## 3 Noise Improvement Plan
### Sound Design
#### 3.2.3 Determination of Sound Design  
To analyze the frequencies of the sounds produced by the milling machine, a 16-bit 44.1 kHz noise was digitally converted into frequency data while applying fast Fourier transform (FFT). The frequency spectrum with the strongest energy was extracted and selected as F0, the fundamental frequency. The second and third orders were assigned as F1 and F2, respectively. The spectrum was analyzed with the structure having one independent pitch and timbre constructing the harmonic series. The frequency components of the milling machine sounds after applying the noise-reduction method are shown in Figures 5 and 6. The musical instrument sounds were compared to those of the milling machine dust collector.  
A number of wind instruments were selected as candidates, and the harmonic waveform was compared to that of the noise of milling machines. As shown in Figure 5, for example, the alto horn's fundamental harmonic amplitudes in in Bb1 (about 58 Hz) is at least 5 dB weaker than its following harmonics. If the sound is synthesized similar to horn instruments, it can produce louder noises, as shown in Figure 5C. For this reason, when choosing a musical instrument to refer to the harmonic structure, the following two conditions are considered as priorities: First, the strongest  
Figure 5: Harmonic spectra. (**A**) Alto Horn at Bb1, (**B**) milling machine after noise reduction, and (**C**) the synthesis of alto Horn and milling machine after noise reduction.  
noise was generated at 60 Hz in the milling machine, which showed the greatest decrease after noise control. We selected the musical instrument that had a harmonic structure of overtone greater than the fundamental frequency in order to reduce the effect on the noise after 60 Hz synthesis. Second, we decided to target instruments that could have a similar harmonic structure, except the fundamental frequency, when reconstructing harmonics. Compared to organ pipe harmonics [28] in Figure 6, milling machine harmonics were found to be best suited for making similar in their composition. Therefore, the organ pipe harmonics were used as a musical object to compare to the harmonics of the milling machine dust collector. As a result, as shown in Figure 6, the flue organ pipe has the most suitable structure, and its harmonics were used for musical comparison with the harmonics of the milling machine dust collector. The harmonics of the milling machine frequency bands with insufficient energy were compensated through the design and synthesis of musical instrument sounds. The frequency bands of the additionally designed sounds used in the synthesis were 118 Hz, 180 Hz, 240 Hz, and 299 Hz (see Figure 7). The harmonic structure of the pipe organ was synthesized to appear at the same rate. These sounds were designed to minimize the effects of sound pressure when anti-vibration motion was activated. An overview of the proposed method in this study is shown in Figure 8 as a block diagram.  
Figure 6: Components and sizes of milling machine frequencies vs. harmonic structure of flue organ pipe (Malvern Priory, UK).  
Figure 7: Frequency components compensated by harmonic synthesis.