Acoustic stealth technology is applied in the military to minimize the sound emitted by aircraft and make it less detectable. There are three methods that can be adopted to reduce noise from an aircraft.

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Firstly, the aircraft can fly at a higher altitude outside of hearing range. Most military aircrafts fly at 50,000 - 60,000 feet above the ground which is safe. However, for combat aircrafts, most of them must eventually fly at low-attitude to fight accurately.



Another way is to fly at a speed faster than that of sound (supersonic flight speed)​. As such, the enemy will not be able to hear the aircraft immediately. However, this technique is problematic as the noise will still be heard eventually and fuel will run out quickly for an aircraft to fly that fast. Most jet can only fly at the speed of sound for 10-20 minutes, with the exception of the F-22 (Figure 1), which is the only military aircraft that has proven to be able fly that fast and lasts its entire mission.







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The last method is to design the aircraft so that it makes less noise, and this is where stealth technology comes in. Most aircraft noises come from the slipstream and the engine. Reducing noises from slipstream also equates to reducing air resistance, so it is taken into consideration while building any aircraft. To reduce engine's noises, there are many techniques available, such as using a silencer-suppressor muffler (Figure 2) or sound-absorbent linings (Figure 3) inside the engine cowlings.











































SONAR EVASION















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Another great discovery in the area of stealth in the military field, would be that of Chinese scientists, who found that thin films of nanotubes can generate sound waves via a thermoacoustic effect. Every time that an electrical pulse passes through the microscopic layer of carbon tubes, the air around them heats up and creates a sound wave. When taken a step further, it was found that nanotube sheets produce the kind of low-frequency sound waves that enable sonar to determine the location, depth, and speed of underwater objects. Speakers made from carbon nanotube sheets that are a fraction of the width of a human hair can both generate sound and cancel out noise -- properties ideal for submarine sonar to probe the ocean depths and make subs invisible to enemies. Hence, speakers can be tuned to specific frequencies to cancel out noise, like the sound of a submarine moving through the depths, allowing for stealth attacks or infiltration on enemies.