Design of an Electronic Muffler - A DSP Based Capstone Design Project

Active control of noise has been an emerging technology for the past two decades. Active noise control (ANC) is an attractive means to achieve large amounts of noise reduction in a small package, particularly at low frequencies, where passive noise control may be impractical. While the concept of ANC has long been established, the technological means for implementing ANC have only recently become available. With the advent of high-speed digital signal processors (DSPs) and modern signal processing methods, ANC is now becoming a reality. This paper presents an interesting undergraduate design project involving ANC. In their capstone engineering design course students designed and built a prototype ANC system that successfully demonstrated the concepts of an electronic muffler. The heart of the project centered on the popular Analog Devices’ SHARC DSP evaluation module (EVM). In this paper we will discuss ANC concepts and the electronic muffler, the students design project, and the pedagogical outcome. 1.0 Introduction The Systems Engineering curriculum at the U.S. Naval Academy is a four-year, undergraduate, ABET accredited, program, where students specialize in the interaction between mechanical, electrical, and computer systems. The curriculum focuses mainly on linear systems theory, feedback control, and mechatronics. An integral component of the Systems Engineering curriculum is its capstone design course. This course provides students with a comprehensive design experience bridging the gap between textbook problems and real life applications. Students work in teams to complete a semester-long design project. Each team must propose a project, prepare schedules and reports of their activities, and design, build, and test a prototype. One of the many challenges facing students in this course is the selection of a project that is meaningful and representative of their engineering discipline. For nearly a decade automotive manufacturers have been experimenting with electronic mufflers for automobile exhaust systems [4] and their realization is near production. Some current electronic muffler concepts are based on active noise control (ANC) that uses destructive interference of sound waves to reduce unwanted noise. An additional advantage of an electronic muffler is the reduction of backpressure associated with conventional mufflers. Reduction of backpressure provides improved fuel economy in most gasoline engines. A relevant on-going project in the Systems Engineering Dept has been the design of an interference-based electronic muffler. The Systems Engineering Department has had several teams of students over the past three years working on electronic muffler designs. Students find the design of an electronic muffler project intriguing because draws upon previous course work in systems engineering and challenges them with advanced topics in active noise control and in digital signal processing. The design and construction of a fully functional electronic muffler is impractical for a one-term design project. However, students do not need to solve every aspect of a complex, real-world project to make it a meaningful design experience. Rather, students can identify one or two key design issues, whose resolution would contribute to the overall objective of the project [1]. The key issue in the design of an electronic muffler is the design of the active noise controller. Therefore the focus of the electronic muffler project is for students to design a prototype ANC system that simulates the behavior of an electric muffler. In the sections that follow, we will discuss the ANC and DSP aspects of this project, the student’s electronic muffler design, and the pedagogical outcome of this project. 2.0 The Electronic Muffler Concept and Active Noise Control An electronic muffler is a practical example of ANC technology. Noise radiating from the outlet of an automobile exhaust system is reduced by transmitting "anti-noise" from a loud speaker located near the end of the exhaust pipe. The acoustic output of the loud speaker is controlled so that it destructively interferes with the automotive noise. The concept of an electronic muffler is shown in Figure 1.