| |
High-quality audio often goes unnoticed, taking a backseat to more visual
portions of productions, but when audio is poorly handled it rapidly
takes center stage. As the first stage in the capturing of audio, the
microphone plays a key role in the ability of other equipment to function
at full potential. While Schriber Acoustic's products are oriented toward
audio professionals, we believe that a firm grasp of the fundamentals
is essential to any user. This educational section attempts to outline
briefly the basics of microphone theory, selection and application.
A microphone is essentially a transducer which converts acoustic energy
(sound waves) into electrical energy. The electrical signal can then
be amplified, recorded, altered or otherwise affected in ways the original
sound wave could not.
The mechanism by which a microphone converts sound energy into an electrical
signal is actually quite simple. There are, however, different methods
of achieving such conversion. The two most widely used microphone categories
for audio-visual, stage and broadcast are dynamic microphones and condenser
microphones.
Dynamic microphones are, in essence, a backwards speaker. In a dynamic
microphone a diaphragm is attached to a coil of fine wire suspended
in a magnetic field. When the diaphragm is struck by sound waves, the
coil of wire vibrates in the magnetic field, inducing an electrical
signal.
Dynamic microphones tend to be very rugged and generally deliver excellent
performance over wide temperature and humidity conditions. Unlike condenser
types, dynamic microphones require no source of external power. These
characteristics have made dynamic microphones a popular choice for demanding
applications such as use on the road, handling by many different users,
or as an audience microphone which may be passed around or used outdoors.
Condenser microphones use an ultra-thin diaphragm stretched tight just
above a flat surface called the back-plate or backplane. A fixed electrical
charge is placed on the backplane so its electrical output varies only
in response to vibrations on the diaphragm. This electrical signal is
at a very low level so it must be amplified by a preamplifier circuit.
This preamplifier may be located in the microphone or an external housing.
Condenser microphones tend to be very sensitive, and are capable of
reproducing a wide range of frequencies with a very flat response. The
low mass typical of condenser diaphragms allows fast transient response.
While some models of dynamic microphones do produce outstanding results,
the clear, smooth, natural sound of a quality condenser generally makes
it the best choice for applications where the highest sound quality
is required. Condenser microphones can also be made very small which
is one reason why all Schriber lavalier microphones are condenser types.
In the end, choosing between a condenser and dynamic microphone requires
consideration of not only the audio capabilities but of the physical
use environment.
Now that you have a basic understanding of the two most common microphone
types, let's take a look at the important characteristics that influence
how a microphone will actually sound.
- Frequency Response
This specification is arguably the most important in determining both
a microphone's basic sound characteristics and suitability to a particular
application. Often the microphone's frequency response is indicated
as a numerical range; however, this number is of little use since it
does not really define the microphone's sensitivity at particular points
within the range. For this purpose a frequency-response graph is most
commonly used. This graph shows the microphone's relative response to
a wide range of frequencies and is invaluable as an assessment tool.
The response graph visually indicates whether the microphone's response
tends to be "Flat" as indicated by a smooth flat line or "Shaped"
as indicated by specific peaks and valleys. A microphone with a "flat"
response can reproduce a wide range of frequencies equally well and
will generally sound natural and uncolored. At first this might seem
ideal for all applications and one might wonder why any other response
characteristic would be desired.
Many times, however, a response can be shaped to greatly enhance the
performance of the microphone in particular applications. For example,
a lavalier microphone designed for speech may roll off the high and
low frequencies so as to improve vocal clarity, and reduce rumble and
feedback. Many times, the microphone manufacturer will shape the frequency
response to give the microphone a distinct sound and allow it to achieve
better performance in its intended application.
The process of choosing a response profile depends on both the sound
source and surrounding environment. Microphones with a wide flat response
are most appropriate for music or situations where a wide range of frequencies
must be captured. Microphones employing a shaped response most typically
benefit very specific applications such as the reproduction of the human
voice. Finally, some microphones allow the best of both worlds, thanks
to user-selectable internal "roll-off" filters, although such
filters are generally found only on professional microphones.
- Directionality
The term "directionality" simply refers to the microphone's
sensitivity to sound arriving from different positions around the microphone
element. Some microphones pick up sounds equally well from all angles
and directions while others favor sound arriving from a particular direction.
Although an infinite number of patterns are possible, these directional
characteristics are generally lumped into three categories: OMNIDIRECTIONAL,
BIDIRECTIONAL, UNIDIRECTIONAL. Just as the frequency response graph
provides a useful visual indication of the microphone's frequency response,
so a "Polar" graph provides a useful visual indication of
the microphone's directional characteristics. A typical polar graph
is shown here. At first the polar graph may appear very confusing. The
simplest way to use the graph is just to look at it as a visual representation
of the sensitivity pattern. In the example it can be clearly seen that
the microphone will be most sensitive to sounds from the front, less
to the sides and comparatively almost none to the rear. While it is
outside the scope of this article, more advanced readers will benefit
from further study of how the polar graph is prepared and represents
graphically complex electrical measurements.
- Omnidirectional Microphones
As the name suggests, an omnidirectional microphone type is designed
to pick up sound equally well from all directions, a characteristic
clearly visible on the adjoining polar graph. At first this might seem
like the most favorable pattern for all applications. And, indeed, it
is the best choice for many applications where sound needs to be picked
up evenly from multiple sources. Examples of this would be a vocal group
gathered around a single microphone, a conference room, or a reporter
and interviewee sharing the same microphone.
In reality, however, omnidirectional microphones also have some drawbacks.
First, since sound is picked up more or less equally from all directions,
there is not the option of aiming the microphone to favor the desired
sound source; thus, there is a much greater risk of picking up extraneous
location sounds. In addition, they can pick up a greater amount of sound
from different sources, which can produce a hollow, reverberating sound.
A final drawback is that, when connected to a loudspeaker for sound-reinforcement
applications, there is a much greater risk of feedback.
Despite these limitations the omnidirectional pattern is a must for
many applications and should be a part of most audio arsenals.
- Bidirectional Microphones
Far less common is the biderectional pattern. The term "bidirectional"
refers to a microphone which is equally sensitive to sound from two
different directions, most commonly the front and rear of the microphone.
This pattern is not frequently used but can be appropriate for applications
such as capturing the performer and audience with a live ambience or
for capturing audio for stereo productions.
- Unidirectional Microphones
The final and diverse class of microphones is referred to as "unidirectional"
because they are most sensitive to sound coming from one direction (typically
the front of the mic). This characteristic helps to eliminate the limitations
of the omnidirectional patter discussed above. For example, a unidirectional
microphone can help eliminate sound from undesired sound sources. Its
pattern also generally reduces the possibility of feedback in sound-reinforcement
applications. Another useful characteristic of most unidirectional types
is the "proximity effect". This term refers to the additional
emphasis of low frequencies when the sound source comes into close proximity
to the microphone. The proximity effect results in a warm, full sound
and can be used as an artistic tool by knowledgeable vocalists but it
can also result in changing sound character.
There are many different kinds of actual patterns which fall under the
unidirectional label including: CARDIOID, SUPERCARDIOID, HYPERCARDIOID,
SHOTGUN, PARABOLIC. By now you may be getting a bit overwhelmed by the
terminology, but stick with us as we take a brief look at each.
The Cardioid mic is the most popular of unidirectional patterns. Mics
using a cardioid pattern have about half the sensitivity to sounds on
the sides and less than a tenth the sensitivity to sounds at the rear,
as compared to sounds arriving from the front. This results in the heart-shaped
polar pattern from which the cardioid name was derived.
The Supercardioid and Hypercardioid types have a progressively higher
directionality and rejection of sounds occurring to the sides of the
microphone. This can be very useful for isolating the desired sound
source and avoiding unwanted ambient sounds including reflections and
reverberation; however, increasing care must be given to orientation
of the microphone to assure that the performer remains on the ic axis
to avoid variations in sound levels. It will also be noted from the
polar patterns that, while these styles have a narrower coverage pattern
than a cardioid, they also begin to pick up sounds to the rear of the
microphone.
Other unidirectional microphone types such as the Shotgun and Parabolic
have extremely narrow, highly directional pickup patterns and are designed
for distant sound pickup and excellent isolation of the desired sound
source. The shotgun microphone is widely used for on-location work,
video, broadcast and film production. The Parabolic mic is used at extreme
distances of several hundred feet or more and, while not practical for
general field use, is excellent for specialized applications such as
sports, or wildlife recording.
Another important characteristic of a microphone is its output impedance.
This is a measurement of the AC resistance to current flow that would
be observed looking back into the microphone. Source impedance determines
the size of the load that the microphone can comfortably drive. It is
important to recognize that the impedance of a microphone should not
be matched to the impedance of the device to which it is connected;
in fact doing so will cause a significant loss in signal level. Ideally,
a microphone should be connected to a load whose input impedance is
roughly ten times the mic's output impedance.
Microphones are usually divided into two basic classes: low impedance
50-1,000 ohms (also called Low-Z) and high impedance 10,000+ ohms (also
called High-Z). Most professional microphones designed for long cable
runs are low impedance devices. This means their source impedance is
below 600 ohms since, properly connected, they are far less susceptible
to extraneous noise pickup in the cable and can be used with long cable
runs (over 1,000 feet) with very little loss in sound quality. High
impedance mics are limited to about twenty feet before degradation.
High impedance microphones require a buffer amplifier or transformer
when using low impedance inputs and/or long mic cables. Most Schriber
mics are of the low-impedance type except those designed for very short
cable runs such as on-camera microphones.
A microphone's impedance is not necessarily an indicator of quality
or performance. it is simply a factor that must be weighed for a given
application and the characteristics of the input to which it will be
connected.
A two-wire audio connection in an unbalanced connection. One wire carries
the audio signal, and the other is connected to ground. A balanced connection
utilizes a three-wire system. Two separate wires carry the signal in
equal but opposite polarity while the third is again connected to ground.
A balanced output offers real advantages to the audio professional.
The balanced system is more immune to noise, and thus is by far the
preferred method in professional audio, especially over long cable runs
or electrically noisy environments.
Unbalanced connections are often used for high-impedance microphones
and pickups. Low-impedance mics are used in consumer and semi-professional
equipment. Good performance will generally be obtained with short cable-runs
in environments free of electrical noise.
The last electrical consideration is that of microphone power. As we
learned earlier, condenser microphones require electrical power to operate.
This power can come from different sources. The power may be on-board
the microphone in the form of a small battery which often lasts for
many years. Other times the power is delivered through the microphone
cable from a remote supply, commonly called "Phantom Power".
This power supply can be a tiny belt pack, table-top unit or may be
incorporated into the audio mixer, video, camera or wireless transmitter.
You may remember that dynamic microphones do not require such external
power; however, the slight inconvenience of powering may be outweighed
by the advantages of condenser microphones for many professional applications.
There is more to choosing an appropriate microphone than electrical
considerations, of course. In the end, issues such as the physical shape,
size, mounting method, and visibility can be just as critical as electrical
issues in determining the microphone's appropriateness in a situation.
Let's take a look at some common package styles.
- Lavalier Style
Lavalier microphones are tiny microphones which are generally clipped
to a pocket, tie, or lapel. They are easily concealed and free the wearer
to move, gesture or demonstrate at will. In some theatrical applications
they are even woven into actors' hair or taped to the actor's body.
Lavalier mics produce extremely consistent results since the distance
from the microphone to the speaker's mouth is fairly uniform. These
favorable characteristics have made lavalier mics one of the most widely
used microphone styles. While most commonly used for speech applications,
some types of lavaliers are also useful for unobtrusive miking of wide-range
sources, such as acoustic instruments. The most common polar pattern
for lavaliers is omnidirectional because it has less sensitivity to
breath, handling and clothing noise. Unidirectional lavaliers are often
employed to control excessive ambient noise or feedback problems.
- Hand-Held Style
By far the most common and popular style of microphone is the hand-held.
As the name implies, the hand-held is often held during use; however,
it can also be clipped to a stand, flexible gooseneck or a boom. A good
quality hand-held will be internally designed to resist handling noise
and be ruggedly constructed to withstand the inevitable drops onto stage
or concrete. If you are just getting started or can only afford one
microphone, make it a hand-held. It can be used for interviews, speeches,
vocals, and voiceovers as well as general recording applications.
- Boundary Microphones
These microphones are designed to work on hard, flat surfaces such as
table tops. The microphone element is typically mounted in a sleek,
unobtrusive package and works by picking up both direct and reflected
sound waves. The microphone element is so close to the reflective surface,
both sound paths are in phase. Because of this, the sensitivity of a
surface-type microphone can be far greater than a hand-held at the same
distance (when the surface is sufficiently large). While you man not
use them for general applications they are incredibly handy for board
meetings and legal depositions. Just place one or two on the conference
table and you are ready to go.
- Shotgun
The Shotgun gets its name from its long, formidable-looking isolation
tube. These microphones, shaped like the barrel of a shotgun, are designed
to be highly directional and have high rejection to sounds originating
from the sides. Shotgun mics are one of the most commonly used types
for on-location work due to their ability to isolate the talent from
off-camera sounds. A common misconception is that shotgun mics somehow
zoom in on a remote sound source. This is generally not the case. To
illustrate, imagine looking through a paper-towel tube. You are not
actually seeing the image more closely, only isolating some specific
portion of it. In the same way, shotgun mics do not really "zoom"
in on the remote sound source, they only isolate it. Some shotgun microphones
do include on-board amplification or processing circuitry which helps
draw in distant sounds more effectively. Schriber Acoustic also offers
a model with selectable pattern characteristics to give the microphone
increased application versatility.
By this time we trust that you have gained a better understanding of
the electrical and physical characteristics of microphones, and will
be better equipped to make an appropriate choice from our products or
those of other manufacturers.
 
|
|
