Instruction Manual

Sound Level Meter Types 2245 – Instruction Manual Page 23 of 110
2.8 Placing the Microphone (Sound Level Meter)
The sound level meter must be placed away from shielding, reflecting, or absorbing
objects. In a diffuse sound field, absorbing objects will reduce the measured sound
levels. In a free sound field, reflecting objects can change the measured sound levels.
Typically, the sound level 0.5 m from a plain reflecting wall is 3 dB higher than if there
was no wall.
The operator of the system may be personally shielding, absorbing, and reflecting, and
the operator can also be an additional noise source.
The optimum position for the microphone is best found by trying different positions
and observing the resulting sound levels.
For outdoor noise measurements (or indoor measurements exposed to air movement)
mount Windscreen UA-1650 onto the microphone and preamplifier combination,
making sure it snaps into place over the windscreen sensor.
It is good practice to measure downwind in dry conditions with a wind speed less than
5 m/s.
It is recommended to place the sound level meter on a tripod.
2.9 Mounting the Sound Level Meter on a Tripod
To minimise the influence of the operator on measurements, the sound level meter
should be mounted on Tripod UA-0750, possibly using the tripod adapter UA-2237.
For all practical purposes, the sound level meter fulfils the requirements of IEC 61672–
1 in this configuration, with or without Windscreen UA-1650 fitted. However, tripod
mounting still presents some major difficulties in the measurement of acoustical
characteristics of sound level meters and it is, therefore, normally beyond the scope of
type approval for sound level meters.
2.10 Measuring Low-level Sounds
If the measured sound level is within the Linear Operating Range or, for C-weighted
peak sound levels, within the Peak C Range given in the specifications (see section 4.9.7
and section 4.9.8), then the self-generated noise and level linearity problems can be
ignored.
It is possible to correct the measured sound levels, except peak levels, for the typical
self-generated noise, found in the specifications, see section 4.8.2. The correction for
self-generated noise can be made by subtracting self- generated noise, L
inh
, from the
total sound level, L
tot
, using the following equation:
If L
tot
– L
inh
is less than 3 dB, the sound level is too low to be compensated.