Condenser microphone

Schematic structure of a condenser microphone (audio circuit, without impedance converter)

A condenser microphone ( English microphone condenser , capacitor microphone ) is an electro-acoustic transducer that converts sound into a corresponding electrical signal.

The microphone uses the changes in capacitance caused by changes in the distance between the (movable) membrane and the counter electrode to convert sound pressure or sound velocity into an electrical signal, depending on the design .

Mechanical structure and operating principle

In the condenser microphone, an electrically conductive membrane a few micrometers thick is attached to a metal plate, which is often perforated for acoustic reasons, in an electrically insulated manner. From a technical point of view, this arrangement is a plate capacitor with air dielectric, which has an electrical capacitance C of 4 pF ( ¹ ⁄ ₈ ″ capsule) up to max. 100 pF (1 ″ capsule). This depends on the plate area A and the distance d between the capacitor plates .

Impacting sound causes the membrane to vibrate, as a result of which the distance d between the membrane and the counter electrode changes and thus also the capacitance of the capacitor.

Acoustic properties

Since the diaphragm deflection and not the diaphragm speed leads to the signal, the condenser microphone is technically an elongation receiver . The very low mass of the membrane enables particularly precise reproduction of the sound, which with this type of microphone results in good impulse behavior and brilliant highs.

Condenser microphones are available as both pressure microphones and pressure gradient microphones . They come in a wide variety of manifestations, as this term only describes the converter principle. However, the term has established itself as a microphone class, as sound properties are closely linked to the principle of conversion.

Some condenser microphones allow the directional characteristic to be changed electronically. This is made possible by combining two pressure gradient microphones to form a double diaphragm microphone . The membranes stand "back to back" with a common counter electrode in the middle; the output signal is formed as the sum of both individual signals. Each individual system has a cardioid polar pattern . Changing the polarization voltages results in different directional effects for the entire system

The current state of the art is the use of microphone arrays with digital beamforming in order to reduce environmental disturbances. This is used in smartphones, headsets and notebooks.

Electrical circuits

There are two options for evaluating the change in capacity:

Low frequency circuit

Explanation of sizes: ${\displaystyle C=\varepsilon \cdot A/d}$

In order to convert these fluctuations in capacitance into an electrical voltage signal, the capacitor in the low-frequency circuit is charged with a bias voltage via a high-value resistor. This bias is also called polarization voltage . The resistor forms an RC element with the capacitance of the capacitor film . Its time constant must be selected to be sufficiently high so that the charge does not change appreciably during a period of the lowest tone that the microphone is supposed to pick up. Depending on the capacity of the microphone capsule and the lower cut-off frequency, there is a resistance in the range of a hundred megohms to a few gigaohms.

A microphone amplifier ( impedance converter ) adjusts the impedance for the signal transmission (cable) directly in the microphone. The signal voltage is not amplified. ${\displaystyle \Delta U}$

${\displaystyle U={\frac {Q}{C}}={\frac {Q\cdot d}{\varepsilon _{0}\cdot A}}}$

With

U - voltage across the capacitor

Q - charge stored in the capacitor (assumed to be constant for short periods of time)

C - electrical capacitance of the capsule

d - distance between membrane and counter electrode

${\displaystyle \varepsilon _{0}}$- Electric field constant

A - Effective field area between the membrane and the counter electrode

The signal voltage at the output is the difference between the voltage on the capacitor and the bias voltage from the power supply. It depends linearly on the foil spacing or its change . If the sound pressure is high, distortions can still occur because the restoring force of the membrane is non-linear and the membrane is not deflected linearly to the sound pressure, and because the membrane clamped at the edge is not deflected in parallel, but rather arches. This can be remedied by a larger distance between the foil, which however leads to a lower signal voltage for the same area. The signal voltage can be increased by increasing the bias voltage. ${\displaystyle \Delta U}$ ${\displaystyle d}$ ${\displaystyle \Delta d}$

In order to exclude electromagnetic interference in the microphone cable, signal balancing is often carried out using a transformer or an electrical balancing stage with transistors .

A type closely related to the NF condenser microphone is the electret condenser microphone . Here the capacitor charge is “frozen” in a coating with electret foil; capsule pre-tensioning is not required. Nevertheless, this microphone also needs an impedance converter and a voltage supply for it.

High frequency circuit

In a high-frequency condenser microphone, the capacitance of the microphone capsule, together with a small coil, forms an oscillating circuit whose resonance frequency is changed by the changes in capacitance of the capsule. Operated as an oscillator or phase shifter, the resonant circuit emits a frequency-modulated or phase-shifted signal with center frequencies between 7.68 and 27 MHz, which is demodulated directly in the microphone. Here to frequency modulation , phase modulation and amplitude modulation are used.

HF condenser microphones require more extensive electronics than the NF design (housed in one or two ICs), but are superior in the following aspects:

• lower noise, especially in the low-frequency range below 3 kHz (no 1 / f noise ), 5 ... 10 dB (A) compared to 15 ... 20 dB (A) are achievable,
• Insensitive to electrical LF interference, as the microphone capsule does not use LF signals,
• electrically symmetrical output without AF output transformer despite electrical asymmetry of the microphone capsule, low coupling capacitance between capsule and cable, very high CMRR,
• Frequency response up to 0 Hz possible (no high-pass due to capsule capacitance and resistance of the capsule / supply resistance), but switchable high-pass filters are common.
• Sensitivity is independent of air humidity and temperature (because with a few hundred ohms impedance it is low in the working area, as opposed to hundreds of megaohms in the low frequency range),
• no capsule preload required, power supply only for the electronics also possible with less than 48 V phantom power and
• no electrostatic force effect on the membrane (no collapse tension problem).

The first condenser microphone in RF circuitry was built in 1923 with a double triode, but this only gained importance with the advent of transistor technology in the 1960s, which are now found in ICs.

Note

The term "high frequency" (English. Radio frequency or RF) is not only used for the detection of changes in capacitance of the microphone capsule, but is also used for the wireless transmission of the microphone signal. However, higher frequencies above 400 MHz are used there.

Power supply

Condenser microphones always require a power supply, as a built-in impedance converter ( microphone amplifier ) is always required. Non-electret microphones also require a polarization voltage between the capacitor plates.

The following options are available for supplying the microphone preamplifier:

Tone supply

Main article: Tone supply

When the tone wire is powered , the operating DC voltage of usually 12 V is applied between the two signal lines that connect the microphone to the mixer. The power supply causes interference with other not intended (e.g. dynamic) microphones and is therefore rarely used today.

The term plug-in power is often used in addition to cartridge power supply. This is a high-impedance tone supply with 2.7 V to 5 V, which enables electret microphones with a built-in FET to be operated without their own power supply.

The DC voltage must be separated from the actual audio signal on both the microphone and the mixer.

Phantom power

Main article: Phantom power

With phantom power, the DC operating voltage is applied between the screen and the signal lines that connect the microphone to the mixer. There is no supply voltage between the signal lines. A third connection is required compared to the audio lead, so it cannot be used for asymmetrical connections without an additional screen. It differs from the power supply to the cartridge in that one operating voltage pole uses the two signal lines for symmetrical signal transmission, the other uses the ground (the shield). 48 V are common, more rarely (historically) 24 V and 12 V, as a makeshift for simple devices also 15 V. Other microphones symmetrically connected to the mixer (e.g. dynamic microphones) work even if the phantom power is not switched off.

Some microphones transform the phantom power voltage using DC-DC converters to voltages of up to 120 V in order to increase the sensitivity and to be independent of the specific level of the phantom power voltage.

With asymmetrical signal transmission (for example on a jack microphone input), there is often a small supply voltage on the signal line via a series resistor, which is only required by electret microphones . Other microphones short-circuit the voltage (safely).

Battery feed

In addition to an external power supply, the microphone can also be self-powered by an internal battery. With electret microphones, a 1.5 V battery is usually used for the impedance converter. With classic condenser microphones, both the impedance converter and the polarization voltage must be provided, so that higher-voltage batteries are used, such as B. 9 V , 12 V or 22.5 V (2x in the Neumann U87). Devices with battery power can often also be switched to phantom power.

External phantom power

If the microphone requires phantom power, but the recording device does not provide this, there is the option of switching an external phantom power supply (or audio power supply) between the two, which in turn can be mains or battery powered. In addition to providing the supply voltage, these devices can also contain further impedance converters and amplifiers for the audio signal.

Mains connection

Thiele M4 PGH with double triode ECC83 as cathode follower, without output transformer, switching between cardioid and omni possible

Older condenser microphones with tube preamplifiers (see picture) require a power connection in addition to the signal cable. Signal voltage (s) and mains voltage run over a common, multi-pole cable. A power supply in the microphone housing supplies the heating circuit of the tube and generates the anode and bias voltage of the tube amplifier. Since there is a network connection, these microphones are not entirely safe in the event of a malfunction.

Small and large diaphragm condenser microphone

Small diaphragm microphone

As is customary in the industry, a small diaphragm microphone is all those microphones with a microphone capsule with a diaphragm diameter of less than 1 "(25.4 mm). Typical for condenser microphones are diameters of ¹ ⁄ ₂ ″ (12.7 mm) and ¹ ⁄ ₄ ″ (6.35 mm).

Compared to larger membranes, the transmission factor is lower because the area exposed to the sound field is smaller. A subsequent amplification in turn increases the noise. However, with current amplifier technology, this disadvantage no longer plays a role in practice.

The acoustic advantages of a small capsule diameter are in the range of higher frequencies. Below a wavelength that corresponds to twice the membrane diameter, special effects such as partial vibrations and a complex directivity result from interference . The wavelength of the airborne sound at 10 kHz is about 34 mm, so that a ¹ / ₂ - inch membrane up to this frequency a uniform course of the sensitivity function of the angle of sound incidence may have. In addition, the sound field is only slightly disturbed by the usually small design of these microphones, which is advantageous, for example, in stereo microphone arrangements with two or more microphones.

Because of their tonal neutrality, small diaphragm microphones are preferred for music productions and broadcasts that require sound authenticity.

Large diaphragm microphone

Neumann large-diaphragm condenser microphone U87 with microphone spider

Capsules with a diaphragm diameter of 1 inch or more are called large diaphragm microphones. In practice, diaphragms with 0.85 inches (just under 22 mm) are used as large diaphragms.

Although the size is often presented as a quality feature, large diaphragms are not superior to small diaphragms in every respect. Rather, they have their own sound character, often subjectively described as "warm", which determines their area of ​​application:

• A larger capsule has a direction-dependent amplitude frequency response for higher frequencies (smaller wavelengths) , because increased interference occurs.
• Large-diaphragm microphones represent a relatively large disruptive body in the sound field, which significantly influences the sound propagation in the immediate vicinity of the microphone - reinforced by the usually large-volume housing of these microphones.

Together with the close-up effect , large diaphragms offer special design options due to these acoustic conditions, even during the recording, which cannot be reproduced to this extent by subsequent processing.

Comparison between small and large diaphragm microphones

	Diaphragm size
	small	big
Self-noise	higher	less
sensitivity	less	higher
Level stability	higher	less
upper limit frequency	higher	lower
Sound field influence	less	greater
dynamics	higher(?)	less (?)

See also

• Transfer factor
• List of audio terms
• Electret microphone

literature

• Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr (eds.): Manual of the recording studio technology. 8th revised and expanded edition, 2 volumes. Walter de Gruyter, Berlin / Boston 2014, ISBN 978-3-11-028978-7 or e- ISBN 978-3-11-031650-6 .
• Thomas Görne: Microphones in theory and practice. 8th edition. Elektor-Verlag, Aachen 2007, ISBN 978-3-89576-189-8 .
• Norbert Pawera: Microphone Practice . 4th edition. Franzis Verlag, Munich 1993, ISBN 3-932275-54-3 .
• Fritz Kühne: mono, stereo and transistor microphones. 7th edition. Franzis Verlag, Munich 1966.
• Andreas Ederhof: The microphone book. 2nd Edition. Carstensen Verlag, Munich 2006, ISBN 3-910098-35-5 .

Web links

• Condenser microphones with high frequency circuit (PDFi; 57 kB)
• Condenser microphone and the voltage divider principle (PDF; 91 kB)
• Change of the limit sound pressure level for microphones (PDF; 31 kB)
• Sennheiser whitepaper about their MKH microphones
• Data sheets for Senneheiser microphones

Individual evidence

1. ^ Michael Dickreiter: Handbook of the recording studio technology. 6th edition 1997, Volume 1, p. 182.
2. ↑ Thomas Görne: Microphones in theory and practice. 2nd edition 1996, p. 87.
3. ↑ Thomas Görne: Microphones in theory and practice . 8th edition. Elektor-Verlag, 2007, ISBN 978-3-89576-189-8 , pp. 49 .
4. ↑ Eberhard Sengpiel, Manfred Hibbing: Condenser microphones with high-frequency circuit. Retrieved October 24, 2019 . 
5. ↑ Volker Metz: Historical development of condenser microphones. TU Berlin, 2005, accessed in 2020 . 
6. ↑ Marc Füldner: Modeling and production of capacitive microphones in BiCMOS technology. University of Erlangen-Nuremberg, 2004, accessed on January 20, 2020 . 
7. ↑ Riegger, Hans; Trendelenburg, Ferdinand: Method for the distortion-free electrical transmission of acoustic vibrations, Austrian Patent No. 103098, 1924
8. ^ Michael Dickreiter: Handbook of the recording studio technology. 6th edition 1997, Volume 1, p. 174.
9. ^ Ernst Erb: M4 microphone / TA Thiele; Leipzig Ostd., Build 1954 ??, 12 images. Radio-Museum Meggen / Switzerland, 2004, accessed on July 13, 2020 .