Acoustic Measurements Using Common Computer Accessories: Do Try This At Home

Abstract

This paper presents some simple techniques for acoustic measurements using common, readily available computer accessories together with intuitive LabVIEW™ software. The methods used are designed to capture the interest of a wide range of engineering and science students. The speed of sound in air can be determined with good precision using only the sound card, microphone and speakers found with many personal computers (IBM or Mac). Using the computer’s sound card under LabVIEW control, a sophisticated acoustic measurement system can be created. Appropriate sound signals (including music or other sound clips) are generated, transmitted, received and processed to determine the speed of sound. After the speed of sound is known, relative positions of the speakers and microphone can easily be determined for various geometries using similar techniques and algorithms. By using familiar hardware and sound clips from their favorite music, students are motivated to experiment with these techniques at home. The methods employed are rich in content for courses involving the areas of acoustics, signal processing, instrumentation and measurement or combinations of these areas. The hardware and software to implement these measurements and illustrative examples are presented in detail. The speed of sound is used in a variety of distance and fluid level measurement applications. Usually, the transit time (time-of-flight) of an acoustic burst is measured and the distance is calculated using the speed of sound in the media of propagation (usually air). Students are usually referred to a table in the appendix of a textbook to find the value of this quantity. The speed of sound however is one of the few physical quantities that can actually be experimentally determined by the students with an acceptable degree of accuracy using equipment that they probably already have at home. This capability greatly empowers the students to explore the concepts on their own. 1,2 A personal computer with a sound card, external speakers and an external microphone is all the equipment that is needed to perform these measurements. LabVIEW software, which is quite prevalent in engineering education, provides an excellent means of controlling the computer hardware and processing the acoustic signals. Various experiments can be constructed to measure the speed of sound in air using common laboratory equipment such as signal generators and oscilloscopes together with speakers and microphones. Some of these experiments exploit the relationship between the wavelength, frequency and velocity of propagation of a sound wave: