Recording Instruments for Frequency and Intensity

Abstract

An inexpensive recorder which employs a spark to puncture a moving strip of chart paper has been constructed. One of the sparking electrodes is a light pointer attached to a short-period D'Arsonval galvanometer element. The other is a curved metal guide which bends the chart paper to the arc described by the tip of the pointer. In conjunction with the logarithmic vacuum-tube voltmeter already described (Rev. Sci. Inst. 4, 672, 1933) this instrument comprises a level recorder having a uniform decibel scale (60 db) and capable of following sound decay curves as steep as 150 db/sec. A new electronic frequency meter has been developed which is capable of following rapid variations of frequency. This device employs gas-filled discharge tubes and yields an output that is strictly linear with frequency from zero to 8000 cycles and independent of the input intensity over a range of at least 35 db. The instrument is flexible and any portion of the frequency range may be fitted to the indicating instrument permitting high sensitivity to small frequency changes. In combination with the spark recorder, or any other type of recorder, this instrument places the delineation of time variations of frequency on an equal footing with time variations of intensity. For example, the melodic curves or intonation of speech and music, vibrato, and attack may be recorded directly.