| Using
SLMs in the field
The
next thing to do is to
note the directional
response characteristics
of the microphone.
Ideally the
microphone will respond
equally to all sound
waves hitting it,
irrespective of the
angle of incident, ie.,
it will be
omnidirectional.
However, in
practice it is not
possible to achieve
this, and the person
making the measurement
has to choose between
two general types of
microphone:
Pressure
response microphones are
adjusted to give as
nearly as possible a
uniform frequency
response in a diffuse
sound field (ie. where
the sound waves arrive
equally from all
directions). For most accurate results, this type of microphone should be
aligned with its axis at
right angles to the
line(s) linking it to
the main source(s).
A term with
similar meaning is
random incidence
response.
Free
field microphones are
designed for se where
the sound arrives from a
single direction.
These have as
uniform as possible a
frequency response to
sound arriving along the
axis of the microphone,
but for sound arriving
from other directions,
the response is less
good.
For most accurate
results, this type of
microphone should be
pointed directly at the
main source.
This type of
field is usually
applicable near a source
or out of doors where
there are not
reflections of the sound
from walls or other
surfaces.
Misalignment
of the microphone when
taking readings can well
result in slight
over-reading with
pressure types and
under-reading with free
field types.
The
directional response of
either type of
microphone is also
affected by its size and
the frequency of the
noise, the response
being more uniform where
the microphone is small
and the frequency low.
Under industrial
conditions, noise may be
radiated from several
sources, and
consequently it may be
difficult to decide how
the microphone should be
aligned, or to maintain
the alignment if the
equipment generating the
noise moves around.
The HSE advise
(in Noise Guide 3) that
where a small microphone
is used (12 mm or less
in diameter) the effect
will be reduced and
precise alignment will
not be important except
where the sound is of
high frequency.
The example they
quote as an exception is
the measurement of
audible noise emitted by
ultrasonic equipment.
In
practice, the user will
often simply measure
with whatever instrument
and microphone are
available and little
significant error will
arise.
However, it is as
well to be aware of the
issues just described.
In
addition, the effect of
wind should be
considered.
Wind
is obviously a
particular problem
outdoors, depending on
the prevailing
measurement conditions,
but can also be a
problem with indoor
measurements.
Depending on the
degree of air movement,
the error created may be
10 dB or more.
The error can be
minimised by using a
wind shield.
This is a spher5e
of foam or wire mesh
covered in a thin cloth
which is placed over the
microphone.
It will stop the
wind from reaching the
microphone but
shouldn’t block the
sound waves to any
serious degree.
In case the
windshield does have a
slight effect on the
measurement, you should
note in your records
when you use it,
especially when you are
doing precision
measurements.
In
practice, the windshield
is often kept in place
even for measurement
indoors, as it provides
physical protection to
the microphone.
The exact
measurement position
chosen will depend on
the reason for measuring
the noise.
Measurements
should not be made too
close to the source, the
sound field is very
complex here and moving
the microphone a short
distance is likely to
produce marked changes
in the level.
As a rough guide,
the microphone should be
at least 1 metre from
the noise source.
The
sound level meter should
be held at arms length,
or better still, mounted
on a tripod.
An operator
standing immediately
behind the instrument
may reflect sound waves
back towards the
microphone and so
produce a false reading.
Mounting the
instrument on a tripod
also leaves the
operator’s hands free
for noting down the
measurements.
It
is important always to
check that the correct
measurement parameter,
response time, weighting
network, etc. have been
selected.
It is very easy
to miss-set one thing,
especially with some of
the newer
multifunctional
instruments with
electronic displays.
A double check at
the start could save the
need to repeat a series
of measurements.
»
Sound
Level Meters
»
Microphones
»
The
Dynamic Range
»
Calibration
»
Equipment
Standards
»
Using
SLM's in the Field
»
Sound
Level Instruments
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