For 8
KHz
L
L
L
ANSI Standard RTL for 0.
dB
HL
=
13.0
dB
SPL
(+)60
dB
HL
=
(+)
60.0
dB
SPL
60.
dB
HL
=
73.0
dB
SPL
When calibrating using a Sound Level Meter (SLM) it is important to add or subtract the
appropriate microphone correction.
When the microphones are calibrated the
manufacturer or calibration facility should supply the microphone’s frequency response
curve.
The microphone is then calibrated to the SLM which has a flat frequency
response by using a piston phone or similar standard device. Below is an example of
a microphone frequency response curve.
MICROPHONE
CALIBRATION
CHART
l
5
dB
+1
0
-1
-5
-10
-15
Provo,
Utah
I
.
.._,...,
.q
,
,
,
!
I.
._
!
i,
_
._,‘_.,..,
1,.
__,
_,._
,
.,-_i..,
.I
11
1
--1-\I--s’
20
Hz
50 1DO
200
'
500
1K
2K
5K
10K
20K
50K
100k
'
200K
The curve shows that the microphone response is flat between 20
Hz
and 1.5 KHz,
high between 2
KHz
and 6
KHz,
and low at frequencies 8 KHz or greater. Using 4 KHz
as
an example when a standard force is applied to the surface of this microphone its
output is 1
dB
high. Therefore, when calibrating 4
KHz
using this microphone we must
add
1
dB
to our expected value. Our formula for determining the proper calibration
level for 4 KHz is as follows:
. 41
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