B&K P-161-2 User manual

INSTRUCTION MANUAL
P-161-2
AUDIOMETER CALIBRATOR

:a &0 K INSTRUMENTS. INC.
INSTRUCTION MAN U AL
P-161-2
AUDIOMETER CALIBRATOR
February 1975
Defense Activities
Shippsburg Cont~ol Ctr.
Medical Repair Branch,Bldg. 406
Mechanicsburg, PA 17055

E &. ~ INSTRUMENTS , INC .
I.
II.
TABLE OF CONT EN TS
GE NERA L DE TA ILS
TABLE 1.
TABLE 2.
IB
lC
ID
IE
THE CALIBRATION SIGNAL
CORRECTION FOR THE ACTUAL MICROPHONE
SENSITIVITY
MEASUREMENT OF 2ND HARMONIC DISTORTION
THE 2425 VTVM
CALIBRATIN G THE SY STE t'-1
2A
2B
NORMALIZING THE 2211 AMPLIFIER GAIN
SETTING IN THE LOUDNESS BALANCE CURVE
MODEL 4220 PISTONPHONECALIBRATION CHECK
III. OPERATING INSTRUCTIONS
•
3A MEASUREMENT OF FREQUENCY AND OUTPUT
FROM AN AUDIOMETER
B &K 2211 AMPLIFIERS AND FILTERS
B & K 2425 VTVM
H.P. 5221 B ELECTRONIC FREQUENCY METER
1022 OSCILLATOR
PAGE
1
1
2
3
6
7
7
7
8
11
12
12
12
13
13

E ... K INSTRUMENTS , INC .
TABLE OF CONTENTS (Continued)
MISCELLANEOUS
DRAWINGS: SK-A-288, SK-A-290, A-772
SCHEMATICS: ZG 0001 - AC Power Supply
ZE 0003 - Input Amplifier
ZE OOO~ - Output Amplifier
Electronic Voltmeter
•

13 "" ~ INSTRUMENTS. INC .
Page 1
I. GENERAL DETAILS
lAo LOUDNESS BALANCE DATA
TABLE 1.
Before the P-15l-2 Aud i o meter test stand can
be pl ac ed int o o pera t ion the test channels
must be calibrated to the requir ed loudness
balance curve. The AN SI 50 db hearing le ve l
figures are gi ve n bel ow. These levels
TEST
NORMAL
TONE
CALIBRATI ON
FREQUENCY
LEVEL
125
111.8
250 99.5
500 84.1
750 78.5
1,000
77 ; 2
1,500
78.0
2,000
78.0
3,000
75.6
4,000
74.3
6,000
79.5
8,000
86.8
are the desired acoustical outputs of the
TDH-38 and 39 Earphones. \I t would be difficult
~
to generate these sound pressures acoustically
with a great degree of accuracy and so the
•

:s & K INS~RUMENTS . INC.
Page 2
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TABLE 2.
P-161-2 is calib r ated fr om an oscill a tor
whicn is adjusted to gi ve an o utput equal to
that of the microphone. The calibration
procedure includes a correction factor t o
mo dify th e o scill a tor output t o com pe ns a te
for t he m icr o ph one s en siti vi ty . The f re qu ency
range of t he "4144 m icrophone wh ic h is us ed
in - the system begins t o fall o ff at 3 k Hz
and an additional correction is added to
- the loudness balance data to include this
factor. Correcting the data in Table 1
from the B & K 4144 frequency response curve
(Fig. 1) and incl udin g the microphone
sensitivity co rr e ction we h ave :
ANSI 60 dB HEARING LEVEL CORRE C TION OF CALIBRATION
LEVELS
/"
Microphone " Corrections Corrected Cal.
ANSI C:§ O
cm - ...... H.L.
r'
r ~
Fre quen cy
Level on
Test Tone I
Sensit ivity " R
esponse
1022
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12 5
Hz
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99.5
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77.4' '1. '2.
78.2
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78. 4 ".,
1500
2 kHz 78.0
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3 kHz 75.6
"10
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76. 3 7~.7
4 kHz
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•

E ok K INSTRUMENTS . INC.
Page 3
A} Frequency response data obtained from 4144
calibration curve supplied w ith micr o phone.
B) Sensitivity cor r ec ti on obtained fr om mi cropho ne
K factor (2.0 db) less cath o de fo llo w er
attenuation (0. 8 db) w hich is co mmon to the
microphone and oscillator inputs. (See
Section lC).
)Z NOTE: The corrected calibration figures refer to
~ microphone Serial No. 100871. Any other
microphone requires a new correction table
for ASA 60. Other tables can be developed
for other loudness balance requirements.
IB THE CALIBRATION SIGNAL
Bruel & Kjaer ITT microphones have a nominal
sensitivity of 5 mV per microbar or 5 mV output
> at 74 db SPL. This equals 10 mV at 80 db SPL
and offers a convenient relationship with the
-- £ruel & Kjaer Model 1022 Oscillator. The
oscillator is fitted with a precision output
attenuator cal . ibrated in 10 db steps. A
continuously variable amplitude control
adjusts the voltage applied to the attenuator

E "" K. INSTRUMENTS. INC.
Page 4
x
an d the attenuator input is indicated on a
voltmeter dial which is calibrated linearly
and also in db. The meter scale has 0 db
at the 10 % scale pos ition and ex tends up scale
-20 db at the f ull scale pso itio n .
An extra
2 db calibration above f ull scale i s included
to improve the accuracy of settin g the o utput.
When the meter pointer is at 0 db (10 %) and the
a- ttenuator is set to 100 mV, the actual output
signal is 10 mV thus the 100 mV position
of attenuator can be labelled 80 db SPL.
Similarly the othe r positions of the attenuator
can be marked from 40 to 100 db and intermediate
values obtained by setting the output meter
to db values on the meter scale and these are
added to the attenuator scale. As an example
an equivalent SPL of 98-db can be set at
90 db attenuator plus 8 db on the meter scale
or better still 80 db attenuator and 18 db
meter scale, (the db scale can be read more
' accurately at the higher scale readings).
lC CORRECTION FOR THE ACTUAL ~lICROPHONE SENSITIVITY
When a condenser microphone is connected into
a cathode follower, the microphone open circuit

E .Ie K INSTRUMENTS. INC.
Page 5
sensitivity is modified by TIvO effects. The
first concerns the effect of capacity loading
due to the input cap acity of the cathode follower
ac~ing as a voltage divider with the micro p hone
electrode/diaphragm capacity. The output is
also reduced by the attenuation of the cathode
follower. These effects amount to:
Capacity loading effect 0.4 db
Cathode follower attenuation 0.8 db
The
calibration method used in the P-16l-2
connects the oscillator output into the cathode
follower via a large coupl~ng capacitor thus
the capacitive loading effect does not alter
the oscillator level. The cathode follower
does not alter the oscillator level. The
, cathode follower does att ' enuate the signal
by 8.0 db. A K Factor is given for each
microphone and states the correction in db
which is necessary to make the output from the
cathode follower equal to 5 mV/pbar. If the
cathode follower attenuation (0.8 db) is
added to the K Factor, the new K Factor
refers to the cathode follower input i.e.,
the same condition as the test oscillator.
Avplying the K Factor to the oscillator output
corrects for the microphone sensitivity and the

E &. ~ INSTRUMENTS, INC .
Page 6
oscillator then gives the same output as the
microphone at the cathode follower. The
microphone sensitivity correction applied in
Table 2 were obtai ned by this logic.
ID MEASUREMENT OF 2ND HARMONIC DISTORTION
The selecti ve amplifiers have numerically
related center fre quencies thus channel 2 (250 Hz )
can be used to measure the 2 nd harmonic content
of channell. Because the , ~ ac-oustical levels
are different f or each channel (Table 2) the
ZE 0004- post filter amplifiers have different
gain settings. As an example, the 2 nd harmonic
output of 125 Hz measured on channel 2 w ould
have an extra 12.3 db of gain for the ANSI
loudness balance curve. To overcome this
limitation, each ZE 0004- amplifier is fitted
with a second attenuator knob (clear plastic)
and by setting this control to the same level
as the 125 Hz selective channel and switching
attenuator connections bya remote switch, the
250 Hz channel can be reset to the same gain
as channell. If the output of channel 2
is now measured on a VTVM, the second harmonic
content can be obtained directly from the VTVM

E &0 K . INSTRUMENTS , INC .
Page 7
.
scale wi t h out con s ide r ati on of the gain diff -
erences in the post fi lter amplifier.
IE THE 24 25 VTV t-l
A Bruel & Kjaer 24 25 VTVH is built int o the
system and is connected at the rear to th e
channel selecti ng switch mounted in the 2211
noi s e limit indicator frame.
A second input
connection is available on the front panel of
the 2425. When the signal plug is inserted
into the front input, the rear inp ut is a ut om -
atically disconnected. A separate error indicat or
calibrated ±6 db is op erat ed fro m the 24 25
· meter circuit and the 0 db error position
---- corresponds to 10 db on the 2425 meter.
II CALIBRATI NG THE SY STE M
2A NORMALIZING THE 2211 AMPLIFIER GAIN
After repairs or interchange of ZE 0004 amplifiers
not necessary on weekly calibration checks.
1) Switch on power to all instruments and allow
10 minutes to stabilize.
2) Sw itch the 2211 input amplifier (ZE 0003) to ITRef.1f
.. .

13 &.: ~ INS'rRUMENTS . INC .
Page 8
3)
Select "Fi lter inputfT on the 2211 monitor me ter
-:.,-.
and adjust the ZE 0003 fine g ain control until
the monitor meter indicates on the lower red
reference mark.
4) Select pos iti on 1 on t he m onitor m eter, swi tch
the 125 Hz chan n el (ZE 0004 Ncr: 1) to "Lin. I I
5) Turn t he blac k attenuato r knob to 0 db and
adjust the fine gain control so that the
monitor meter coincides w ith the upper red
reference mark.
6) Repeat for the other 11 channels.
7) Reset all filter switches to ITFilter-infT
position (to the left).
THE AMPLIFIER GAINS ARE NOW NORMALIZED.
2B
SETTING IN THE LOUDNESS BALANCE CURVE (\veekly calibration)
1) Patch the 1022 oscillator attenuator output into
the cathode follower using the JJ2612 adaptor
and test cable supplied.
2)
z and adjust the
output to the required db settin g using the
clear plastic overlay on the 1022 osciJ~ator output

E &. K INSTRUMENTS, INC .
Page 9
control. The db setting is given on the Table 2
for a specific microphone.
3) Set the 2211 input amplifier (ZE 0003) ga in
control t o 110 db.
Advance the o utput sel ector to channel 1
using the foot operated switch.
5) Check the oscillator frequency on the
counter and increase the ZE 0004 gain using
the black knob until the error meter indicates
o db. Use the screw adjustment for the final
correction.
6) Switch to channel 2, set the ZE 0004 for that
channel to TTLin
TT
•
7) Select IIHarmonic distortion
TT
on the toggle
switch (lower left bay) and adjust the clear
plastic gain control on ZE 0004 ch~nel 2
until the error meter is approximately at
o db. The nearest attenuator setting is
adequate. Do not change the screw adjustment
of channel 2 - ZE 0004.
8) Return channel 2 ZE 0004 to IIFilter
ll
and turn
the harmonic distortion switch to IINormal
TT
•
Set the oscillator to 250 Hz , set the oscillator

13 & K INSTRUMENTS, INC. Page 10
output to the -250 Hz sound pressure level
on the corrected loudness balance curve (Table
2) and adjust the black attenuator knob on
the ZE 0004 (channel 2) until the error meter
reads 0 db. Use the screw adjustment i f
necessary.
9) Repeat step 7 for channel 3. ,
10) Repeat step 8 for chann el 3 using 500 Hz and
the appropriate oscillator output.
11) Channel 5 is the second harmonic of channel
3 and the procedure for channel 3 and higher
channel numbers is slightly different. Proceed
as below:
12) Select channelS with the os . cillator set at
500 ~ Hz.
13) Set channelS to ITLlrf.1T Select harmonic distortion
on the toggle switch and adjust the clear
-----------..
plastic gain control of channel 5 until the
error meter indicates approximately 0 db.
Do not change the screw adjustment.
14) Swi tch Channel 5 to ITFil ter IT •

E & K INSTRUMENTS , INC,
Page 11
15)
1 6)
Cycle the fo ot oper ated selector to c ha nnel
4 and repeat the sequence for associated
channels.&.!-e-, 5-7, 6-'&, 7-9, 19: 11. '-II
':1-", ,-g-~ g-ID//#-/"2...
J.}.4.C
Usi ng ~.O db at 250 Hz in put, set the 7.E OOO~
to 120 db and ad ju st the black at tenuator of
channel 12 to in d icate 0 in the linea r position.
Reset the ZE 0003 gain to 110 db w hen completed.
The system is now calibrated to the loudne s s
balance curve and channel 12 is adjuste d for
pistonphone calibration checks.
~ - .
MODEL 4220 PISTONPHONE CALI BRA TION CHECK
1) Remove the couplter from the artificial ear
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and push the 4220 pistonphone onto the micro p hone.
2) Select channel 12 and turn the filter switch
to linear.
3) Turn the ZE 0003 to 120 db.
4) Turn on the p istonphone and th e error meter
should read 0 db ± .2 db.
5) Reset the ZE 0003 to 110 db.
THE SYSTEM ·HAS NOW BEEN CHECKED ACOUSTICALLY.

E "" ~ INS'rRUMENTS. INC.
Page 12
2C METER PROTECTION
All meter circ uits in th e P-161-2 audiometer
calibrat or are e lectrically protected and ar e
not damaged by off scale meter indications .
III OPERATING INSTRUCTIONS
3A MEASUREHENT OF FREQUENCY AN D OUTPUT FROM AN
AUDIOMETER
1) Turn the system on and allow 10 minutes to
stabili ze . Set the various cont ro ls as
listed below:
B & K 2211 AMP LI FI ER S AND FI LTERS
ZE 0003
ZE 0001+
Normal/Distortion
(switch)
B & K 2425 VTVH
Range
Meter Switch
110 db position
"Filter-in positionlT
Gain as set during calibration
ITNormal
lT
position
(VU Damping Meter function)
NO PLUG SHOULD BE LEFT IN THE FRONT INPUT

> E
&.:B: IN STRUM EN TS , INC .
H.P. 5221 B E LECT R ON IC FREQ UENCY METER
Gate Se lector
Sensi t ivity
1022 OS CIL LA TOR
1 sec.
1 V.
Page 13
2) Set th e system to selective channe l 1 using
the f o ot switch.
3) Fit th e left or right earphone of the audio-
mete r o nto the artificial ear and lo wer the
weighted arm onto the earphone.
4) Sele ct the correct TTLoudness curve setting
TT
on th e audiometer and set the frequency dial
to 12 5 Hz.
5) Read th e audiometer error on the error meter.
If the error meter is off scale, a broader
range can be obtained by looking at the 2425
mete r. This meter indicates at 10 db when the ·
error meter is at 0 db thus 0 db on the 2425
corr es ponds to -10 db error and 20 db
corresponds to +10 db error.
6) Advance the audiometer to 250 Hz , step the
calibrator to channel 2 and read the error
from the meter.

13 &. ~ INSTRU::M:ENTS. INC .
Page 14
7) Continu e for all channels .
3B MEASUREMENT OF SECOND HARMONI C OUTPUT
1) Select "Distorti o n" on the system front panel .
2)
Advance to channel 2.
3) Set the audiometer to 125 Hz.
4)
Set the 2425 to 1 V RMS.
5) Read the 2425 meter. The meter scale w ill
show the harmonic outpu t. The 10 db scale
point indicates -20 db of second harmonic
distortion and the 5 db po int sho ws -25 db of
harmonic distortion. It is only necessary to
ensure that the 2425 VTVM indicates in the
greet portion of the meter scale. (below 5 db)
6)
Continue through the frequency range of the
audiometer using the following calibrator
channels and audiometer frequencies.
AUDIOMETER
FREQUENCY
HARMONIC DISTORTION
MEASURED ON CHANNEL
125
250
500
750
1
1.5
2
3
4
6
8
12
Hz
kHz
kHz
kHz
kHz
kHz
kHz
kHz
kHz
2
3
5
6
7
8
9
10
11
12
THIS COMPLETES THE HARMONIC DISTORTION NEA S URE~lEN TS!

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