中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Xie Jinlai, Tao Zhongda, Xie Zhaohua Institute of Acoustics, CAS, Beijing , China A Infrasound Condenser Microphone with IMS

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Recently we study IMS and observatory of geophysical phenomena in infrasound field, such as solar eclipse, earthquake, bolides thunder storm, severe weather, etc. According to infrasound station specification for IMS and other infrasound measurement, the new type infrasound condenser microphone has been made in Institute of Acoustics, CAS. Various design factors are considered, including protection against mechanical distortion, thermal expansion, vibration and level change, and the adjustment of frequency response. The operational principle, structure and performance were instructed in this paper. I. Introduction

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) II. Mechanical Structure of Condenser Microphone The pressure-sensitive element is membrane or a diaphragm. The change of pressure-sensitive element, i.e. displacement or velocity, is drawn by a recorder. That is an infrasonic signal.

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Fig.1 Sketch for configuration of condenser type infrasonic microphone

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Fig.2 Sketch map of infrasonic microphone structure Atmospheric inlet Acoustic resistance R1 Front cavity acoustic compliance C1 Membrane acoustic compliance Cs Positive plate Backing cavity acoustic compliance C2 Pressure equalizing capillary acoustic resistance R2

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Fig.3 Equivalent circuit for condenser microphone R1 R2 C1Cs C2 P Q

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) III. Circuit Structure of the Infrasonic Microphone According to measurement slow change of capacitance value, the high frequency voltage was added to transducer and reality bridge modulation amplitude principle shown as Fig.5

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) oscillation output modulation Amplification demodulation microphone Fig.4 Operation diagrams for condenser microphone

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Fig.5 Capacitance bridge modulation-amplitude principle C0 C0+CΔ uiui uouo M

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) IV. The Operational Principles The operational principle of infrasound condenser microphone shows as fig.5. The head capacitance is Co, and change volume is ΔC when infrasound waves act on it.

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Parameter relative of fig.5 is:

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Under unconsidered R 1 and C 1, the sensitivity S Q can be represented by Q.

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) In high frequency:

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Low frequency cutoff period T N is: Note: where τ=R 2 (C 2 +C S ) is time constant.

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) The sensitivity S C in capacitance variable is: Note: where d is distance between membrane and positive plate, A is active area of transducer capacitance.

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Fig.6. Map of CC-1T infrasonic condenser microphone

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) Fig.7. Response curves for CC-1T infrasound condenser microphone

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) V. CC-1T Infrasonic Condenser Microphone Features ① The sensitivity is high, 0.1μ bar infrasound pressure can be received; ② Frequency band is wide, cover all infrasonic frequency range,from 10Hz to Hz bellow; ③ Output volt is large, such as 400mV/Pa, then can be directly converted by A/D;

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) ④ Dynamic range is wide, the most high sound level≥108dB; ⑤ Frequency response is good flat in frequency band necessary; ⑥ Long-term stability of operation is good. The outside of back volume keeps vacuum and prevent with plastic(freedom from zero drift); ⑦ Consistent is good, the head capacitance C 0, the first resonance frequency of stress of membrane and circuit can be adjusted, so can be used to in the infrasound array;

中国科学院声学研究所 INSTITUTE OF ACOUSTICS ACADEMIA SINICA 17 Zhongguancun Road Beijing , China Tel:(86+10) Fax:(86+10) ⑧ Volume is small, φ80×220mm, weight is light, 1.6kg, and easy to portable and installation. ⑨ Membrane mass is small, freedom vibration, no sense to shock; ⑩ Forms a complete configuration, can be connected with PC and structured infrasound monitoring system. Adding triangle array, it can be got infrasonic arrival time, the most amplitude and period of it, wave direct, wave horizontal velocity and real-time 3D dynamic spectrum.