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
f: 0,5dB. Attenuation of frequencies
outside 5x and 1/5 of the band
centre frequency is better than
75 dB.
Octave Band Pass Filters
Octave Filters are formed in both
Band Pass Filters by electronically
altering the components and charac-
teristics 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 1617 and Type 1618 conform
to IEC 225 — 1966. DIN 45651,
and ANSI S1.11-1966 Class II,
which are the strictest standards for
octave filters. Fig.4 shows the top
of a typical octave filter characteris-
tic, attenuation outside 8x and 1/8
of the band centre frequency is bet-
ter than 60dB. Peak-to-valley ripple
is less than 0,5dB, 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
0
-10
-15
-20
-25
-30
00
O
..
-2
0
4
6
8
10
12
14
16
18
20
22
0.4
Max. Ripple
(peak-to-valley)
0,5 dB
IEC
(DIN 45 652)
ANSI Class III
Typical B & K
1/3 Octave
Filter
0,6 0,8 9'0 1,4 1,8 2,5
0,5 0,7 0 9 ,2 1,6 2,0
Frequency relative to centre frequency fillm
110 1 2 4 / I
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-
CO -2
g o
12
19
20
22
04 05 06 07 cia 00 10
Max Ripple
(peak-to-valley)
0,5 dB
IEC
ANSI
Class II
Typical
A B & K
octave
Filter
12 IA lb 18 20 25
Frequency relative to centre frequency f/fm
7101250
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
1617, selected by the Range
switch, and hence two frequency
Typical Frequency Characteristics
.88181111....
MI I 11■
Linear 1 Hz 200 kHz
Ailehl MI•11111 MIS
to
Ilk. Linear 1 Hz to 200 kHz ME
1111111111111.1111191111•PPA
8 +`��,111e 11618) 11
-rnmirnor
11115111k
0.2 05
2
10
100 200
500 1 k 2k 5k 10 k 20k.
Fig.5. Weighting networks and Linear functions
50 k 100 k 200 k
500 k
'0 os
4
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