BRUEL & KJAER 1615 User guide

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. Instructions
" and Applications ~
I
Octave and 1/3 Octave
Filter Sets 1614 and 1615
The Filter Sets are compact
instruments
of plug-in unit
construction. Types
1614 and 1615
filters conform to the IEC 225 and
ANSI Sl. II. standards and cover
the 2 Hz-160 kHz and
25 Hz-20 kHz
ranges respectively. "A", "B",
"C", "D" and "Linear" networks are
also included. Both the filters and
the networks can be automatically
scanned.

Octave and 1/3 Octave Filter Sets
Types 1614 and 1615
Reprint may 1970

CONTENTS
1. INTRODUCTION • • • • • • • • • • • • • • • • • . • • • • • . • • • • 3
2.
TECHNICAL DESCRIPTION • • • • • • • • • • • • • • • • • • • • • 5
Input Amplifier . . . . . . . . . . . . . . . . . . . . . . 5
1/3 Octave Filters . . . . . . . . . . . . . • . . . . . . . 5
Octave Filters ......... . .............. 10
Effective Bandwidth . . . . . . . . . . . . . . . . . . . 12
Phase Response ...................... 12
Filter Transient Response ............... 13
Weighting Networks .................... 14
Linear Ranges . . . . . . . .............. 14
Output Amplifier . . . . . .............. 14
Remote Control . . . . . . . . . . . . . • • • . . . . . . 14
3. CONTROLS •.•••••..••••.•....•••...••••.. 16
4. OPERATION ••••••.•..•••.•.•.••••••••.••• 19
General ...................•••..... 19
Conversion of 1615 to 1614 ............... 19
Use with Microphone Amplifier 2603/04 ..•..... 21
Use with Amplifier 2606 ..........•...... 23
Use with Frequency Analyzer 2107 ........... 25
Use with Precision Sound Level Meter 2203 and
Impulse Precision Sound Level Meter 2204 ..•... 27
Use with FM Tape Recorder 7001 ........... 28
Use with Random Noise Generator 1402 ........ 28
Use with Level Recorder 2305 • . . . • . • • • • . • . . 31

5. TYPICAL MEASUREMENT AND ANALYSIS SET -UPS USING
THE TYPES 1614/15 .....•.......... . ...... . .. 36
Direct analysis of sound
.... 36
Direct analysis of vibration . . . . . . . . . . . . . . 37
Bands
of Random Noise . . . . . . . . . . . . . . . 38
Analysis from Tape Recorder ...... . ....... 38
Low Frequency Analysis ............... . . 38
Analysis of Transients . . . . . . . . . . . 38
Recording
of Low Frequency Sound ........ . . 39
6. USEFUL FILTER INFORMATION AND DEFINITIONS ..... 41
Ideal Filter (Band-pass)
Effective Bandwidth
Octave
Filter Centre Frequency .. .. .. .
Filter Band-Edge Frequencies
Bandwidth
......... .
...... 41
...... 42
...... 42
. 43
. 43
. 43
Spectrum . . . . . . . . . . . . . . . . . . . . . . 43
Noise
............................. 44
7. SPECIFICATIONS ..........•................ 45

1. INTRODUCTION
The Filter Sets Types 1614 and 1615 are a development of the older design
(Type 1612).
The filter performance is generally improved and the Type
1614 includes extra filters at both low and high frequencies. Both 1614 and
1615 contain the sound weighting networks A, 8, C, and the more recently
introduced D network.
The Filter Sets are primarily designed for vibration and acoustic frequency
analysis, and have specifications and tolerances which conform
to the
following standards:
IEC 225 1966
ANSI S1.11-1966 (1/3octaveCiasslll, 1/1 octave Class II)
DIN
45652
The filters are band-pass filters, with 1/3 octave bandwidths. The filter
centre frequencies are spaced
1/3 octave apart, and filter selection is by a 50
-position switch on the front panel. In the octave mode, 3 adjacent 1/3
octave filters are connected such as to produce a 1/1 octave bandwidth.
Filter selection
is either manual or by remote control, enabling synchroniza-
tion with other equipment.
Linear ranges and
the A, 8, C, and D networks are also selected by the 50-
position switch.
The input and output to the filters can be connected to a wide range of
terminating impedances. For Field measurements provision is made for 12 V
DC operation.
Frequency analysis over a very wide frequency range can be accomplished
with
the Filter Set in conjunction with a 8 & K Sound and Vibration
Measuring amplifier
Type 2606 or Type 2603 or the 8 & K Sound Level
Meters. 1/3 octave or 1/1 octave bands of random noise may also be
produced by using
the Filter Set in conjunction with the 8 & K Random
Noise Generator
Type 1402. These and other typical uses of the Filter Set
are briefly mentioned in Chapter 4 of this Manual.
3

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Input
01 Bandwidth
1: 1/3 Oct.
2: 1/1 Oct.
02 Frequency Range
1: 25Hz-20kHz D-A-8-C-Lin
2: 2Hz-160kHz
03 50 Position Filter Switch
04 Scanning Selector
1: Manuel
2: Remote
05 Linear t8Hz-200kHz
• 1: On
2: Off
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1'7oo15

2. TECHNICAL DESCRIPTION
The block diagram in Fig.2.1 shows the basic design of the instrument. The
signal is fed via an input amplifier to the filter inputs, which are connected
in parallel. The input amplifier has a high input impedance, and drives the
filters from a low output impedance. The filter outputs are selected by the
50 -position switch. When in the octave mode, the outputs of three
adjacent 1/3 octave filters are connected together (via resistors). The signal
from the filters is fed to two inputs of a buffer amplifier.
A subsidiary range of filters, from 25 Hz to 20 kHz with D, A, B, C and
Linear filters
can also be selected. This is also the only frequency range of
the 1615, which does not possess the low and high frequency filters.
The 50 position switch can be operated by actuating the electromagnetic
drive unit from the remote control socket.
The Filter set can be operated from normal mains supplies or from a 12 V
DC accumulator, for which purpose there is a DC- AC converter.
INPUT AMPLIFIER
The input amplifier provides the low impedance (rv0.8Q) drive required for
the filters. It has an input resistance of 125 KQ in parallel with 55 pF, and a
gain
of 0 dB. Its frequency response is flat from 1.8 Hz- 200kHz within
±0.1 dB.
1/3 OCTAVE FILTERS
The 1/3 octave filters are all 6-pole Butterworth networks and have the
centre frequencies and bandwidths given in Table 1.
The 2 - 200 Hz filters are active circuits consisting of 3 stagger tuned
Wien-bridge amplifiers, with an emitter follower output to reduce the out-
put impedance.
The other 1/3 octave filters are passive networks. Schematic diagrams for
both active and passive 1/3 octave filters are shown in Fig.2.2 and 2.3
5

Centre
Centre
Frequency Bandwidth Frequency Bandwidth
Hz
Hz Hz Hz
2 0.46 630 145
2.5 0.58 800 183
3.15 0.73 1000 230
4 0.92 1250 290
5 1.16 1600 370
6.3 1.45 2000 460
8 1.83 2500 580
10 2.30 3150 730
12.5 2.90 4000 920
16 3.70 5000 1160
20 4.60 6300 1450
25
5.8 8000 1830
31.5 7.3 10000 2300
40 9.2 12500 2900
50 11.6 16000 3700
63 14.5 20000 4600
80 18.3 25000 5800
100
23
31500 7300
125
29
40000 9200
160
37
50000 11600
200 46 63000 14500
250
58
80000 18300
315
73
100.000 23000
400
92
125.000 29000
500
116
160.000 37000
Table I. Filter centre frequencies and bandwidths.
6

<>--1
Input
c 201
<>--:-1
Input
L201
Ground
Fig.2.2 Schematic diagram of a single 1/3 octave active filter
network
L 202 p
201 c 202 p 203 L 203
c 203
Ground
Fig.2.3 Schematic diagram of a single 1/3 octave passive filter
network
The overall characteristics conform to the existing standards for 1/3 octave
filters. Typical examples
of filter characteristics are shown in Fig.2.4, 2.5;.
Peak-valley ripple in the pass band is less than 0.5 dB and rejection is greater
than 75 dB at frequencies of five times and one fifth of the centre frequen-
cy.
A typical set of adjacent filter characteristics is shown in Fig. 2.6 The freq-
uency characteristics cannot be expected to intersect exactly at the 3.7 dB
point because of the steep slope at this point. A very small displacement of
the filter centre frequency will cause a large displacement of the point of
intersection.
A buffer amplifier is placed directly after the 1/3 octave filters. This is
necessary because the filter characteristics are very dependent on the
terminating impedance.
7

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~Typical B&K _
\ 1 /3 Octave Fi lte
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', ANSI
, class
m
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20
Frequency Hz
-17o1 :l~
Fig.2.4 Typical 1/3 octave filter frequency response and tolerances

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0.5 0.6 0.7 0.8
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1.2
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ANSI Class m
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1.4 1.6 1.8 2.0
Frequency relative to centre frequency tftm
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Fig.2.5 Top of typical 1/3 octave frequency response and tolerances
cocococoooocoooacooo aoooooooocooaooc ooooocoooocoooooooo
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OP1102 10Hz
Frequency-
100Hz
Fig.2.6 Frequency response of several adjacent 1/3 octave filter
9

OCTAVE FILTERS
The octave filters are formed from three adjacent 1/3 octave filters, con-
nected in parallel with resistors in series with other. Octave filter centre
frequencies are given
in bold type in Table I.
Typical octave filter response is shown in Fig.2.7. All filter responses are
measured with single frequency sinusoidal voltages.
The peak-valley in the pass band is better than + 0.5,- 1.0 dB, and the
rejection outside the passband is greater than 60 dB at eight times and
one-eighth
of the centre frequency.
10
-2
-1
0 dB
Attenuation max rippl• (p•ak-to-vaU.yl
1.5 dB (- 0.5 to +1.0 dB)
10
11
12
13
14
15
16
17
18
19
ANSI
Class II
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 13 1.4 1.5 f/fm 2.0
Fr•qu•ncy
r•latin to c•ntr• fr~u.ncy
Fig.2.7 Top of typical octave filter response
Typical B&K
Octave Filter

c
0
:...
0
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c
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c coooooo o oooaooo oooooooooooooooooooa ooooaoooooooono o ooo
--
~ : ______ __ ~4 ~ k ~ H ~ z ---- ---~ ~a~ ·k ~ H~ ~ z ________ ~ 16 ~ ~ ~ H ~· z ~ __ ~ F ~ r ~ e
2
q ~ u ~ e;- ~ c ~ y ________ ____ ~_ , _·~_
0
_,_- ____ ~
Fig.2.8 Frequency response of several adjacent 1/1 octave filters
dB
c
0
0
10
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15
20
270°
180°
goo
lag
c oo
0
0 lead
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goo
Ql
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0
..c 180°
ll.
270°
1fJ
1
/s
1!s
1!3 octave
Frequency --
1 63 6+1
Fig.2.9 Phase response of Filter Set in 1/3 octave condition
11

EFFECTIVE BANDWIDTH
Table I gives the bandwidth at the -3.7 dB points. This bandwidth is 1/3
octave and is also the effective bandwidth of the filter. See "Definitions".
When filtering white noise, the RMS noise voltage from the filter can be in
error ±0.4 dB when using the effective bandwidth of 1/3 octave, because of
the production tolerances on the filters.
PHASE RESPONSE
The filters, by their nature, produce considerable phase distortion in the
pass band. Figs.2.9, 2.10, show the limits within which the different filter
curves for the whole apparatus are expected to lie.
12
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dB
10
c
0
c
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c
Ql
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30
6300
540°
360°
t 1800
log
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:0 180°
Ql
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a. 360°
540°
6300
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% "S :Ys +'s 1fs -o+ 1/s 2fs :ts "6 !j(; octave
Frequency - /6f9 :1 f
Fig.2.10 Phase response of Filter Set in 1/1 octave condition

FILTER TRANSIENT RESPONSE
The filter inputs are all connected to the input terminal in parallel so that
during an analysis when the filter set is being scanned, there are no filter
transients.
The transient response of filters is actually quite complex,
depending
on the input waveform as well as the type of filter, but normally
need not be taken into account for measurements with the 1614/15.
However,
the response of the filters to a suddenly applied sinusoidal signal
has been determined, and
is shown in Fig.2.11.
The filter output normally requires about 15 periods to stabilize to the
steady state. This means about 8 sees. for the 2 Hz filter, so this could
become a difficulty when making low frequency measurements. If scanning
starts
at 2 Hz and is too fast, and the input signal is started at the same time
as scanning,
then the filter output will not have reached the steady state.
8 sec. would be required in the 2 Hz filter, from the time of switching on.
Input Waveform Input Waveform
-~~~-- \\\\111\\~,.....-
Output Waveform
Output Waveform
at Filter Centre
Freguency fo
at Filter lower
Bandedge Freguency f1
Fig.2.11 Filter transient response to sinusoidal burst (shown in
correct time relationship to input signal
410 0 78
13

WEIGHTING NETWORKS
The A, B and C weighting networks are internationally standardized (IEC
179 for precision sound level meters) and their use is well-known in sound
measurement.
The D network was more recently introduced, and complies
to the DIN standard and to the proposed IEC standard. The frequency
responses of these filters are shown
in Fig.2.12. They are selected by the 50
position switch when the Frequency Range selector is in position I.
Am
plificotion
dB
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linear 2 Ha- 200 kH•
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lin• or 2 Hr -200 kHr
.....:::::::
~ 1\ 22 . HH•
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B+C
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169003
Fig.2.12 Frequency response of A, B, C, D and linear Scales
LINEAR RANGES
The linear ranges are 22.4 Hz to 22.4 kHz on the 50 position switch, and
1.8
Hz to 200 kHz on the pushbutton switch.
See specifications for further details.
OUTPUT AMPLIFIER
The output amplifier compensates for an attenuation in the 200 kHz LP
filter of about 6 dB and presents the correct output impedance to the
output sockets (i.e. 600i1).
REMOTE CONTROL
The Filter Switch can be manually or automatically operated, the latter
being achieved by a remotely controlled electromagnetic drive unit. To
obtain remote control an external DC supply (24 V, 180 mA) in series with
a single-poled switch
is required. The connections to be made to the jack
REMOTE CONTROL can be seen in Fig.2.13.
14

NOTE: Take care that the +terminal of the external DC source is connect-
ed with pin 2 and the -terminal with pin 1. If inversely connected
a spark-suppressor diode will be damaged.
When the switching action is controlled from the Level Recorder Type 2305
a special cable AQ 0019 has to be applied between the two instruments.
2Hz s wit ch
posi t ion
24 v
Re-mote- Co ntr ol
Vie-we-d e-xte-rna lly
Fig.2.13 Connections to socket REMOTE CONTROL for remote control
of Filter Switch.
15

Power-IF:i•••i w
Scanning --.----..J
3. CONTROLS
Fig.3.1 Front Panel
hll/;t<)el , ~ ~ . tM.
~f,, Out~Od . r 1r U:l-h w ~ lin.
~----.-Range
ln1/30ct.
Out1/1 Oct.
CONTROLS AND TERMINALS, FRONT PANEL
POWER
SCANNING
16
Push-button on-off switches.
With
the Manual button depressed the Filter
switch may be turned freely to any position.
When the Remote button is depressed the
electromagnetic drive unit is switched in and
the filter switch can be operated from the
Remote control socket on the rear panel.