2.3, Breath sound analysis method
Breath sound analyses were performed for all participants, as described previously13, 16). Breath sounds were recorded using a handheld microphone for ≥10 seconds. The microphone was placed on the right upper anterior chest at the second intercostal space along the mid clavicular line. A sound analysis of the inspiration phase was performed using an LSA-2012 sound spectrometer (Kenz Medico Co., Saitama, Japan).
The sound-amplifying unit was found to be effective for analyzing sounds in the range of 100-3,000 Hz. The recorded sounds were analyzed according to the fast Fourier transformation. The sampling frequency was 10,240 Hz and the spectra were obtained using a Hamming window. The sounds were displayed as a sound spectrogram. The point of the maximum frequency (Hz) in the shape during inspiration was used for the sound spectrum analysis.
To evaluate the dBm-based spectrum images, we used a new analyzing program (LSA 2020/ANA, Kenz Medico Co., Saitama, Japan). Using this program, we decided to set the zero point of the Y-axis (dBm) based on the mean of the background noise of all subjects. In this report, the zero level (0 dB of breath sound spectrum) was visually corrected based on the breath sound spectrums in each sample before the zero point (the frequency at 0 dB) was decided22). The zero level and the zero point were used to calculate of the area under the curve (AUC) of the sound spectrum.
From the breath sound spectrum, the power of low frequencies (LF) (100-200 Hz) and middle frequencies (MF) (200-400 Hz) in the inspiration and the expiration sounds were calculated17). In addition, the inspiration-to-expiration sound power ratio in the low- and mid-frequency ranges (E/I LF and E/I MF, respectively) were also calculated17).
The sound spectrum curve indices of the inspiration were determined by one point of the maximum frequency (Hz) in the sound spectrum. The data were automatically calculated using a custom software program22). The spectrum curve indices (A3/AT,B4/AT, RPF75 and RPF50) were measured according to the methods of a previous report13, 16). The RPF75 is the ratio of power at F75 (dBF75) to the frequency value (F99-F75), and the RPF50 is the ratio of power at F50(dBF50) to the frequency value (F99-F50). The total sound spectrum was divided into three or four sections from low to high frequencies, which allowed for the quantitation of the energy distribution within the spectrum13, 16). The AT, A3 and B4 were conventionally calculated by dBm and Hz (1 arbitrary unit [dBm・Hz] on a spectrum image. The A3/AT and B4/AT values are the ratio of the higher-frequency areas (A3 and B4) to the total area (AT) of the sound spectrum13, 16).
A five-point moving average was used as a smoothing technique to determine the suitable dB value for identifying the 0 dB in the slope of each sound spectrum. Each personal breath sound parameter was analyzed conventionally, using a sample with a median value from three tidal breaths.