A single 5.25 ″ x 3.25 ″ oval structure is a straight pipe closed at one end, and many small holes are drilled on the pipe wall. The principle is to replace it with many small nozzles according to the spectrum change of jet noise, while keeping the total area of the nozzle unchanged. When the air flow passes through the small hole, the spectrum of jet noise will move to high frequency or ultra-high frequency, so as to significantly reduce the audible components in the spectrum, so as to reduce the interference and harm to the human body.