ous standards applying
to
third-
octave
filters
. The response
curve
for
a typical
third-octave
filter
is
shown
in Fig.2, and
the
top of
the
curve
in
the
enlarged
view
in Fig .3 .
The
IEC,
DIN ,
and ANSI
limitations
are also
indicated in both Figures. Peak-to-
valley ripple in
the
pass
band
is less
than
0,5
dB
with
attenuation
within
±
0,5
dB
.
Attenuation
of
frequencies
outside 5 x and 1/ 5
of
the
band
centre
frequency
is
better
than
75dB
.
Octave
Band
Pass
Filters
Octave
Filters
are
formed
in both
Band Pass
Filters
by
electronically
altering
the
components
and
charac-
t~stics
of
the
Filter
circuits.
This
ar-
rangement
gives a
flat
crest
to
the
characteristic
curve, and
low
peak-
to-valley
ripple
.
The
octave Filters
cover
the
frequency
range
from
2 Hz to
20
kHz
centre
frequencies,
selectable at
third-octaves
in
the
preferred series.
There
is
no
provi-
sion for
connection
of
full-octave
fil-
ters at
higher
frequencies
in
either
Filter Set.
All
octave
Filters
contained
in
the
Type 1 61 7 and Type 1 61 8
conform
to
IEC
225
-
1966,
DIN
45651
,
and ANSI S 1.
11-1966
Class II,
which
are
the
strictest
standards
for
octave
filters.
Fig .4
shows
the
top
of
a typical octave
filter
characteris-
tic,
attenuation
outside
8 x and 1/ 8
of
the
band
centre
frequency
is bet-
ter
than
60
dB . Peak-to-valley ripple
is less
than
0,5
dB,
while
attenua-
tion
in
the
pass
band
is
within
±
0,5
dB
.
The
total
integrated
random
(white) noise
power
passed by
the
practical octave and
third-octave
Fil-
+5
a
Linear 1
Hz
to
200 kHz
/"
!-
/V
V
C
-5
-10
-15
'g
- 2
c
o
.
~
:J
C
e
;;.
o
2
4
6
8
~;
I
16
18
20
22
L-
__
~~~L_~~~~~~~
0,4 0,6 0,8 1,0 1,4 1,8 2,5
0,5 0,7 0,9 1,2 1,6 2,0
Frequency relative
to
centre frequency flfm
170124/1
Fig.3.
Top
of a
third-octave
filter
characteris-
tic
ters
in
the
Type
1617
and Type
1618
is equal to
that
which
would
be passed by an ideal octave
or
third-octave
filter.
Weighting
Networks
In
addition
to
the
Linear
re-
sponse, and
A-weighting
network
in-
cluded in
the
Type
1618,
the
Type
1617
contains
B- and
C-weighting
networks,
plus
the
D-weighting
net-
work
specified in
IEC
537
for
meas-
urement
of
aircraft
noise. The
fre-
quency
responses of
the
four
weighting
networks
are
shown
in
Fig.5
. The Figure also
indicates
the
Linear
range 1 Hz
to
200
kHz
obtain-
able
from
the
Type
1617,
and
the
Linear
range 1 Hz
to
40
kHz
from
the
Type
1618.
Filter
Selection
and
Scanning
Ranges
Filter
switching
is
accomplished
electronically
by
FET
switches
in
the
Filter
Selectors
that
are
regu-
~~
~i!all
Fre~Ue~cy
Char~ctJri
!
ti~sl
V
lI::
,.......f-'
b=
I:?""
~
./
B
/"
V
0
~
~
/,
~
/
-20
-2
5
-30
0,2
V
a,s
V
V
10
,/
V
/
20
50 100 200
500
1 k 2 k
:-
Fig.5
.
Weighting
networks
and
linear
functions
4
'"
-2
"0
_,
§ 0
'';::; I
..
:J
2
a;
3
:::
.
« 5
0.5 0.6 0.7 Q.8
0.9
1.
0
1.2
1.-4
1.6
1
.8
2
.0
2.5
Frequency relative
to
centre frequency flfm
170125/2
Fig.4 .
Top
of a
typical
octave
filter
charac-
teristic
lated by
the
Digital
Controller
acting
on
instructions
from
internal
or
re-
mote
control
settings
.
Bandwidth
of
the
Filter
in
use is selected by
the
three
position
Selectivity
switch,
giv-
ing a choice
of
third-octave
band-
width
scanning
in
third
-octave
steps, octave
bandwidth
scanning
in
third-octave
steps
(with
adjacent
bands
partially
overlapping).
or
oc-
tave
bandwidth
scanning
in
full-
octave steps.
Manual
selection
of
any
particular
Filter
band is
made
by
turning
the
Manual
Filter
Selector
control
to
the
required
position
.
On
the
Band Pass
Filter Type
1618
the
Manual
Filter
Selector
has a scale graduated in
third-octave
centre
frequencies
from
2 Hz
to
20
kHz,
plus
the
two
positions
"A-weighting"
and
"lin-
ear"
.
There
are
two
measuring
ranges on
the
Band Pass Filter Type
1 61
7,
selected by
the
Ra
nge
switch
, and
hence
two
frequency
0
I'
"'-
~
"'-
"-
"'"
~
B+C
r\
5 k 10 k 20 k
Linear 1
Hz
to
200 kHz
1'.
~
1'\
\
1161
1
S1
I'"
1\
'"
\
\
~l\
"-
~
"
i
~
1\
50 k lOa k 200 k
500
k
770105
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