Measuring the Speed of Sound through Gases Using Nitrocellulose

Abstract

Measuring the heat capacity ratios, γ, of gases either through adiabatic expansion or sound velocity is a well-established physical chemistry experiment. The most accurate experiments depend on an exact determination of sound origin, which necessitates the use of lasers or a wave generator, where time zero is based on an electrical trigger. Other experiments use loudspeakers as the sound source, which eliminates the ability to accurately measure time zero of sound generation. To date, experimental heat capacity ratio data have been reported for measurements at room temperature. We have designed an apparatus to directly measure the speed of sound generated as a result of nitrocellulose ignition via two microphones. Our experimental design also provides the ability to measure the speed of sound at various temperatures and thus determine the heat capacity ratio as a function of temperature. When implemented in a junior-level physical chemistry laboratory course, students learned to use equipment with which they were unfamiliar, such as home-built ignition circuits, a vacuum pump, thermocouple temperature and vacuum gauges, gas cylinders, and an oscilloscope. Students used the data to determine the speed of sound and heat capacity ratio through nitrogen, carbon dioxide, atmospheric air, and argon gases both at 298 K and approximately 253 K. Error analyses of the experimental speed of sound and heat capacity ratio using percent error and propagation of error were performed to ensure a high level of accuracy and precision.

Determining the Speed of Sound and Heat Capacity Ratios of Gases by Acoustic Interferometry

“…The high sensitivity and flat frequency response of the microphone and cavity generates a large signal-to-noise ratio and highly accurate data. Molek et al 15 describe in this journal an alternative method for determining the speed of sound in gases in which a shock wave is generated by the detonation of nitrocellulose in a tube containing a sample of gas. The time delay of the resulting sound wave between two fixed microphones in the tube is measured with an oscilloscope and used to determine the speed of the wave directly as the ratio of the distance traveled to the time delay.…”