Brüel & Kjær 2240 Guide De L'utilisateur page 33

Where to Place the Instrument
Legislation often specifies where measurements should be made, for example, at property
boundaries or at the complainant's property. Other factors also need to be taken into account
when measuring, because sound levels vary at different heights above ground level. They will
also vary depending on the distance between the measurement point and facades and obstacles.
These requirements must be noted and applied.
This will often mean making measurements:
• away from reflecting surfaces (for example, facades)
• with the microphone 1.2 – 1.5 m above ground level
• downwind
• in dry conditions with a wind speed of less than 5 m/s
Reflective Objects
When sound waves impact upon a surface, part of their acoustic energy is reflected from it, part
is transmitted through it, and part is absorbed by it. Generally in the case of buildings, most of
the sound energy is reflected, which makes the measured sound pressure level higher. Regula-
tions often require the exclusion of the effect of reflection from reported measurement results
(free-field conditions). Therefore, in both indoor and outdoor situations, you should try to keep
a distance of 3 m or more from reflective surfaces such as walls or large objects. Hold the
sound level meter at a natural position of 1.2 to 1.5 m above the ground/floor, since it also
reflects sound.
Special Considerations for Measurements of Noise at Work
If you are to measure the noise level exposure at a workplace, it may not be easy to stay away
from reflective surfaces, nor may you want to as you need to measure the sound levels to which
the workers are actually exposed – this includes reflections. General guidelines are as follows
(from the ISO 9612 standard):
• Place the microphone where an operator's head would normally be positioned
• If measurements cannot be made without an operator present, place the microphone close to
(approximately 10 cm from) the ear of the operator. Do this at the ear receiving the highest
noise levels
Wind, Temperature and Humidity
Type 2240 operates within the tolerances of the applicable standards over a wide range of
temperature and relative humidity. You will seldom have any problem staying within the limits
required. Always check with the relevant standards and guidelines to see which measurement
conditions must be followed. If no guidelines exist, good rules of thumb for outdoor measure-
ments are as follows:
• Measure in relatively calm conditions such as a light breeze – wind speeds of 5 m/s or less are
often required
• Always measure in downwind conditions, especially if you are at a considerable distance from
the sound source (50 m or more)
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