BRUEL & KJAER 4220 - Reference Level for Recordings; Tape Recording Calibration

recorder, making possible the automatic recording of the amplitude frequency
curve (Fig. 16).
When usmg the Pistonphone, an absolute calibration is immediately obtained
on the recording paper. Attention must be paid, however, whether the Re-
corder measures the RMS, the arithmetic average, or the peak value of the
signal. The Pistonphone is calibrated in RMS value. The peak value is 1.41
times,
the RMS value (peak = RMS + 3 dB), and the average value is o.90
times the RMS value (av. = RMS- o.9 dB).
The zero level on the level recorder should be chosen so as to avoid over-
loading of the amplifier of the preceding instrument. Calibration consists
of setting, by means of the Level Recorder input potentiometer, the recording
stylus to any suitable deflection corresponding to the reference S.P.L . pro-
duced by the Pistonphone .
N.B. When using the fonner Level Recorder Type 2304, which is a peak
rectifying instrument (if not modified) a correction should be made,
equ&l to the practical crest factor difference between the measured
signal and the calibration signal. Since the crest factor of the acoustical
noises encountered in practice is generally around 8 dB (with respect
to the 2304) the correction to be made is about -5 dB (i.e. the re-
ference level ""n the paper is 119 dB S.L.).
Level calibration of Tape Recordings.
The Pistonphone is well suited for supplying a reference sound level for
sound measurements which are recorded on magnetic tape. The calibration
signal is recorded at the beginning and at the end of the tape. In some cases,
the calibration lev ~ l of 124 dB will fall in the S.P.L. range of the measure-
ment. If not, the level of the tape calibration can be modified a certain
amount of dB by means of the microphone amplifier attenuator, but this
must be taken into account when the measurements are evaluated and fre-
quency analysed by playing back the tape in the laboratory.
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