| Japanese | English |

(Aircraft noise analysis 9)

Jet noise is generated on the boundary of the fluid blew off at high speed, and stationary air. Each of turbulent flows becomes a noise source because of its the viscosity. In the area near a nozzle, disorder of a turbulent flow is fine and generates high frequency sound. In area distant from a nozzle on the contrary, with reduction in the flow velocity, disorder becomes large and low frequency sound is generated. Therefore, the sound generated by the jet engine becomes the broadband random noise.

Representation frequency is proportional to the outflow speed in a nozzle, and in inverse proportion to the diameter of a nozzle. The strength of noise is proportional to the eighth power of outflow speed. 
(Reference work : ISBN 4-339-00353-0)



air8.wav (44.1kHz / Stereo / 15sec / 2.52MB)

The example in Aircraft noise analysis 8 was measured with the Integration time of 0.01 second. Analysis results are summarized as:


  1. From the graph of F(0) it is a high sound pressure level from 2 seconds till 11 seconds. The maximum sound pressure level was found at 6 seconds.
  2. The graph of t1 and f1 shows that the peak of a high frequency ingredient exists between 7 and 8 seconds.
  3. From the graphs of F(0), t1 andf1, it is from 5 to 6 seconds and at 11 seconds that the representation frequency ingredient has clarified.
  4. From WIACC, a high frequency ingredient becomes the maximum in 8 seconds, and there are many low frequency ingredients after 7 seconds and 8 second.


The ACF below shows the most exciting noise ingredient that have the minimum t1 value and the maximum f1 value.

The fundamental frequency is seen at 1.22 ms that corresponds to 800 Hz. The t1 value is seen at 0.14 ms that is 7 kHz.

The time delay for the first peak in the ACF is defined as t1. This value shows the strongest high frequency component included in the signal. The time delay for the maximum peak in the ACF shows the fundamental frequency.

This graph shows the time change of the t1 value between 7.8 and 8.25 seconds after the measurement started. That shows the variation of pitch between 5.5 and 6 kHz.

Although the engine power is suppressed to the minimum at the time of landing, that sound is the jet noise with a high sound of 7kHz. The outline of the jet noise at the time of landing is summarized as follows. 

At the time of approaching to the airport, the representative (the most prominent) frequency of jet engine noise rises because of the Doppler effect. However, if it approaches more, the Doppler effect decreases and the pitch of sound falls since the difference of the direction of the plane and the direction of sound becomes large. When the plane approaches more and the body comes just ahead, there will be the sound of the fundamental frequency and the high frequency. When the plane is passing, it is influenced of both the high frequency breathed out from a jet nozzle and the decreasing low frequency by the Doppler effect. When the airplane goes away, the energy will decrease sequentially from the high frequency to the low frequency. 

October 2002 by Masatsugu Sakurai