Final answer:
Older turbojet engines used a noise suppressor technology based on active noise reduction by destructive interference, which creates an anti-noise sound wave to cancel out the incoming engine noise, effectively reducing the noise levels by at least 30 decibels.
Step-by-step explanation:
The noise suppressor in older turbojet engines operates using a concept called active noise reduction by destructive interference. This method involves a fast electronic analysis of the incoming noise, followed by the generation of a second sound wave with its maxima and minima exactly reversed from the incoming noise. The two sound waves cancel each other out when they meet, effectively reducing the noise level. The underlying principle is based on Pascal's principle, in which pressures from two layers of sound in fluids (like the air in an airplane cabin) can add and subtract like simple numbers, thus creating a much quieter environment.
For example, the noise-canceling technology in headphones creates an anti-noise sound wave that destructively interferes with the incoming engine noise, which can be more effective than passive attenuation methods, as demonstrated by their use in the record-setting nonstop flight of the Voyager aircraft in 1986. These techniques can reduce noise levels by at least 30 decibels, helping to protect the hearing of passengers and crew in commercial aircraft.