Absorbent Ducts

Practical problems:
Settling of liner fill materials
The fibres of absorbent materials have a tendancy to settle with time resulting in the properties of the absorbent chamber as a whole changing. This may not be for the worse.

Temperature and fire retardation
Materials used for acoustic absorption typically have a low melting/flash point (~500C) or degrade if exposed to a high temperature for a long period of time. Hence changing the acoustic properties of the material. Therefore, in the case of exhaust design it is important that a temperature survey is conducted prior to positioning any absorptive treatment to ensure that the material will survive without degradation. Typically the absorptive treatment is more likely to survive the further it is away from the engine, although, temperatures of 300 to 450C are not uncommon in the rear exhaust silencer-can.

Accumulation of deposits
Exhaust gases are a mixture of many chemicals. A lot of which only exist in gaseous forms at the high temperatures at the front of the exhaust system. As the exhaust gas cools on its way down the exhaust system these condensate and collect at the lowest points, often the bottom of exhaust silencers. These chemicals are highly corrosive. This condensate will also be absorbed by the fibrous absorbent material, so reducing its effectiveness.

Erosion by air flow
The erosion of absorbent liners by the flow of gas through the system is a common problem. A number of actions may be taken to reduce this problem.
  • Introduce an erosion resistant barrier between the air flow and the absorption. In exhaust design stainless steel wire wool is commonly used as such a barrier.
  • Reduce the diameter of the holes used in the perforate. Perforate holes of <5mm diameter are used in exhaust absorptive silencers.

Chemical interactions
The chemical cocktail within exhaust gases can be very corrosive and highly likely to react with any absorbent material.

Given the above factors that affect the acoustic performance of absorption material within dissapative silencers it is important to thoroughly test any absorbent chambers within exhaust systems to ensure that they remain at an acceptable working level for the life of the exhaust system.

See also: Absorbent Material.

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Subjects: Noise & Vibration