Microphones. Reference data. Part 2

Encyclopedia of radio electronics and electrical engineering / Reference materials

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Microphones are classified on the basis of the conversion of acoustic vibrations into electrical ones and are divided into electrodynamic, electromagnetic, electrostatic (condenser and electret), carbon and piezoelectric.

Microphones are characterized by the following parameters:

1. Microphone sensitivity is the ratio of the voltage at the output of a microphone to the sound pressure acting on it at a given frequency (typically 1000 Hz), expressed in millivolts per pascal (mV/Pa). The larger this value, the higher the sensitivity of the microphone.
2. Rated operating frequency range - the frequency range in which the microphone perceives acoustic vibrations and in which its parameters are normalized.
3. Frequency response flatness - the difference between the maximum and minimum sensitivity level of the microphone in the nominal frequency range.
4. Electrical impedance modulus - normalized value of the output or internal electrical resistance at a frequency of 1 kHz.
5. Directivity - dependence of the microphone sensitivity (in the free field at a certain frequency) on the angle between the microphone axis and the direction to the sound source.
6. Microphone noise floor - expressed in decibels, the ratio of the effective voltage value due to pressure fluctuations in the environment and thermal noise of various resistances in the electrical part of the microphone, to the voltage developed by the microphone on the load at a pressure of 1 Pa when the microphone is exposed to a useful signal with an effective pressure of 0,1 Pa.

In telephone sets, electrodynamic, electret and carbon microphones are mainly used. But, as a rule, in 95% of push-button TAs, electret microphones are used, which have increased electro-acoustic and technical characteristics:

• wide frequency range;
• small uneven frequency response;
• low non-linear and transient distortions;
• high sensitivity;
• low self-noise level.

Fig 1. Scheme for switching on a condenser microphone

On fig. Figure 1 is a diagram explaining how a condenser microphone works. The membrane (1) and electrode (2) made of electrically conductive material are separated by an insulating ring (3) and represent a capacitor. A rigidly stretched membrane, under the influence of sound pressure, oscillates relative to a fixed electrode. The capacitor is connected in an electrical circuit in series with a DC voltage source GB and an active load resistance R. When the membrane vibrates, the capacitance of the capacitor changes with the frequency of the sound pressure acting on the membrane. An alternating current of the same frequency appears in the electrical circuit and an alternating voltage appears on the load resistance, which is the output signal of the microphone.

Electret microphones, according to the principle of operation, are the same condenser ones, but the constant voltage in them is provided by an electret charge deposited on the membrane with a thin layer and retaining this charge for a long time (over 30 years).

Since electrostatic microphones have a high output impedance, to reduce it, as a rule, a source follower on an n-channel field-effect transistor with a pn junction is built into the microphone housing. This allows you to reduce the output impedance to a value of no more than 3 + 4 kOhm and reduce signal loss when connected to the input of a microphone signal amplifier. On fig. 2 shows the internal circuit of an electret microphone with three leads MKE-3.

Rice. 2. Internal circuit of the MKE-3 electret microphone

For electret microphones with two leads, the microphone output is made according to the open-drain amplifier circuit.

Rice. 3. Internal circuit of the electret microphone MKE-389-1

Rice. 4. Wiring diagram for electret microphones with two leads

On fig. 3 shows the internal circuit of an electret microphone with two leads. FEM-389-1. The connection diagram of such a microphone is shown in fig. 4. According to this scheme, you can connect almost all electret microphones with two leads, both domestic and imported.

The table shows their technical characteristics

Microphone options

The current consumption of the IEC-1 microphone is not more than 0,2 mA, MKE-377-1 and MKE-378 is not more than 0,35 mA. The current consumption of the M1-A2, M1-B2 and M-7 microphones is not more than 70 μA.

The difference between the MKE-332 microphone and the MKE-333 is that the MKE-332 is unidirectional, and the MKE-333 is non-directional.

Harmonic coefficient at a frequency of 1000 Hz at a sound pressure of 3 Pa for MKE-377-1 and MKE-389-1 microphones is not more than 4%, MKE-378 is not more than 1%.

The unevenness of the frequency response of sensitivity in the nominal frequency range for the MKE-3 microphone is not more than 12 dB, and for M1-A2, M1-B2, IEC-1 and MKE-389-1 is not more than ±2 dB.

Rice. 5. Tolerance region of the frequency response of the MKE-377-1 microphone

Rice. 6. Tolerance region of the frequency response of the MKE-378 microphone

Publication: cxem.net

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