1.10 MUFFLERS

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An addition to a sound pathway can reduce the sound levels that pass. These are commonly used with internal combustion engines to reduce sound, along with other devices. An example of muffler systems is given below,

After the fan there is typically a length of 5 to 10 diameters to allow the flow to settle. This makes the fan more efficient, and reduces noise from fan turbulence. This length between elements will also help improve performance.

1.10.1 Lined Ducts and Baffles

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Absorbs a portion of the sound

The more material, the better these devices absorb sound. Typical materials include mineral wool, and may use a stainless steel or mylar cover to protect the materials from high velocities and corrosive gases.

These are good at high frequencies.

Typical duct liner sound absorption characteristics are given below,

1.10.2 Bends

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As sound must turn corners, it attenuates.

For a 90° bend in a pipe we can expect attenuation as given below. These numbers assume that the bend section is round or rectangular with the diameter or widest dimension being that given. A lining is assumed with a thickness of 10% of D.

For a 180° bend we can find the attenuation by looking at the absorption coefficient, diameter of duct, and length of bend section.

1.10.3 Plenum

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An offset in a pipe of the right dimensions can reduce the sound well. The plenum itself is a large volume compared to the duct.

The reduction is a function of both the absorption and reflection path.

Typical attenuations are 10 to 20 dB.

Try problem S39

1.10.4 Expansion Chamber

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When a particular frequency of sound is to be muffled, a tuned cavity can be used.

These use a change in impedance at the inlet and outlets.

1.10.5 Cavity Resonator (Helmholtz)

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Another type of tuned resonator is a Helmholtz resonator.

This is basically a cavity added onto the side of a pipe.

The air inside the cavity acts as a spring. When the quantity of air is a specific volume, it will resonant with a specific frequency.

These are typically best at low frequencies.

The device can be used at higher frequencies, but it is typically lined with fiberglass or other porous materials.