their own diagnostic tests.
2.1 WORKING PRINCIPLE OF AN AUDIOMETER
Any audiometer that is used to test hearing should be
calibrated
regularly
to ensure that the level shown on its display is equal to the actual stimulus the
subject is exposed to. Accurate and reliable measurements are the first stage in
characterising and quantifying hearing loss; furthermore, proper calibration
ensures that the measurements are consistent, regardless of the clinic or indeed
the district or nation where the measurements are carried out. Audiometers are
calibrated using an Ear Simulator System (or ‘Audiometric Calibration
System’). For testing air-conduction hearing mechanisms, the systems include
an ear simulator or acoustic coupler, conforming to either IEC 60318-1 (a.k.a.
artificial ear) or IEC 60318-3 (reference coupler) respectively. These devices
essentially consist of a
calibrated microphone
with an associated coupling
volume, which is open on one side to allow application of
headphones
when
testing. For testing bone-conduction hearing mechanisms, the ‘system’ will
include a mechanical coupler (a.k.a. artificial mastoid) conforming to IEC
60318-6. This device uses a series of rubber layers to couple the bone-vibrator
(which in testing delivers the stimulus to the patient) to a force transducer. This
enables the device to mimic the way in which sound is transferred through the
mastoid part of the temporal bone
, to which the bone-vibrators are applied when
testing.
7
2.2 ARTIFICIAL EAR / REFERENCE COUPLER (AIR-CONDUCTION
EAR SIMULATOR SYSTEMS)
Both the ‘artificial ear’ and ‘acoustic reference coupler’ are designed for
the purpose of
calibrating
(air conduction) audiometer outputs. When
calibrating, the audiometer headphones are applied to the open side of the
coupler, and the internal (calibrated) microphone is used to detect the resulting
sound pressure level
in the cavity. The electrical output from the microphone is
usually measured with a
sound level meter
, and gives an indication of the sound
pressure level that a real listener’s
eardrum
would be exposed to by the same
signal and headphones when used in hearing tests. It is the complete
combination of artificial ear, sound level meter and associated leads etc. that is
referred to as the ‘Audiometric Calibration System’.
The
microphone
and associated coupling volume comes in two distinct types:
the IEC Acoustic Reference Coupler (specified in IEC 60318-3) is simply a
cylindrical coupling volume (volume depending on application) with a 1-inch
microphone at the far end. The second, the IEC Artificial Ear (specified in IEC
60318-1) is designed to be used with a ½-inch microphone, and has various
extra cavities and portholes adjoining the main coupling volume between the
headphone and microphone. These serve to alter the acoustical transfer
impedance (with varying
frequency
) in the main cavity in a way that mimics
that of a human
outer ear
. Though both devices are still relevant, and comply
with ISO 389-1 (for the calibration of supra-aural headphones), the more
sophisticated Artificial Ear provides a better indication of the actual
sound
pressure level
at the average patient’s
eardrum
.
The calibration of audiometers using an ear simulator is a form of
secondary calibration
; the ‘audiometric calibration system’ (ear simulator
system) is used to validate / calibrate the output from an Audiometer. Bear in
mind, however, that the
sensitivity
of the testing system itself is liable to drift
with time (due to factors such as instability in the
sound level meter
,
microphone drift) and hence also requires regular calibration. The calibration of
ear simulator systems is often carried out by specialist test and calibration
laboratories, or a National Measurement Institute, such as
NPL
in the UK.
Calibration is by comparison; in the case of air-conduction systems this means
comparison of the ‘system’ output against known levels as measured by a
calibrated reference microphone
. Note that this includes specifying a method for
setup of the
sound level meter
, as well as testing the
sensitivity
of the
microphone and associated connectors and
pre-amplifier
. Once the ear simulator
system has been calibrated, it can be used to calibrate audiometers ‘in the field’.
8
A similar calibration procedure to the above takes place for bone conduction
audiometry, using a device called an artificial mastoid. As per air-conduction ear
simulator systems, the system can be used to test the force hearing level
delivered by an audiometer at a particular frequency, but the artificial mastoids
must themselves be regularly calibrated.
2.3 ARTIFICIAL MASTOID / MECHANICAL COUPLER
(BONE-CONDUCTION EAR SIMULATOR SYSTEMS)
An artificial mastoid (or IEC Mechanical Coupler, as specified in IEC
60318-6) is an acoustic device that is designed to present a
frequency
dependent
mechanical impedance approximating that of the
mastoid part of the temporal
bone
. They are primarily used to
calibrate
the bone-conduction channel of an
audiometer, which themselves are used to test patient’s hearing in response to
bone-conduction mechanisms by presenting known force levels at specified
frequencies.
The device consists of a ‘dome’ that represents a mechanical simulation of the
human mastoid, in terms of its frequency dependent mechanical impedance, and
has a force transducer to measure the force level delivered when bone-vibrators
are applied. The electrical output from the force transducer is usually measured
with a
sound level meter
, and gives an indication of the force level, (or force
hearing level when referred to the RETFL, ‘Reference Equivalent Threshold
Force Level’) that a real listener would be exposed to by the same signal and
bone vibrator when used in hearing tests. It is the complete combination of
artificial mastoid, sound level meter and associated leads etc. that is referred to
as the ‘Audiometric Calibration System’.
Whilst the artificial mastoid, as part of an audiometric
calibration
system, is
used to calibrate the bone vibrator force output of audiometers, the sensitivity of
the testing system and the mechanical impedance of the ‘dome’ is prone to drift
and hence also requires regular
calibration
. The calibration of artificial mastoid
systems is usually carried out by specialist test and calibration laboratories, or a
National Measurement Institute, such as NPL in the UK.
A bone-conduction audiometric calibration system must be calibrated in
two specific ways: firstly to ensure that the mechanical impedance of the
‘dome’ is within the tolerance specified in IEC 60318-6, and secondly to ensure
that the reading on the
sound level meter
is representative of the force hearing
level which would be delivered to the patient. Calibrating the system includes
specifying a method for setup of the
sound level meter
, as well as testing the
sensitivity of the force transducer and associated connectors and pre-amplifier.
Once the ear simulator system has been calibrated, it can be used to calibrate
audiometers ‘in the field’. It is also worth noting that the artificial mastoid is
9
very sensitive to
temperature
; special precautions such as temperature isolated
enclosures are usually necessary when transporting the device in hot or cold
conditions.
A similar calibration procedure to the above is used for air conduction
audiometry, using either an Artificial Ear or Acoustic Reference Coupler. As per
bone-conduction ear simulator systems, the system can be used to test the sound
pressure level delivered to the patient by an audiometer at a particular
frequency, but the system itself requires regular calibration.
10