How do condenser microphones work

How does a condenser microphone work?

© Burkhard Heise, 2011



Condenser microphones (capacitive or electrostatic converters)

If you talk about condenser microphones, you describe the transducer principle of the microphone (dynamic, condenser, etc.).
The Converter principle Has none Influence on the directional characteristic, it only describes how sound is converted into an output voltage.

Condenser microphones are the standard in the studio. They are low-noise, pulse-stable, very sensitive and deliver high levels at the output (about 20dB more than dynamic microphones). Their transmission behavior is linear over a wide range, but there are weaknesses in the low frequencies (from approx. 100 Hz downwards) as well as in the high frequencies (from approx. 10-12 kHz upwards) in the functional principle.

Condenser microphones are available with both small and large diaphragms. The realizable directional effects are spheres, different kidney shapes and the figure eight, depending on the design. Condenser microphones are more expensive than dynamic microphones and less robust, and they also require an external power supply to operate the capacitor circuit (phantom power).



Converter principle of the condenser microphone:

The electrical principle of condenser microphones is based on changes in the capacitance of a capacitor - what a surprise! This is why this principle is also referred to as a capacitive converter. And because relatively little moves, they also say electrostatic converters - so much for vocabulary training.

You still know what a capacitor is? A quick reminder: two opposing electrically conductive plates are the electrodes. In between is a non-conductive medium, for example air. The capacitor can store electrical charges. If you move an electrode, the capacitance of the capacitor changes, and electrical charges flow in or out. This can be converted into an electrical voltage via a circuit, which changes proportionally to the capacitance. Okay, that was the brief backlash to the physics class, now it goes on.

What does something like that look like with a microphone? A metalized (gold-vaporized) plastic membrane is clamped in a ring, it is the moving "plate" of the capacitor. Movement means that it arches back and forth a little, nothing more. Opposite is the fixed capacitor plate (counter electrode). If the moving plate is deflected by the sound pressure, the capacitance of the capacitor changes proportionally to the deflection of the membrane. Everything else is a matter for specialists: The change in capacitance is converted into an output voltage by an electronic circuit with the help of a high-value resistor (see above). Finally, a tube or transistor serves as an amplifier.

The distance between the two capacitor plates is only a few hundredths of a millimeter. The deflection of the membrane at high sound pressures is often only a few ten thousandths of a millimeter. The structure is a mechanically highly sensitive system. The following picture shows the principle of the condenser microphone. A cardioid microphone is shown, which the experts recognize from the open design and the sound passage holes in the fixed electrode. I explain that exactly on the subject of the recipient principle. First of all, note the structure with the movable and the fixed membrane.


                Principle of the condenser microphone as a pressure gradient receiver (cardioid)



Features of the condenser microphone:

The frequency response of a condenser microphone is quite even compared to dynamic microphones, especially if it is an omnidirectional microphone. You can see an almost linear frequency response, which, however, has a slightly higher sensitivity at around 10 kHz, the so-called height hump (diffuse field equalized characteristic). This particular microphone is more sensitive to high frequencies. The height hump, which makes sense for recordings in the diffuse field, can also be suppressed and at high frequencies an almost horizontal line is obtained. If you compare this frequency response with that of the dynamic microphone, you will see the much better linearity of the condenser microphone.

Frequency response of a typical condenser microphone (cardioid = pressure gradient receiver)



Phantom power for operation:

To operate the condenser microphone (charging the condenser), a direct voltage is required, the so-called phantom voltage (usually 48 volts). It is fed to the microphone from the external preamplifier or mixer. This voltage is not applied between the two tone wires, but with a different polarity between the tone wire and the ground shield, so it does not affect the electrical signal from the microphone. That's why it's called phantom power. It is harmless and ineffective for dynamic microphones (in contrast to the tone supply).

The level of the phantom voltage arose for very banal reasons. When the Neumann company presented their new series of condenser microphones to Norwegian radio in 1966, the existing operating voltage for the additional lighting in the studios should be able to be used as phantom voltage - it was 48 V.

The microphones usually achieve their characteristic values ​​with these 48 V. Check the technical data of your microphone and mixer to avoid uncertainties about the voltages. The quality of the circuit for generating the phantom voltage has a significant influence on the sound quality of the microphone. In particular, the distortion-free transmission of high sound pressure levels is not possible with low-current phantom power supplies, the microphone cannot then play to its sonic strengths and the linearity of the transmission suffers.

Insufficiently paired resistances in the circuit lead to lower interference suppression of the symmetrical line. Poorly executed phantom power supplies can therefore be the reason for inadequate interference suppression on the microphone line. If you operate your good condenser microphone with a good preamplifier, this problem shouldn't affect you.



Attenuation switch:

The attenuation switch on the microphone, which is often present, attenuates the useful signal without, however, reducing the interference voltage on the cable (interference), i.e. the interference voltage ratio deteriorates when the pre-attenuation is switched on. The rare cases in which the microphone signal overdrives the following amplifier can be better treated with an attenuator at the amplifier input, i.e. at the end of the microphone cable. This means that the useful signal and interference voltage are equally attenuated.

It is better to leave the attenuation switch on the microphone on "Off" and, if necessary, use the input attenuation on the preamplifier or mixer.



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