# Development and Evaluation of Light-Weight Active Noise Cancellation Earphones
## 3 Real-Time Experiments
### Real-Time Noise Reductions Results  
In the first set of experiments, we use stationary primary noise for evaluating noise reduction at different frequencies. As shown in Figure 4, the primary loudspeaker is used to generate single tone primary noise with frequency increasing from 100 Hz to 3000 Hz with the step of 100 Hz. The light-weight ANC earphones are placed on the KEMAR's ear for experiment, and the identical experiments for the commercial ANC headphones and earphone (types A, B and C) were also repeated using the same noise level for comparison. The signal sensed by the KEMAR's microphone resembles the noise that will be perceived by the ANC earphones (or headphones) users. The power difference between the ANC OFF and ON is computed to obtain the noise reduction achieved by the ANC. The satisfactory performance can be achieved by using step size 0.0041, filter length 100 for \(\hat{S}(z)\) to model the secondary path, and the ANC filter \(W(z)\) uses step size 0.0081 and filter length 450. Figure 6 summarizes the noise reduction achieved by six ANC earphones and headphones from 100 Hz to 3000 Hz with step of 100 Hz. This clearly demonstrates that the developed light-weight ANC headphones achieved better noise reduction than the commercially available ANC headphones. Especially, the in-ear earphone obtained the best performance of noise reduction. The average noise reductions for the clip phones is 29 dB, the earbud earphone is 35 dB, and the in-ear earphone is 50 dB. These three ANC earphones are effective over the wide frequency range from 100 Hz to 3000 Hz. As a comparison, the commercial type A headphone failed to reduce noise for frequency above  
Figure 5: Magnitude responses, (**a**) the recorded rain sound (red line) and white noise (blue line); (**b**) results of \(\hat{S}(z)\) using white noise (solid) and natural sound (dotted) for the in-ear (red), earbud (blue), and clip earphones (green). 1000 Hz, the commercial type B headphone failed to reduce noise for frequency over 1500 Hz, and the commercial type C earphone failed to reduce noise for frequency over 2200 Hz. In the second set of experiments, the recorded MRI noise is used as the primary noise [16]. First, we measure the MRI noise at the KEMAR's ear without placing the earphone or headphone on it. After that, the developed light-weight ANC earphones and commercial ANC earphones and headphones are placed on the KEMAR to measure the MRI noise sensed by the [PERSON]'s microphone without turning on the ANC. This stage evaluates the passive noise reduction achieved by the earphones and headphones. Finally, we turn on the ANC and measure the residual noise for every ANC earphone and headphone. Figure 7a shows the measured noise at the [PERSON]'s ear for the ANC clip earphones. It shows that the ANC clip earphone cannot passively attenuate the MRI noise, but there is 20 dB noise reduction from 1100 Hz to 3200 Hz after ANC is turned on. However, there is no noise reduction below 500 Hz due to the poor loudspeaker response at low frequency range as shown in Figure 5 of the clip earphone. Figure 7b shows the measured noise at the [PERSON]'s ear for the earbud ANC earphone. This shows the earphone cannot passively attenuate the MRI noise neither, since the spectrum without placing the earphone is similar to the spectrum with earphone. After ANC is turned on, there is almost 20 dB noise reduction from 1000 Hz to 3200 Hz. However, there is also no noise reduction below 500 Hz due to the poor loudspeaker response at low frequency range as shown in the blue lines of Figure 5 for the earbud ANC earphone. Figure 7c shows the measured noise spectra of the in-ear ANC earphone from these three steps. Comparing the black line and blue lines, we show that the in-ear earphone can attenuate high frequency MRI noise from 500 Hz to 2500 Hz by its plastic earpad without using ANC. In addition, comparing the blue line (ANC OFF) and the red line (ANC ON), there is about 15 dB noise reduction for frequency range up to 2400 Hz, which is achieved by ANC.