Silencers
(Diagram: A combined reflection /
absorption silencer for a forklift
truck)
A significant proportion of noise from an engine is transmitted through the exhaust,
as a result of pressure pulses which originate when the exhaust valves open and
high pressure gas leaves the cylinders. The purpose of a silencer (also known as
a muffler) is to smooth these exhaust gas pulses as far as possible, and hence make
them quieter. The two most common principles used in silencers are reflection and
absorption.
Reflection, as its name suggests, involves reflecting pressure waves
against a fixed surface so that the reflected wave ‘interferes’ with the original
wave and partially cancels it out. For this reason a reflection silencer is also
known as an interference silencer. A reflection silencer consists of several chambers
which are connected by tubes protruding into the chambers. Reflection silencers
are usually chosen where reducing low frequency noise is the priority and hence
they are widely used in heavy duty sectors.
Absorption silencers follow a different
principle and use a cavity packed with heat-resistant compressible fibres to damp
the pressure waves. This type of silencer is also known as a ‘straight through’
silencer because in many cases it consists of a single perforated tube surrounded
by a body containing the fibres. Absorption silencers are more effective at dealing
with high frequency pulses and are often used on motorcycles and as the rear silencer
on cars. In many cases reflection and absorption principles are combined within
a single unit.