# Engineering Acoustics/Microphone Design and Operation

![](Mic_Title.jpg "Mic_Title.jpg")

## Introduction

Microphones are devices which convert pressure fluctuations into
electrical signals. Two main methods of achieving this are used in the
mainstream entertainment industry today - **dynamic microphones** and
**condenser microphones**. Piezoelectric
transducers
can also be used as microphones but they are not commonly used in the
entertainment industry.

## Dynamic microphones

Dynamic microphones utilise \'Faraday's Law\'. The principle states that
when an electrical conductor is moved through a magnetic field, an
electrical current is induced within the conductor. In these microphones
the magnetic field comes from permanent magnets. There are two common
arrangements for the conductor.

![](Moving_coil.JPG "Moving_coil.JPG"){width="300"}

Figure 1: Sectional View of Moving-Coil Dynamic Microphone

The first conductor arrangement has a moving coil of wire. The wire is
typically copper and is attached to a circular membrane or piston
usually made from lightweight plastic or occasionally aluminum. The
impinging pressure fluctuation on the piston causes it to move in the
magnetic field and thus creates the desired electrical current.

![](Ribbon.jpg){width="300"}

Figure 2: Dynamic Ribbon Microphone

The second conductor arrangement is a ribbon of metallic foil suspended
between magnets. The metallic ribbon moves in response to a pressure
fluctuation and an electrical current is produced. In both
configurations, dynamic microphones follow the same principals as
acoustical
transducers.

## Condenser Microphones

Condenser microphones convert pressure fluctuations into electrical
potentials by changes in electrical capacitance, hence they are also
known as capacitor microphones. An electrical capacitor consists of two
charged electrical conductors placed at some relatively small distance
to each other. The basic relation that describes capacitors is:

:   **$Q=C\times V$**

where Q is the electrical charge of the capacitor's conductors, C is the
capacitance, and V is the electric potential between the capacitor's
conductors. If the electrical charge of the conductors is held at a
constant value, then the voltage between the conductors will be
inversely proportional to (a) the capacitance and (b) the distance
between the conductors.

![](Condenser_schema.jpg "Condenser_schema.jpg"){width="600"}

Figure 3: Sectional View of Condenser Microphone

The capacitor in a condenser microphone consists of two parts: the
diaphragm and the backplate. The diaphragm moves due to impinging
pressure fluctuations and the backplate is held in a stationary
position. When the diaphragm moves closer to the backplate, the
capacitance increases and a change in electric potential is produced.
The diaphragm is typically made of metallic coated Mylar. The assembly
that houses both the backplate and the diaphragm is commonly referred to
as a capsule.

To keep the diaphragm and backplate at a constant charge, an electric
potential must be presented to the capsule. There are various ways this
can be achieved. The first uses a battery to supply the needed DC
potential to the capsule (figure 4). The resistor across the leads of
the capsule is very high, in the range of 10 mega ohms, to keep the
charge on the capsule close to constant.

![](Voltage.JPG "Voltage.JPG"){width="500"}

Figure 4: Internal Battery Powered Condenser Microphone

An alternative technique for providing a constant charge on the
capacitor is to supply a DC electric potential through the microphone
cable that carries the microphones output signal. Standard microphone
cable is known as XLR cable and is terminated by three pin connectors.
Pin one connects to the shield around the cable. The microphone signal
is transmitted between pins two and three.

![](XLR.jpg "XLR.jpg"){width="700"}

Figure 5: Dynamic Microphone Connected to a Mixing Console via XLR Cable

### 48V Phantom Powering

The most popular method of providing a DC potential through a microphone
cable is to supply +48 V to both of the microphone output leads, pins 2
and 3, and use the shield of the cable, pin 1, as the ground to the
circuit. Because pins 2 and 3 see the same potential, any fluctuation of
the microphone powering potential will not affect the microphone signal
seen by the attached audio equipment. The +48 V will be stepped down at
the microphone using a transformer and provide the potential to the
backplate and diaphragm in a similar fashion as the battery solution.

![](Powering.jpg "Powering.jpg"){width="600"}

Figure 6: Condenser Microphone Powering Techniques

### 12V T-Powering

A less popular method of running the potential through the cable is to
supply 12 V between pins 2 and 3. This method is referred to as
T-powering. The main problem with T-powering is that potential
fluctuation in the powering of the capsule will be transmitted into an
audio signal because the audio equipment analyzing the microphone signal
will not see a difference between a potential change across pins 2 and 3
due to a pressure fluctuation and one due to the power source electric
potential fluctuation.

### Electret Condenser Microphones

Finally, the diaphragm and backplate can be manufactured from a material
that maintains a fixed charge, known as \'electret\' (from
electric+magnet, because these materials can be seen as the electric
equivalents of permanent magnets). As a result, these microphones are
termed electret condenser microphones (ECM). In early electret designs,
the charge on the material tended to become unstable over time. Advances
in science and manufacturing have effectively eliminated this problem in
present designs.

## Conclusion

Two types of microphones are used in the entertainment industry.

-   Dynamic microphones, which are found in the moving-coil and ribbon
    configurations. The movement of the conductor in dynamic microphones
    induces an electric current which is then transformed into the
    reproduction of sound.
-   Condenser microphones which utilize the properties of capacitors.
    The charge on the capsule of condenser microphones can be
    accomplished by battery, by phantom powering, by T-powering, and by
    using \'electrets\' - materials with a fixed charge.

## References

-Sound Recording Handbook. Woram, John M. 1989.

-Handbook of Recording Engineering Fourth Edition. Eargle, John. 2003.

## Microphone Manufactures Links

AEA

AKG

Audio
Technica

Audix

Bruel & Kjaer

1

Neumann

Rode

Shure

sE Electronics

Back to Engineering
Acoustics

