INTRUSION DETECTION BASED ON THE SOUND FIELD VARIATION IN AUDIBLE FREQUENCY - BROADENING SECURITY AREA

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1 INTRUSION DETECTION BASED ON THE SOUND FIELD VARIATION IN AUDIBLE FREQUENCY - BROADENING SECURITY AREA Sung Q Lee and Kang-Ho Park IT Convergence & Components Laboratory, Nano Convergence Sensor Team, Electronics Telecommunication Research Institute (ETRI), 138 Gajeongno, Yuseong-gu, Daejeon, , Republic of Korea, hermann@etri.re.kr In this paper, intrusion detection technique based on the sound field variation of audio frequency is proposed. In previous research, the feasibility of the proposed detection algorithm was verified in the anechoic room in 2012, and was verified experimentally in general sound space in Based on the customer's requirement of broadening the security space, in this paper, the sound source manipulation technology is proposed. In general, sound space such as the office room and the conference room, there are sound reflections and reverberation that make the interference of the sound more complex. The multi-tone sound source composed of sine waves with evenly spaced frequencies is employed. The multi-tone is modulated as the sound source, and the room transfer function is calculated first at the security space without the intruder, and second with the intruder. Through the difference between room transfer functions, it is possible to detect the intrusion. The security sensor module has two speakers with the distance of 30 cm and one microphone in between two speakers. Speakers make the multi-tone sound to the security space and a microphone detects the reflected sound simultaneously. Multi-tone sound source is manipulated for two speakers with difference phase and magnitude that makes more complex reflections in security space, so that it is possible to detect the intrusion with higher sensitivity than single or mono speaker module. As a result, the security space is broadened in to 5m x 5m. This results show the potential of commercialization of intrusion detection in general spaces such as offices and living room. 1. Introduction Owing to the increasing requirements of both private and public security, including crime prevention, the security industry is growing increasingly larger and more important. High technologies in sensor and security software have also added to the volume of devices on the security market. Security technologies are moving from the use of direct/indirect human security guards to automatic guard/alert systems 1. The sensors used in security systems are varied according to their application. It is proper to make a complementary system from a combination of several different sensor types based on their particular merits and limitations. Sensors typically used in security systems include optical cameras, infrared cameras, microwave sensors, and radar sensors. Among these, CCTV (Closed Circuit Tele- ICSV22, Florence (Italy) July

2 Vision), as a representative optical camera, is a candidate intrusion detection sensor that utilizes the image processing of captured video images, but may provide an unwarranted alarm owing to lens contamination, direct light, or shadows of objects of non-interest. Moreover, CCTV requires a light source in a dark security space for a clear video image. Other sensor types used in security systems have their own advantages and disadvantages. Therefore, a combination of several types of sensors is desirable for a high-level security system. 2, 3 In the previous paper, the intrusion detection method based on the sound filed variation is proposed. It can be made of low cost speakers and microphones that have the audible frequency range. It has the advantage of not only low cost but also of capability in dark environment that pro-vide low energy consumption during intrusion detection. It also has advantage over infrared, ultra-sonic or microwave security sensors in the point of detecting the intruder behind obstacles due to the diffraction of sound whose wavelength is longer than obstacles. The experimental results per-formed in anechoic room and general acoustic space showed the feasibility for the intrusion sensor for space guardian. In general, sound space such as the office room and the conference room, there are sound reflections and reverberation that make the interference of the sound more complex. The multi-tone sound source composed of sine waves with evenly spaced frequencies is employed. The multi-tone is modulated as the sound source, and the room transfer function is calculated first at the security space without the intruder, and second with the intruder. Through the difference between room transfer functions, it is possible to detect the intrusion more practically. 4-7 In this paper, the manipulation of two multi-tone sound sources method is proposed to enlarge the security area for more practical applications. In previous method, the only one speaker and one microphone is used. However, it is possible to make null of acoustic wave with two speakers, so that the reflected multi-tone signal is relatively stronger. It helps to increase the sensitivity of proposed sound field variation based security sensor. 2. Intrusion detection sensor based on sound field variations 2.1 Theoretical background A security space can generally be described as a space governed by the wave equation 2 2 ( P k P 0 ) with the boundary condition of pressure ( P(r 0 ) ) or/and velocity ( U(r 0 ) ), as shown in figure P + k 2 P =0 P(r o ) S u S p U(r o ) S Figure 1.Wave equation and boundary conditions in an acoustic space When a sound is generated within a security space, a sound field that differs from the normal acoustic characteristics of the space is created. For example, if a speaker is positioned in an anecho- ICSV22, Florence, Italy, July

3 ic room, it acts as a monopole sound source, and a pressure field is developed according to the following green function: (1), exp(jk r) P(r) ju q 4 P(r) exp(jk r) G(r) j q 4 where r is the position vector in a free field, is the angular frequency, is the density of the medium, k is the wave number, and G(r) is be a transfer function of input q to output P(r) in an acoustic-free field. The proposed sound field variation based intrusion detection method can be set in a security space using a sound source and several acoustic sensors. A speaker as a sound source generates sound, such as white noise, pink noise, a swept sine signal, or an impulse signal. Additionally, a microphone used as an acoustic sensor receives the pressure information, such as the magnitude and phase, at each point to calculate the room transfer function of the space from the sound source and the acoustical sensors. When an intruder enters a security space, the sound pressure distribution is changed according to the reflection, absorption, and/or diffraction of the sound. These phenomena can be described as a transfer function. That is, if we denote the source as Q(s), the transfer function of the space as H(s), and the pressure at a certain point as P(s), the transfer function of the acoustic space and its log scale expression will be (2) X 20 log( H( s)) 20 log( P ( s) / Q( s)) When an intruder enters the acoustical space, the transfer function will be changed to (3) X ' 20 log( H' ( s)) 20 log( P' ( s) / Q( s)) The variation in the transfer function from the presence of the intruder can be defined as the difference in the transfer function between a normal security space and the same space with the intruder present. (4) E X ' X ' P 20 log( P ( s ( s j ) ) j ) (5) E ' P 20 log( P (j ) ) (j ) It is notable that the transfer difference in the logarithmic expression is equivalent to the pressure ratio between the initial and intrusion states. In addition, phase and magnitude information are included in the transfer function. Both of these, equally or combined, can be used in the sound field detection method. In this paper, the absolute magnitude of the pressure ratio is used as intrusion detection criteria to clearly explain the principle and feasibility of the proposed method. ICSV22, Florence, Italy, July

4 2.2 Discrete type multi-tone source In previous paper 6, it was reported that the transfer function measurement resolution should be at least 20Hz widths. The more detail frequency resolution, the more the sound energy is required in order to produce sound wave. Through checking the optimal frequency interval experimentally, 16Hz frequency interval is used. The centre frequency for the measure is used as 1 khz that is most central frequency for the speaker just for checking operation of proposed method. It is notable that the other centre frequency such as 4 or 8 khz can be used to avoid the interference with other sound noise. The multi-tone sound source can be composed of central frequency and 17 frequencies around it. The one example used in this paper is shown in figure 2. It has central frequency of 1 khz and 16 Hz interval frequency. The time domain signal for the sound speaker is the shape of figure 2 (b). Since it has the combination of several frequencies near centre frequency, this signal sounds like a cricket. Figure 2.Multi-tone signal component of 1 khz centre frequency and 17 components with 16 Hz interval in frequency domain and time domain 2.3 Manipulation of two multi-tone sound source In previous paper 6, one sound source and one microphone is used to implement the sound field variation based security sensor. However, in this paper, two sound sources method is applied. Two sound source is positioned apart from each other by the λ distance(regarding centre frequency). If two sound sources make the same sound, a microphone gets a double power of multi-tone signal, so that the reflected multi-tone from the boundary condition received by the microphone is relatively low compared with signal received by the two multi-tone sources in both sides. On the other hand, if two sound sources make the opposite signed signal (that is 180 degree phased difference), a microphone positioned at the centre of two sound source gets zero power of multi-tone signal, theoretically. Therefore, the reflected multi-tone from the boundary condition received by the microphone is relatively high compared with signal received by the two multi-tone sources in both sides. It is notable that it is possible to enhance the sensitivity of sound field variation according to the manipulation of multi-tone sound sources with more than two sources. In this paper, in order to check the feasibility of enhancing the sensitivity through the manipulation of sound sources, two sound sources method is applied. In figure 3, Received signal from two sound sources are plotted. Spk A means the received signal from one sound source from 1/2λ distance. 1/4λ and 1/8λ means distance of microphone position from the centre of two sound sources. As the microphone is positioned to the right (or left) from the centre, the received signal from sound sources becomes bigger and bigger. If compared with reflected signal from boundaries, the received signal is very big except the ICSV22, Florence, Italy, July

5 null position (centre position between two sound sources). Theoretically, there will be enhancement of sensitivity at centre and 1/8λ position rather than 1/4λ position or mono source.. Figure 3. Comparison between received signals and reflected signal: The magnitude of received signal from mono source (spk A) is much bigger than reflected one. At the centre of two sound sources, reflected signal is bigger than received signal (theoretically zero). 3. Experimental results 3.1 Experimental setup The experiments are performed in general office as security space. The size of security space is 5 m times 5 m, and the sound source is 5 W powered loud speaker (Britz Co.) that makes the 94 dbl at 10 cm of 1 khz sound source. The sampling frequency for sampling the acoustic information is 25,600 Hz, the resolution of frequency for transfer function is 16 Hz. Algorithm is imported NI DAQ and Labview module. One pair of speakers and a microphone is setup for practical implementation in office environment that has desk and several chairs. The microphone is positioned right centre of two speakers those have 34cm distance. The intrusion is assumed to be open the door. The microphone will be moved from the centre to right by 1/8λ. And the sound field variation method is applied at each position when the door is closed (before intrusion) and opened (after intrusion). ICSV22, Florence, Italy, July

6 Figure 4.Experimental setup: one pair of speaker and a microphone are located on the desk. The door is closed (before intrusion) at first and opened (after intrusion) for intrusion situation 3.2 Experimental results After the sound source is active, each microphone measures the sound signal for 0.5 second. The measured data is converted into frequency domain to get the transfer function of the security space. The detail procedure for deriving S/N for the proposed intrusion method is explained in detail in previous paper 5, 6. In this paper, manipulation of sound sources method that has the effect of S/N sensitivity enhancement is tested. In theology, even it does not have limitation in source numbers, just two sound sources are tested in this paper. In figure 5(a), only left sound source is activated to make multi-tone signal. As the microphone received signal that is mixed with the reflected signal and source signal, source signal gives more contribution to received signal. The power of first spectrum could be 65dB that is quite big compared with reflected signal. When the door is opened (after intrusion) the S/N value is around 10.0 that is the two times of initial S/N value (~4.5) If two sound sources makes multi-tone signal with opposite sign and the microphone is positioned at the center, this point would be null point (figure 5(b)). The power of first spectrum is around 51dB that is quite small compared with mono source. When the door is opened (after intrusion) the S/N value is around 18 that is the five times of initial S/N value (~3.5). It is notable that the null point has quite much enhancement in sensitivity. ICSV22, Florence, Italy, July

7 Figure 5.Effect of manipulation of sound sources: (a) mono sound source, (b) null position of microphone, (c)1/8 λ left side position, (d) 1/4λ left position When the microphone is move to have offset of 1/8λ, the power of first spectrum is around 60dB that is a little reduced power compared with mono source case (figure 5 (c)). When the door is opened (after intrusion) the S/N value is around 13 that is the two times of initial S/N value (~6.0). It is the mid-level sensitivity between null and mono source case. Figure 5 (d) shows the case of 1/4λ movement of microphone to the left side. The power of first spectrum is around 65dB that is similar to mono source. The S/N value is around 13 when the door is open that is the three times of initial S/N value (~4.0). It is similar to the case of 1/8λ offset. The result is coincided with theoretical prediction as in session 2.3. It is not sure how big the reflected signal is. However received signal is minimum at the null position (microphone is positioned at the centre and the two sources have opposite signed signal), so that the reflected signal become strong relatively. 4. Conclusions and Discussions The sound field variation detection bases security sensor technique is proposed for the intrusion detection in security space through detecting the variation of transfer function when the intruder comes in. For higher sensitivity and broad detecting area, manipulation of sound source method is applied. If two sound sources makes multi-tone signal with opposite sign and the microphone is positioned at the centre, null condition of sound wave is met. The power of received signal become small compared with mono source. When the door is opened (after intrusion) the S/N value is ICSV22, Florence, Italy, July

8 around 18 that is the five times of initial S/N value (~3.5). It is notable that the null point has quite much enhancement in sensitivity. As a result, the security space is broadened in to 5m x 5m. This results show the potential of commercialization of intrusion detection in general spaces such as offices and living room. Acknowledgement This work was supported by R&D program of Ministry of Trade, Industry & Energy. [ , Development of Smart Video/Audio Surveillance SoC & Core Component for Onsite Decision Security System] and by R&D program of ETRI [15ZB1500, Development of environment and user adaptive MEMS microphone solution] REFERENCES YongTak Jung, The present and future in security sensor, The institute of Electronics Engineers of Korea, 36 (10), , (2009) Junhee Han, Video analytics and algorithm: states of art, The institute of Electronics Engineers of Korea, 36 (10), , (2009) Victor Kremin, Ultrasound motion sensor, Cypress Microsystems, Application note, 1-11, (2002) Sung Q Lee, Kang-Ho Park, Woo Seok Yang, Jong Dae Kim, Daesung Kim, Kihyun Kim and Semyung Wang, Intrusion Detection Based on the Sound Field Variation of Audible Frequency Band, Transactions of the Korean Society for Noise and Vibration Engineering, 21 (3), , (2011) Sung Q Lee, Kang-Ho Park, Woo Seok Yang, Jong Dae Kim, Daesung Kim, Kihyun Kim and Semyung Wang, Intrusion Detection Based on the Sound Field Variation of Audible Frequency, Proceedings of the 18 th International Congress on Sound and Vibration, Rio de Janeiro, Brazil, July, (2011). Sung Q Lee, Kang-Ho Park, Kihyun Kim, Homin Ryu and Semyung Wang, Intrusion Detection Based on the Sound Field Variation of Audible Frequency-General security space, Proceedings of the 19 th International Congress on Sound and Vibration, Vilnius, Lithuania, 8 12 July, (2012). Sung Q Lee, Kang-Ho Park, Kihyun Kim, Homin Ryu and Semyung Wang, Practical Implementation of Intrusion Detection Method Based on the Sound Field Variation, Proceedings of the 20 th International Congress on Sound and Vibration, Bangkok Thailand, 7 11 July, (2013). Yang-Hann Kim, Lecture note on acoustics, Chungmoongak, (2005) ICSV22, Florence, Italy, July