Acoustic measurements with photographic-schlieren techniques are limited to regions of strong density gradients such as those found in shock waves and at the leading edge of intense sound waves. To study waves of less-than-finite amplitudes, a system employing photomultiplier-schlieren techniques was constructed. With this system, acoustic pressures as low as 20 dyn/cm2 at 6 kHz were measured in air. A unique feature of the design is that the knife edge is vibrated at 300 Hz. This permits the absolute value of the pressure to be determined from known constants and a few length measurements. Microphone calibrations obtained using values of the pressure determined with this equipment agree with pistonphone calibrations to within ±1 dB.