Development |
Historically, several methods of Artificial Mastoid calibration
were used before the IEC, ISO & ANSI settled on a method based
on the “Constant Dynamic Force” applied by the bone
conductor. The earlier “Constant Acceleration” method
producing values that are not too different from the IEC constant
force method.
The output from the artificial Mastoid can be measured with a high
impedance voltmeter, or the Sound Level Meter can be used - provided
that it is calibrated in a manner that makes the voltage output
from the artificial mastoid meaningful. The Sound Level Meter is
usually designed to be calibrated in dBµV (1µV
gives a reading of 0dB)
External Signal Calibration of the Sound Level Meter to read the Artificial Mastoid Output
The absolute response of each mastoid is measured at the factory, and is stated as a Force Sensitivity Constant (Kf) it is typically 150mV/Newton. Ours, after NATA calibration, is 132mV/N. at 1 kHz.
From dBµV = 20.log(Kf /Vref) - Where Vref = 1x10-6
We find that the typical value will read 103.5dB on a meter reading in dB referenced to 1µV (dBµV ), and our mastoid at 132mV will give 102.4dBµV (So when calibrating with an external signal generator, that 18 milli-Volts difference only produces 1dB change, which when compared to the required final accuracy of ±5dB for a bone vibrator, is not a large error!
But Reference Equivalent (hearing)Threshold Force Levels (RETFLs)
for bone conductors applied to the mastoid process are given in
dB referenced to 1micro-Newton. So how many millivolts will our
mastoid produce at 1µN?
We can say if 132mV is 1 Newton or 120dBµN, then
0dBµN is produced by 132 mV x 1x10-6
or 132 nano Volts (132x10-9 V).
Our sound level meter can not be adjusted to read dBN or dBµN directly, as the calibration pot range is limited, so we will calibrate in dBµV as it was designed to be, and will allow for this difference when using it to calibrate audiometer bone vibrators.
We can approach it like this:
• The voltage difference in dB between the µVolt scale
and the µNewton scale is
20 log(1x10-6/132x10-9) or 17.6dB for our SLM.
To read in dBµNon the SLM dBµV
scale, add 17.6dB to the sound level meter reading. So 20dBµV
is 37.6 dBµN
Or better, like this:
• Factor this into the RETFL calculations as described on the next page,and go for a direct reading in dBµV.
| Any suitable precise RMS sinusoid voltage level at 1 kHz can be used calibrate the SLM in dBµV. We adjust the SLM to read the appropriate calculated dBµV plus the losses in dB of any coupling cables and adaptors.
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| External Signal Calibration Setup |
The Artificial mastoid sensitivity figure includes the insertion loss of the supplied connecting cable, so we only need to add in the sound level meter coupling adaptor loss. (typically about 1 dB, and in the overall scheme of things can be ignored if you don’t know it.) The loss for the adaptor we have (B&K coupler J2614) is stated as 0.8dB in the manual.
For example If an external 132mV RMS signal is applied to the input
of the sound level meter, we set the SLM to read
102.4dBµV + 0.8dB = 103.2dBµV
Using the Internal Reference
Alternatively the SLM can be calibrated using the internal reference
(SLM 2203 manual, sect. 3.5.3, page 20). This assumes an input –
still from the standard microphone – of 50mV. Our coupling
device as above has an insertion loss ( “K” factor of
+0.8db) The SLM is set to read 0.8dB higher to compensate
for this. As before, this calibrates the SLM as a microvolt
meter, showing readings in dBµV
Note:
Once this procedure has been performed, a reading of the SLM sensitivity
setting can be taken using the piston phone and standard 1”
microphone, and thereafter used as a substitute calibration method.
Having calibrated the Sound Level Meter, we need to derive the
Sound Level Meter readings for the bone vibrator when it is applied
to the Artificial Mastoid and driven by the Audiometer. This is
detailed next. This calculation only needs to be done once for the
specific artificial mastoid –phew! Then calibration
can commence!
The following table includes formulae for use in EXCEL