Electronic mufflers

The function of a vehicle’s muffler is to minimize the sounds coming from the vehicle exhaust system. These sounds originate from the combustion process within the engine.

Exhaust noise pollution becomes an issue as:

• Vehicle systems become generally quieter
• The number of vehicles on our roads increase

To understand the operation of modern exhaust noise reduction systems, it is helpful to understand what sound is:

• We sense sound from our eardrum within the ear.
• The eardrum is made to move by variations in air pressure.
• Variations in air pressure can be created when a force is placed upon an object.
• An example would be clapping your hands. As the two hands collide they push the air surrounding them, away.
• This creates a moving wave of air pressure or a sound wave.
• This sound wave moves your eardrum, which is interpreted as sound, by the brain.

Noise cancellation is a system that prevents the sound waves leaving the exhaust system by canceling them out inside the muffler.

These systems create gas pressures that are equal in force but opposite in direction to the noise source.

• These generated pressures are known as anti-noise.
• Any remaining sound is referred to as residual noise.

Anti-noise pressures can be generated by various means, such methods include:

Baffles and chambers

This relies upon variations in exhaust back- pressure to cancel pressure variations in exhaust flow. This system is only effective over a limited range of engine speeds and loads.

Variable flow exhaust

A moveable valve fitted within the exhaust system is used to change the amount of exhaust back-pressure. At higher engine speeds when exhaust noise levels are unacceptable the valve is closed, thus reducing the bore of the exhaust. This creates greater back-pressure and noise cancellation is the result. The valve can be operated by

• Pneumatics - exhaust gas pressure
• Electronics - a computer

When a variable flow exhaust is added to the baffle and chamber system, quieter noise emissions are the result. This is because the system can partially respond to changes in engine speed and load.

Electronic mufflers

Any restriction to exhaust flow in the exhaust system creates back-pressure. Whilst some back-pressure can be beneficial, excessive back-pressure reduces volumetric efficiency. This in turn reduces engine efficiency.

Electronic mufflers are designed to produce anti noise without restricting exhaust flow. This computer-controlled system uses a microphone to detect the sound waves produced within the exhaust system. As the exhaust gas leaves the tail pipe, computer driven loudspeakers are operated to generate the correct amount of anti-noise.

The result is a virtually silent exhaust without generating additional and unwanted back-pressure across all engine operating conditions. This increases fuel economy and reduces exhaust emissions.