Method |
Given: Reference Equivalent Threshold Force Levels
in AS1591.(ISO 389) as dB ref. 1µNewton as applied to typical
subject by a bone vibrator. (Column B)
Find: The output level a bone vibrator will produce
in our artificial mastoid at these RETFL settings:
• The first step is to convert RETFLs dB Ref.1 micro Newton
to Newtons, including the Audiometer dial setting of 40dB which
is to be used during the calibration procedure. (Column C, D)
• The sensitivity of the Artificial Mastoid, (from supplied
Calibration chart, below) is 132 millivolts per Newton at 1kHz,
24°C. Using this sensitivity figure, find the artificial mastoid
output in mV when the required force (Column D) is applied to it.
(Column E)
• Convert the mastoid output (mV) to the expected Sound Level
Meter reading in dB ref 1µV . (dBµV
) (Column F)
• Because the mastoid does not have a flat frequency response,
we have to adjust the expected dB reading at the other frequencies.
Just add the Mastoid response from the chart, (Column G) to give
the final reading (Column H)
Constant Dynamic Force Derivation of Sound Level
Meter Calibration Levels Using
ISO 389 / AS1591.1 1995 RETFLs
A |
B |
C |
D |
E |
F |
G |
H |
|
Frequency Hz |
Reference Equivalent Threshold Force Levels
ISO 389 AS1591.1 |
Threshold Force +40dB in N
Calibrate the audiometer using a level setting
of 40dB. Add this to RETFLs and then Convert from dBµN
to Newtons: |
Mastoid output in mV using mastoid force
sensitivity 132mV/N |
SLM Reading. Threshold Output dB for specific mastoid, calibrated In dBµV |
Calibration Chart contour (corrections
to be added to Threshold output) |
SLM reading after Mastoid contour corrections |
|
|
|
Table Values RETFLs dB Ref 1µN |
RETFLs + Setting
dB Ref 1µN |
Alog ((Threshold
force |
="D"
x Force Sensitivity |
=20*(LOG10( "E"x0.001/1µV)) |
From Cal Chart. |
|
125 Hz |
82.5 dBµN |
122.5 dBµN |
1.333 N |
176.02mV |
104.9 dBµV |
-1.0 dB |
103.9 dBµV |
250 Hz |
67.0 dBµN |
107.0 dBµN |
0.223 N |
29.55 mV |
89.4 dBµV |
-1.0 dB |
88.4 dBµV |
500 Hz |
58.0 dBµN |
98.0 dBµN |
0.079 N |
10.48 mV |
80.4 dBµV |
-1.0 dB |
79.4 dBµV |
750 Hz |
48.5 dBµN |
88.5 dBµN |
0.026 N |
3.51 mV |
70.9 dBµV |
-0.7 dB |
70.2 dBµV |
1000 Hz |
42.5 dBµN |
82.5 dBµN |
0.013 N |
1.76 mV |
64.9 dBµV |
0.0 dB |
64.9 dBµV |
1500 Hz |
36.5 dBµN |
76.5 dBµN |
0.006 N |
0.882 mV |
58.9 dBµV |
0.7 dB |
59.6 dBµV |
2000 Hz |
31.0 dBµN |
71.0 dBµN |
0.003 N |
0.46 mV |
53.4 dBµV |
1.3 dB |
54.7 dBµV |
4000 Hz |
30.0 dBµN |
70.0 dBµN |
0.003 N |
0.41 mV |
52.4 dBµV |
1.0 dB |
53.4 dBµV |
4000 Hz |
35.5 dBµN |
75.5 dBµN |
0.005 N |
0.79 mV |
57.9 dBµV |
-2.5 dB |
55.4 dBµV |
6000 Hz |
40.0 dBµN |
80.0 dBµN |
0.010 N |
1.32 mV |
62.4 dBµV |
-9.0 dB |
53.4 dBµV |
8000 Hz |
40.0 dBµN |
80.0 dBµN |
0.010 N |
1.32 mV |
62.4 dBµV |
-8.5 dB |
53.9 dBµV |
|
EXCEL |
|
=("B"+40) |
=POWER(10, |
="D"x132 |
=20*(LOG10("E"x0.001/ 0.000001 |
|
="G"+"F" |
|
 |
Frequency Response at Constant Dynamic
Force |
Calibration using constant acceleration follows a similar method, and produces results that are similar, with some discrepancy at higher frequencies.