Методи за анализ на нискочестотни акустични сигнали от хеликоптери

1. Методи за анализ на нискочестотни акустични сигнали от хеликоптери Analysis methods for helicopter low frequency acoustic signals  Стилиян Георгиев,2, Юри Бижев1, Стоил Тодоров1, Стоян Колев1, Христо Колев3 Институт по металознание, съоръжения и технологии "Академик Ангел Балевски" с Център по хидро- и аеродинамика – БАН, София, България 2 Институт по невробиология – БАН, София, България 3 Институт по катализ – БАН, София, България Трета национална конференция с международно участие Металознание,Хидрои аеродинамика, Национална Сигурност ’2013 24 Октомври – 25 Октомври 2013г. СО ФИЯ, Големия салон на ЦУ на БАН

2. MicrocontrollerMSP430 Output Water proof microphone Band Pass Filter Amplifier MicrocontrollerDSP Output

3. The STFT is based on Discrete Fourier Transformation defined as: For proper STFT calculation a window function g(t) is applied. For windowing function we used Hamming window.

4. The cepstral coefficients spectrum or cepstrum is used for sound and human speech recognition [8]. The cepstral coefficients are calculated as follows: The time-series trial with length 1 s, i.e., 10000 points is transformed with Hamming window and then is Fourier transformed. After that, the power spectrum is transformed logarithmically and the resulting coefficients are inverse Fourier transformed to calculate the Cepstral coefficients results. In addition the Cepstral coefficients spectrum is subdivided into 100 bins with start bin corresponding to the lowest coefficient value and last bin corresponding to the largest cepstral coefficient value.

5. An Autocorrelation is a correlation between the internal elements of the series. The time series are correlated with themselves but only displaced with a certain number of elements - lags. The autocorrelation coefficients are calculated as follows: where and the lag gives the time series displacement to themselves.

6. Panther Bell [Amplitude] [Amplitude] [s] [s] Time-Frequency Plot Time-Frequency Plot [Hz] [Hz] [s] [s] Cepstral Histogram maximums Cepstral Histogram maximums [s] [s] Autocorrelation Autocorrelation [Lag] [Lag] [s] [s] Acoustic signal, time-frequency spectrogram, cepstral coefficients histogram and autocorrelation coefficients of helicopter types Panther and Bell. 1А) Sound signal recorded from flying helicopter. 1B) Time-frequency transformation of the acoustic signal using STFT 1C) Signal cepstral coefficients histogram and its change in time. 1D) Signal autocorrelation coefficients and its change in time

7. Cougar Mi-17 [Amplitude] [Amplitude] [s] [s] Time-Frequency Plot Time-Frequency Plot [Hz] [Hz] [s] [s] Cepstral Histogram maximums Cepstral Histogram maximums [s] [s] Autocorrelation Autocorrelation [Lag] [Lag] [s] [s] Acoustic signal, time-frequency spectrogram, cepstral coefficients histogram and autocorrelation coefficients of helicopter types Mi-17 and Cougar. 1А) Sound signal recorded from flying helicopter. 1B) Time-frequency transformation of the acoustic signal using STFT 1C) Signal cepstral coefficients histogram and its change in time. 1D) Signal autocorrelation coefficients and its change in time

8. Experimentally measured values of fundamental (f0) and harmonic frequencies (f1, f2, f3) of Panther, Bell, Mi-17 and Cougar helicopters inflight, obtained for three different position on the location of the helicopters to the microphone. Autocorrelation coefficients obtained from the analysis of the signal from the four types of helicopters also shows characteristic peaks shifts (lags) corresponding to the fundamental frequency of the helicopters. For Panther helicopter the lag is equal to 38 ms, for Bell to 77 ms, Mi-17 shows 59 ms, whereas for Cougar we obtained 55 ms.

9. The presence of fundamental frequency in 12-26 Hz range in STFT depending on helicopter model is a key component for proper recognition. • The application of all tree methods (STFT, Autocorrelation, Cepstral Coefficients) leads to the best detection and recognition accuracy. • In case of low-power consumption controllers the STFT is the most promising method for proper helicopter detection and identification. • The shift of fundamental frequency and its harmonics due to the Doppler Effect, can be used as marker by which to determine when the helicopter has been on closest distance from the recording microphone.

10. Thank You for Your Attention! Трета национална конференция с международно участие Металознание,Хидрои аеродинамика, Национална Сигурност ’2013 24 Октомври – 25 Октомври 2013г. СО ФИЯ, Големия салон на ЦУ на БАН