# Active Control of Low-Frequency Noise through a Single Top-Hung Window in a Full-Sized Room
## 3 Results
### Attenuation Performance as a Function of Frequency  
To understand the frequency dependency of the attenuation performance for both the ANC system and the passive insulation of the fully shut window, the attenuation level at the seventh microphone was plotted as a function of frequency, as shown in Figure 8. In the low frequencies, the active control performance exceeded that of the passive insulation up to about 250 Hz across all noise types. Attenuation of the aircraft noise between active and passive control was similar across the entire frequency band, with ANC providing better reduction in the lower frequencies, as shown in Figure 8a.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Noise Types** & **Aircraft** & **Motorbike** & **Traffic** & **Compressor** \\ \hline Duration, s & 47.2 & 16.41 & 13.19 & 10 \\ Frequency & 100 to 700 & 100 to 700 & 100 to 700 & 100 to 700 \\ range, Hz & & Industrial & Residential & University \\ Recording location & & & Non-peak traffic & Adjacent to source \\ Description & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Characteristics of the primary noise samples.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Mode of Operation**} & \multicolumn{4}{c}{**Attenuation of Primary Noise Types (dB)**} \\ \cline{3-5}  & **Aircraft** & **Motorbike** & **Traffic** & **Compressor** \\ \hline ANC ON (window opened) & \(\alpha_{active,Error}\) & 9.5 & 8.97 & 7.99 & 12.36 \\ \(\alpha_{active,EA}\) & 5.76 & 4.84 & 4.56 & 10.51 \\ ANC OFF (window closed) & \(\alpha_{passive,Error}\) & 9.98 & 10.09 & 10.77 & 6.47 \\ \(\alpha_{passive,EA}\) & 5.66 & 6.57 & 6.45 & 4.85 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Sound pressure level attenuation in dB of observation microphone 7 near the error microphones, and the energy-average across six microphones (microphones 1–6) across the four noise types, with ANC turned on (window opened) and ANC turned off (window closed).  
There was a significant roll-off in ANC attenuation at 300 Hz for the motorbike noise, as shown in Figure 8b. This dip in ANC performance at 300 Hz was observed for all four noise types, which could be due to the 300 Hz mode observed in the primary and secondary paths. In the case of the compressor noise, active control significantly outperformed passive insulation up to about 250 Hz, with a match in performance between 350 Hz and 500 Hz, which explains the results in Table 3. Moreover, passive control was negligible up to about 150 Hz, as shown in Figure 8d. In general, passive control outperformed the active control system above roughly 500 Hz, which was reflected in all the primary noise types.  
Energy-average attenuation performances (i.e., microphones 1-6) were in line with that found near the error microphone position (microphone 7), where the active control nearly matched that of the passive control up to about 250 Hz, as shown in Figure 9.  
Figure 8: Attenuation in dB at microphone 7 as a function of frequency with ANC ON (window opened) and ANC OFF (window closed) across (**a**) aircraft, (**b**) motorbike, (**c**) traffic, and (**d**) compressor noise types.  
Figure 9: _Cont._