Introduction

A muffler (also known as a silencer) is any section of pipe or duct which has been treated or profiled to reduce the propagation of sound from a source, while allowing the free flow of gas. The performance of a muffler is normally strongly dependent on frequency.

A well‐designed muffler should have (i) an acoustical performance providing a minimum acceptable noise reduction (NR) as a function of frequency, (ii) a maximum allowable pressure drop, (iii) an acceptable shape and volume, (iv) suitable durability, and (v) a reasonable cost.

Ducted sources are commonly found in mechanical systems. Typical ducted‐source systems include engines and mufflers, and fans and air‐moving devices (flow ducts and fluid machines and associated piping). In these systems, the source is the active component and the load is the path, which consists of passive elements such as mufflers, ducts, and end terminations. Active mufflers, which can suppress the noise of sources particularly with strong low‐frequency pure tones are beyond the scope of this chapter.

The designs of both reactive mufflers and passive mufflers and combined types are discussed. Finite element and boundary element models are useful in the study of muffler elements. The acoustical performance of a system with a muffler as a path element is described in terms of the transmission loss (TL), insertion loss (IL), and radiated sound pressure. The acoustical performance of ducted muffler systems incorporating reactive mufflers depends strongly on the source‐load interactions.

This chapter presents a system for reactive muffler modeling based on electrical analogies that has been found useful in predicting the acoustical performance of such systems. The various methods for determining the impedance of a ducted source are described. Theoretical models for the acoustical performance of dissipative mufflers are complicated, particularly above the cutoff frequency, and often practitioners must rely on design charts or mufflers designed and tested by manufacturers. Different theoretical models are reviewed, and some empirical methods are described for simple predictions of the IL of lined ducts.