# Performance Evaluation of Active Noise Control for a Real Device Casing
## 3 Experimental Verification
### Experimental Results  
The performance of the control system was tested for spinning, the loudest phase of washing. Two highest spinning speeds were used, 1000 rpm and 1200 rpm. Table 2 shows the sound pressure levels at different microphones. At all microphone locations a noise reduction was clearly achieved. For 1200 rpm, at monitoring microphones the noise reduction ranged from 2.5 dB at M2, where with control the noise was the quietest, up to 8.4 dB at M0, and without control the noise was the loudest. At 1000 rpm noise levels without control were much lower, and active noise control improvements were also lower; however, still clearly perceptible.  
The average sound pressure level reduction at monitoring microphones, not used for control, was equal to 5.3 dB for 1200 rpm and 4.0 dB for 1000 rpm. Both results from monitoring microphones and hand-held sound pressure meter confirmed that the noise is reduced globally in the whole room [26].  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & **F** & **R** & **L** & **T** & **M0** & **M1** & **M2** & **M3** & **M4** \\  & **[dB]** & **[dB]** & **[dB]** & **[dB]** & **[dB]** & **[dB]** & **[dB]** & **[dB]** & **[dB]** \\ \hline \multicolumn{8}{c}{1200 rpm} \\ \hline without active control & 76.3 & 86.8 & 79.7 & 78.2 & 69.0 & 66.0 & 63.4 & 64.3 & 65.9 \\ \hline with active control & 69.1 & 77.6 & 74.5 & 68.9 & 60.6 & 62.1 & 60.9 & 60.4 & 60.0 \\ \hline difference & –7.2 & –9.2 & –5.2 & –9.3 & –8.4 & –3.9 & –2.5 & –3.9 & –5.9 \\ \hline \multicolumn{8}{c}{1000 rpm} \\ \hline without active control & 69.7 & 79.5 & 74.9 & 72.7 & 63.7 & 61.4 & 60.2 & 59.9 & 59.3 \\ \hline with active control & 66.3 & 76.3 & 68.2 & 64.8 & 57.5 & 57.3 & 58.0 & 57.5 & 54.8 \\ \hline difference & –3.4 & –3.2 & –6.7 & –7.9 & –6.2 & –4.1 & –2.2 & –2.4 & –4.5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The sound pressure levels at all error and monitoring microphones for a different spinning speed without control and with active noise control enabled (F—front, R—right, L—left, T—top).  
Figure 7: Average SPL at error and monitor microphones for reproduced spinning noise at 1200 rpm for a different \(\gamma\) after 15 min, \(\mu=0.001\).  
Figures 8 and 9 show the Power Spectral Densities (PSDs) of signals from microphones and control signals, respectively. Numerous harmonics of drum spinning frequency are clearly visible. The ANC system was able to reduce most important ones, contributing the most to the overall noise generated by the washing machine. The loudest harmonics were observed at frequencies slightly above 100 Hz. It follows also from an analysis of spectrogram presented in Figure 4 that this frequency band is often excited emitting loud noise, probably due to the mechanical structure of the washing machine itself. Given the sweeping nature of the noise generated by the device (due to acceleration and deceleration of the drum) and numerous harmonics, it is very difficult to design a casing that would not represent such resonances; however, by applying the active control system it is possible to mitigate them and significantly reduce the noise emitted to the surrounding environment.  
Figure 8: Power spectral densities of signals from microphones for spinning at 1200 rpm.