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Verifit User’s Guide Version 2.8
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7.7
ANSI test frequencies
FastFacts 7.7: ANSI test frequencies
7.8
HIT calibration facts
FastFacts 7.8: HIT calibration facts
The Frequencies feature allows hearing instruments to be evaluated at the Special
Purpose Average (SPA) frequencies defined by ANSI S3.22, rather than the
default High Frequency Average (HFA) frequencies. The choice of SPA (or HFA)
frequencies influences the average OSPL90, average full-on gain, reference test
gain, bandwidth and equivalent input noise. The frequencies at which distortion is
measured are one-half the selected SPA frequencies. You can also select the
frequencies at which attack and release times are measured.
To change the averaging frequencies:
1)
Press <Setup>, then highlight and <PICK> [Frequencies].
2)
Highlight and <PICK> the HFA/SPA frequencies box, to generate a drop-down
list box of alternate test frequencies.
3)
Highlight the desired HFA/SPA test frequencies and press <PICK> to select.
To change the frequencies for attack and release time measurements:
In the list of available frequencies, highlight and <PICK> the frequencies to be
used to measure attack and release time.
Press <Continue> to exit.
HIT microphone calibration establishes a correction curve for the uncalibrated HIT
reference microphone by comparison with the factory calibrated coupler
microphone. It may be performed as often as desired but will be invalidated at 1
am the following day or at 1 am the following Monday, depending on the choice of
Daily or Weekly calibration interval. After the calibration has expired, you will be
prompted to calibrate whenever you attempt a test that uses the HIT microphones.
Note that there are two styles of HIT reference microphones. The newer style is
larger but responds beyond 16 kHz and has a lower noise floor. The two have
different correction curves, as shown below, but will produce the same test results,
once calibrated. Your calibration curve should be similar to one of these and
should remain stable over time. Irregularities in the calibration curve may be the
result of positioning errors. See
7.9: Calibration of HIT reference microphone.
