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Active Noise control of the farfield noise radiated by a ducted fan  added 11/29/00

        by Daniel Linden Sutliff  North Carolina State Univ    Dissertation 1993

 4. RECOMMENDATIONS and CONCLUSIONS    {Inside the aircraft}

 The data show that digital RPM feedforward active noise control {ANC} applied to a ducted propeller is both feasible and serves as a good method for evaluation of the relative importance of those parameters relevant to active noise control. The use of a second reference signal, the blade position indicator, as a substitute for an acoustic error signal was successfully accomplished. Comparisons of the number of anti-noise speakers and their influence on the directivity of ANC, and the effect of system errors have been documented. The results show.

 1) Feedforward ANC is feasible for ducted fans.

·    •The plane wave was reduced 23 dB.

·    •The [1,0] mode was reduced 17 dB.

·    •Net noise reduction occurred over the farfield angles,  -135° <theta<+135°.

The feedforward results summarized above are easily obtained  when the RPM signal is clean enough to produce BPF {blade passing frequency} coherence levels with y2 above 0.99 for the plane wave and 0.97 for the (1,0) mode.

 2)   Simultaneous control of two tones is possible.  
·         •The BPF and the 1st harmonic were reduced with no loss in reduction or interference at those frequencies.  
·         •No increase in the SPL {sound pressure level} occurred at other frequencies.  
·         •This reduction is also achieved over a wide range of farfield angles.

 3)   RPM Feedforward "converges" fast and is stable.  
·         •RPM response time is equal to the RPM read time (~ 1 second).  
·         •Stability is independent of RPM.

 4)   Multiple ANC anti-sources are required for optimum attenuation.  
·         •The more uniform circumferential anti-source distributions resulted in larger reductions.  
·         •However, significant reduction occurred with one anti-source.

 5)   Accuracy in the acoustic parameters is important, but zero error is not required. 
        A 3 dB loss in reduction potential is caused by:  
·         •A phase error of 10 degrees.  
·         •A phase mismatch error between anti-sources of IS degrees  
·         •An amplitude error of 1 dB.  
·         •A frequency error of 0.I Hertz. (Accurate determination of RPM is a must with this approach.)  

Feedforward ANC with prior system identification as described in this work is ideal for noise reduction of dueled fans, such as those in an aircraft engine. By reducing the noise in the duct, the benefits are realized throughout the environment: lowered flyby noise over neighborhoods, reduced acoustic fatigue on aircraft fuselage, and improved passenger comfort may be derived from one system. The benefit of digital RPM feedforward is simplicity and stability and the ability to respond to frequency changes rapidly, on the order of 1-2 seconds, for the simple desktop PC processors used here.

Transformation ANC methods employing an acoustic error sensor have stability and convergence criteria that may not be useful when applied to real sources (i.e., on the order of several minutes or more).

 4.2 RECOMMENDATIONS

Further study is advised. In order of assumed difficulty:

1)   The effects of static flight conditions, such as altitude should be included.  
2)   Variable loading propeller blades in a shorter duct should be used as the source.  
3)   The effects of freestream inflow should be determined to more closely model realistic flight conditions.  
4)   Higher circumferential modes and simultaneous control of separate modes should be attempted.  
5)   Cancellation of radial modes should be demonstrated.