


                             FP35 Measurement Methods
                                  10/06/00 -med

        Measurement methods:

          Signal   Source Frequency   Method       Window       Mode
         Composite  Off     0Hz     FFT (100Hz)  Raised Cosine  RE+CM
         Composite  On      0Hz     FFT (100Hz)  Rectangular    RE+CM

           Tone     Off   Sweep     DFT (100Hz)  Raised Cosine  RE+CM
           Tone     Off   100Hz **    ERMS       Raised Cosine  RE+CM
           Tone     Off  non-100Hz*    DFT       Raised Cosine  RE+CM

           Tone    <85dB  Sweep     DFT (100Hz)  Rectangular    RE
           Tone    <85dB  100Hz **     DFT       Rectangular    RE
           Tone    <85dB non-100Hz*    DFT       Rectangular    RE
           Tone   >=85dB  Sweep     ERMS (100Hz) Rectangular    RE
           Tone   >=85dB  100Hz **     ERMS      Rectangular    RE
           Tone   >=85dB non-100Hz*    DFT       Rectangular    RE

           Tone    <90dB  Sweep     DFT (100Hz)  Rectangular    CM
           Tone    <90dB  100Hz **     DFT       Rectangular    CM
           Tone    <90dB non-100Hz*    DFT       Rectangular    CM
           Tone   >=90dB  Sweep     ERMS (100Hz) Rectangular    CM
           Tone   >=90dB  100Hz **     ERMS      Rectangular    CM
           Tone   >=90dB non-100Hz*    DFT       Rectangular    CM

          Coupler 
          Mic Cal   Off   1000Hz      TRMS       None           CM
           
        Notes:
        * non-100Hz includes all frequencies above 8000Hz and all 
          frequencies that are not a harmonic of 100Hz.
        ** 100Hz means 100Hz harmonics from 100Hz to 8000Hz.
        RE = Real Ear test mode.
        CM = Coupler (chamber) test mode.
        FFT = Standard FFT using 256 samples over 10mS, 100Hz harmonics.
        DFT = Standard DFT using variable samples and sample rate 
        depending on frequency. 256 samples over 10mS for 100Hz harmonic 
        based signals.
        ERMS = Estimated RMS is done by performing an FFT, correcting for 
        the data calibration and filter errors, then summing the results 
        to obtain the RMS. This gives good results when a full spectrum 
        synchronous source is used, but may not be as accurate in the 
        presence of significant signal noise. 
        CRMS = Corrected RMS is done by performing a standard RMS, then 
        correcting the result for the selected frequency. This gives good 
        results when a strong pure tone signal is present, but may not be 
        as accurate in the presence of strong signal noise. 
        TRMS = True RMS is done by performing a standard RMS without 
        correcting for the frequency. This works well in the presence of 
        strong signal noise, but does not correct for microphone 
        calibration errors at each frequency. The calibration for TRMS is 
        tied to the 1000Hz calibration reference. 



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        The FP35 hardware is calibrated at the following frequencies:

        //Frye calibration frequencies
        INT16 MIC_CalFreqs[HW_CORRECTION_SIZE+1] = {
          HW_CORRECTION_SIZE,  //0: number of frequencies in this list
          50,                  //1: first frequency
          62,79,99,            //2-4
          125,157,198,         //5-7
          250,281,315,354,397,445, //8-13
          500,561,630,707,794,891, //14-19
          1000,1122,1260,1414,1587,1782,  //20-25
          2000,2245,2520,2828,3175,3564,  //26-31
          4000,4238,4490,4757,5040,5339,  //32-37
          5657,5993,6350,6727,7127,7551,  //38-43
          8000,8476,8980,9514,10079,10679, //44-49
          11314,11986,12699,13454,14254,15102, //50-55
          16000,16951,17959,19027,20159        //56-60
        };

        The FP35 hardware is calibrated at the factory. It normally only 
        needs to be recalibrated if it has been repaired. The microphones 
        are calibrated using the above frequency list except that 
        frequencies below 99Hz and above 8000Hz are not calibrated.

        In actual practice, the microphones are calibrated to the closest 
        100Hz value in the above frequency list (from 100Hz to 8000Hz), 
        and the calibration values for the listed frequencies are derived 
        from the results. 

        In addition to the hardware calibration, the FP35 has a System 
        calibration. Although you only need to calibrate the system once, 
        you can improve the accuracy of the system slightly by 
        calibrating the system at the normal operating temperature. The 
        Codec that the FP35 uses to create and measure signals can change 
        calibration by as much as 0.5dB over it's temperature range 
        (0-50C). For best operation, the FP35 should be calibrated at the 
        operating temperature in which it will normally be used.
        The system calibration also compensates for the slight errors in 
        the gain stages and attentuators inside the codec. 

        The external hardware is temperature stable and does not need to 
        be recalibrated for temperature.















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        Microphone Calibration:

        When calibrating the coupler microphone, only the microphone 
        sensitivity at 1000Hz is changed. The frequency response 
        calibration for the microphone will always be the factory 
        generated calibration. The coupler microphone calibration uses 
        the True RMS measurement method. This is done because the 
        Standard calibration method for the coupler microphone uses 
        1000Hz as the 0dB sensitivity reference level, so this will 
        provide the most accurate calibration reference for the 
        microphone. Using True RMS for the measurement allows frequencies 
        other than 1000Hz to be used as the calibration source. However, 
        for best accuracy, it is recommended that a 1000Hz reference 
        source be used for the Coupler microphone calibration.

        When calibrating the probe microphone, it is done with a pure 
        tone sweep using DFTs. The probe microphone is calibrated by 
        comparing the probe microphone response against the coupler 
        microphone response and noting the differences. The frequencies 
        that were used to calibrate the coupler microphone at the factory 
        are used to calibrate the probe microphone. The probe microphone 
        is calibrated to match the calibrated coupler microphone response 
        as closely as possible. 

        While normally the factory supplied calibration for the 
        microphones is adequate for normal measurements, it should be 
        noted that each probe tube will have slight differences, and the 
        microphone itself may drift by a few tenths of a dB over time or 
        if mishandled. You can correct for the slight errors by 
        recalibrating the probe microphone. However, keep in mind that 
        the calibration will be best for the conditions at the time it 
        was calibrated and may actually make the calibration worse if a 
        different probe tube is used. The factory calibration uses a 
        standardized calibration method to insure consistent calibration 
        of the microphones. 

        <eof>




















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