BRUEL & KJAER 2203 Instructions And Applications

5.2. REFLECTION
Whenever an object as large or larger than the wavelength of the
sound is placed in the sound field, it will cause reflections and therefore
irregularity in the field. Normally, when the sound field is diffuse, or
consists of many frequencies, or both, no problem arises, but if the
sound waves are free, plane or spherical with one or two predominant
frequencies the reflections produced can cause considerable errors of
measurement.
To reduce reflections from the Sound Level Meter to a minimum, the
front of the instrument is cone shaped.
An idea of the reflections caused by the operator can be obtained from
Fig.5.6. Investigators have found that anomalies due to such reflections
are usually most marked in the frequency range 200- 4000Hz. Errors
of 2- 3 dB may easily result and around 400Hz (where the maximum
reflections from the human body occur) up to 6 dB may even be experi-
enced.
An excellent treatment of this rather complicated subject of re-
flections has been published by R. W. Young in the journal "Sound",
Vol. 1 1962 page 17.
rr I B=~dJ k4iWv ' *~ ~ f ~· I
--~--l _____ 50 70 100 200 300 500 700 1k 2k 3k 5k 7k 10k 20kHz
50 70 100 200 300 500 700 1 k
2k 3k 5k 7k 10k
20kHz
Fig. 5. 6. Reflections due to the presence of an operator
Whether the presence of the operator has any influence on the sound
level reading or not can be determined by changing the relative position
of the operator to the instrument. Usually, sufficiently accurate readings
are
obtainable by simply fitting the extension rod and holding the sound
level meter forward so the microphone is 1 m away from the body. See
Fig.5.6.
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BRUEL & KJAER 2203 Specifications

General

Brand	BRUEL & KJAER
Model	2203
Category	Measuring Instruments
Language	English