BRUEL & KJAER 2204 - Notes on Reflection

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Fig. 6.4. Frequency response curves of the instrument when employing the
Microphone Type .f.US with Nose Cone UA 0052.
(Publication No. 123), and effectively gives the microphone response in a
diffuse sound field.
2. Another solution is to use a Nose Cone Type UA 0051 in place of the
microphone protection grid. See Fig. 6.3. The Nose Cone is primarily de-
signed to reduce wind noise, and for improving omnidirectivity it is not
as effective as the Random Incidence Corrector.
3. The omnidirectivity of a smaller microphone extends to higher frequen·
cies than that of a larger microphone. Consequently good high frequency
characteristics are obtained by fitting a 1/2'' microphone to the Sound
Level Meter. This involves the use of Adaptor UA 0030. Still further im-
provements are obtained when using the Nose Cone UA 0052 with the 1/2''
microphone. The directional characteristics thus obtained are shown in
Fig. 6.4. Note that the sensitivity of the 1/2'' microphones is approximately
1 mV/p,bar, so that when the 1" microphone is rejlaced by a 1/2'' micro-
phone a K-value of approximately 14 dB must be added to the Sound Level
Meter
reading.
4. Certain arrangements have optimum response for go• incidence, and some-
times it may be possible to arrange that all the sounds measured have
go• incidence. This is the case when the microphone diaphragm is in a
horizontal plane so that all sounds reach it tangentially.
Notes
on Reflection.
The precise measurement of sound with portable instruments is sometimes
hampered by reflections from the operator and also from the instruments
themselves . When the sound field is diffuse or when the sound consists of
many frequencies this presents no problem and the results obtained will
only depend on the accuracy of the instrumentation. However when the
sound waves are planar and the sound consists of one or two single fre-
quencies there is a possibility of considerable reflections and consequent
build - up of sound pressure.
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