BRUEL & KJAER 2204 - Page 41

b) The sound often originates from many sources simultaneously, take for
example a machine shop.
c) Angle of incidence may vary during measurement, for example as a car
passes or an aeroplane flies overhead .
-
DDDDDDODODDDDDODDDDDDDDODDDODDDDDD D DDDDDODDDOOOODDD
Br D:!!,J c.r + .5025 lro.l & l:imt lrG.I & (J- a.o.I .. Ki- 1075
- ~d il e ~ ~ ~~~~~=~~ - ~
22
~ 0 ~ 3~ ~ U ~ A ~ 0 ~ 03 1 9 1 + 1 41 ~ 31 ~ + ~ U ~ A ~ 0 ~ 05 1 5 illlllliil!l!iilil o l o l ,n l c lid ~ en l c i e jlj!jllj !l~l ' ··
With rgodom .a ,
j ncjdence carr
Fig. 6.2. Frequency response of the instrument using a 4t:J1 microphone with
Random Incidence Corrector and Extension Rod.
In all these cases it is desirable that the sensitivity of the measuring instru-
ment should not vary too much with angle of incidence. Therefore when the
sound contains important high frequency components it may be necessary
to improve the directional characteristics of the Sound Level Meter. This
may be done in several ways:-
.
DDO DD.OD O ODDDD D DODDDDD D DDDDD D D D DDDD DD DDD D O DD DDDDOO
~: 11 11 · I I~ 1· 1· 1 1111 1· 1 ·1· 1·1 11~1 1· 1 · ~- ~ l i
,..o .. .. 1nn .. ... 1ooo .... H7 "" 1oooo - "' • ' c ....
160610
Fig. 6.3. Frequency response for various angles of incidence rvllen Nose Cone
U.4 0051 is employed. (Microphone Type 4131 ).
l. Use an Extension Connector UA 0039 and replace the microphone pro-
tection grid with a Random Incidence Corrector Type UA 0055. This
gives the instrument improved directional cha1·acteristics as shown in
Fig. 6. 2. A so- called "random incidence curve" is also shown which has
been calculated from the sensitivities for several well-defined angles of
incidence in accordance with the formula rec11mmended by the I.E.C.
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