One is to use the sudden change of pipeline section (that is, the change of acoustic impedance) to make the sound wave propagating along the pipeline reflect the direction of echo source, so as to reflect the acoustic energy back to its original place.
Second, through the reflection of several interfaces, the first sound wave propagating forward returns to the original point, and then returns to the original point to propagate forward again. This point merges with the second forward propagating sound wave that has not been reflected. The amplitude of the two waves is equal, and the phase difference is odd times of 180 degrees, thus mutual interference is cancelled.
Muffler is a device that prevents sound from spreading and allows airflow to pass through, and it is an important measure to eliminate aerodynamic noise. Muffler is a noise reduction device installed on the airflow channel of aerodynamic equipment (such as blower, air compressor, boiler exhaust port, generator, water pump and other equipment with large exhaust port) or in the air intake and exhaust system.