A Sound Pressure-level Meter Without Amplification
This NACA technical note from 1937, issued by the National Advisory Committee for Aeronautics and preserved through NASA's Technical Reports Server, documents an instrument for measuring sound pressure level without an amplifier, written for engineers and researchers. Its pictures are plain black-and-white engineering images: a wiring schematic with coils, a switch and a meter, a graph trace of microphone response with labelled curves, and photographs of the boxed meter with its lid open. A designer might draw on their clean, diagrammatic line work and the boxed, utilitarian instrument forms.
