How exhaust noise suppressors change jet noise

Aviation Mechanic Powerplant (AMP) practice question 6 of 8 on Engine Exhaust and Reverser Systems. ACS element AM.III.L.K3 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONCorrugated-perimeter and multi-tube suppressors break the single main jet exhaust stream into a number of smaller streams. What is the effect on the noise?

AThe frequency of the noise is lowered
BThe total noise energy is greatly reduced
CThe frequency of the noise is raised

Show the answer and explanation

Correct answer: C. The frequency of the noise is raised

A — incorrect Lowering the frequency would make the problem worse, since low-frequency noise travels farther and reaches people on the ground in greater volume than high-frequency noise.
B — incorrect The total noise energy remains unchanged; the benefit comes from shifting the frequency upward, where the noise is more highly attenuated by atmospheric absorption.
C — correct Breaking the exhaust into smaller streams increases the total perimeter of the nozzle area and reduces eddy size, so that although the total noise energy is unchanged, the frequency is raised considerably.

BACKGROUND

Aircraft powered by gas turbine engines sometimes require noise suppression for the exhaust gases when operating from airports in or near highly populated areas. Of the three sources of gas turbine noise, the engine exhaust dominates, because a high-velocity jet stream mixing turbulently with relatively quiet atmosphere generates most of the sound. Low-frequency noise carries farther and is therefore more objectionable on the ground than high-frequency noise. The noise suppressors in current use are either the corrugated-perimeter type or the multi-tube type, and both split the main exhaust stream into several smaller jet streams. This raises the frequency, which may push some noise above the audible range and leaves the rest more strongly attenuated by the atmosphere.

STUDY TIP

Remember that a suppressor redistributes noise rather than removing it: total noise energy is unchanged, only the frequency shifts upward. Tie that to the fact that high-frequency sound weakens faster with distance.

KEY POINTS

The two suppressor types in use are the corrugated-perimeter type and the multi-tube type.
Both split the main jet stream into smaller streams, raising the frequency considerably.
Total noise energy is unchanged; higher frequencies simply fall off faster with distance.

SOURCES

sources/amp-powerplant/ch03.txt "Engine Noise Suppression"

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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.