# Acoustics/Microphone Design and Operation

![](Acoustics_microphone_design_and_operation.JPG "Acoustics_microphone_design_and_operation.JPG")

## Introduction

Microphones are devices which convert pressure fluctuations into
electrical signals. There are two main methods of accomplishing this
task that are used in the mainstream entertainment industry. They are
known as dynamic microphones and condenser microphones. Piezoelectric
crystals can also be used as microphones but are not commonly used in
the entertainment industry. For further information on piezoelectric
transducers Click
Here.

## Dynamic microphones

This type of microphone converts pressure fluctuations into electrical
current. These microphones work by means of the principle known as
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. The magnetic field within the microphone is created using
permanent magnets and the conductor is produced in two common
arrangements.

!Figure 1: Sectional View of Moving-Coil Dynamic
Microphone{width="300"}

The first conductor arrangement is made of a 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. Figure 1
provides a sectional view of a moving-coil microphone.

!Figure 2: Dynamic Ribbon
Microphone{width="300"}

The second conductor arrangement is a ribbon of metallic foil suspended
between magnets. The metallic ribbon is what moves in response to a
pressure fluctuation and in the same manner, an electrical current is
produced. Figure 2 provides a sectional view of a ribbon microphone. In
both configurations, dynamic microphones follow the same principles as
acoustical transducers. For further information about transducers Click
Here.

## Condenser microphones

This type of microphone converts pressure fluctuations into electrical
potentials through the use of changing an electrical capacitor. This is
why condenser microphones are also known as capacitor microphones. An
electrical capacitor is created when two charged electrical conductors
are placed at a finite distance from each other. The basic relation that
describes capacitors is:

**Q=C\*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 the capacitance. Also, the capacitance is
inversely proportional to the distance between the conductors. Condenser
microphones utilize these two concepts.

!Figure 3: Sectional View of Condenser
Microphone{width="600"}

The capacitor in a condenser microphone is made of two parts: the
diaphragm and the back plate. Figure 3 shows a section view of a
condenser microphone. The diaphragm is what moves due to impinging
pressure fluctuations and the back plate is held in a stationary
position. When the diaphragm moves closer to the back plate, the
capacitance increases and therefore a change in electric potential is
produced. The diaphragm is typically made of metallic coated Mylar. The
assembly that houses both the back plate and the diaphragm is commonly
referred to as a capsule.

To keep the diaphragm and back plate at a constant charge, an electric
potential must be presented to the capsule. There are various ways of
performing this operation. The first of which is by simply using a
battery to supply the needed DC potential to the capsule. A simplified
schematic of this technique is displayed in 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.

!Figure 4: Internal Battery Powered Condenser
Microphone{width="500"}

Another technique of 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. Figure 5 displays the layout of dynamic microphone
attached to a mixing console via XLR cable.

!Figure 5: Dynamic Microphone Connection to Mixing Console via XLR
Cable{width="700"}

**Phantom Supply/Powering** (Audio Engineering Society, DIN 45596): The
first and 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. This
configuration can be seen in figure 6. The +48 V will be stepped down at
the microphone using a transformer and provide the potential to the back
plate and diaphragm in a similar fashion as the battery solution. In
fact, 9, 12, 24, 48 or 52 V can be supplied, but 48 V is the most
frequent.

!Figure 6: Condenser Microphone Powering
Techniques{width="600"}

The second 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** (also known as Tonaderspeisung, AB powering; DIN 45595).
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.

Finally, the diaphragm and back plate can be manufactured from a
material that maintains a fixed charge. These microphones are termed
electrets. In early electret designs, the charge on the material tended
to become unstable over time. Recent advances in science and
manufacturing have allowed this problem to be eliminated in present
designs.

## Conclusion

Two branches of microphones exist in the entertainment industry. Dynamic
microphones 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 utilize the properties of capacitors. Creating the
charge on the capsule of condenser microphones can be accomplished by
battery, phantom powering, T-powering, and by using fixed charge
materials in manufacturing.

## References

-   Sound Recording Handbook. Woram, John M. 1989.
-   Handbook of Recording Engineering Fourth Edition. Eargle, John.
    2003.

## Microphone manufacturer links

-   AKG
-   Audio
    Technica
-   Audix
-   While Bruel & Kjær produces
    microphones for measurement purposes,
    DPA is the equipment sold for
    recording purposes
-   Electrovoice
-   Josephson Engineering
-   Neumann
    (currently a subsidiary of Sennheiser)
-   Rode
-   Schoeps
-   Sennheiser
-   Shure
-   Wharfedale

