Wide Band Standard Condenser Microphone Using Titanium Alloy

Abstract

In this paper a study of wide-band high-reliability standard condenser microphone is described. Frequency band of the condenser microphone is limited by the first mode resonance frequency of the membrane and given by the following equation: f_1 = (α_1)/(2πa )√&lt T/ρ &gt where α_1 : eigen value for the first mode, a : radius of membrane, T : tension of membrane ρ: density of membrane To obtain the wide frequency band, it is necessary to use a membrane material having a large value of the ratio of ultimate tensile strength to density. As a result of the study about several kinds of materials, we found that titanium alloy( 2Al-2Mn-1Mo-95Ti ) is the most suitable one for the membrane material. This alloy is possible to roll up the thickness of 3μm, and after cold rolling an ultimate tensile strength of about 100 kg/mm^2 is obtainable. Density of the material is 4. 52g/cm^3. Furthermore, this material has a special feature that the instability due to the creep after tension is avoided by applying thermal aging 200°C for 5 hours similarly as in metallic titanium. This feature is shown in Figure3. Titanium alloy has sufficient values of Young's modules and hardness, so that it is also suitable as the body material of a condenser microphone. The structure of a newly designed standard condenser microphone named MR-112 is shoun in Figure 5. Tension of the membrance is impressed by a non-rotating ring, and structure contributes to the improvement of reliability. Pressure response of the microphone is shown in Figure8. Followings are the main characteristics of the condenser microphone of MR-112, i. e. , the response at 1 kHz is 54. 5 dB ( 0dB : 1 V/1 μbar ), the resonance frequency is 30 kHz, terminal electric capacitance is 22 pF, the equivalent air volume of membrane is 0. 01 cm^3, and the stability factor of membrane is 7. Value of the free field correction of the microphone used with a cylindrical head amplifier of semi-infinite length are shown in Figure 12. The condenser microphone of MR-112 has a high reliability as in MR-103, as well as wide band characteristics, then it is useful as a standard microphone, especially in the high frequency range.