This article describes the development and field test of the Sound Concept Inventory Instrument (SCII), designed to measure middle school students' concepts of sound. The instrument was designed based on known students' difficulties in understanding sound and the history of science related to sound and focuses on two main aspects of sound: sound has material properties, and sound has process properties. The final SCII consists of 71 statements that respondents rate as either true or false and also indicate their confidence on a five-point scale. Administration to 355 middle school students resulted in a Cronbach alpha of 0.906, suggesting a high reliability. In addition, the average percentage of students' answers to statements that associate sound with material properties is significantly higher than the average percentage of statements associating sound with process properties (p<0.001). The SCII is a valid and reliable tool that can be used to determine students' conceptions of sound.