Basic Microphone Knowledge

Transcription

Knowledge: Microphones
1
Knowledge
Microphones
(What Every Musician Needs to Know)
The microphone is the first element in the audio
chain, and in many respects it is the most important.
Without a good quality microphone, you can't create
an acceptable recording no matter how good the
other elements are. The first advice you'll hear from
any professional about purchasing sound or
recording equipment is "invest in good
microphones."(1)
This chapter deals with some of the basic concepts
all musicians need to know about microphones in
order to select and use them effectively.
It covers:
Microphone construction
Types of microphones
Directionality and pickup patterns
How characteristics are compared
© 2008, Advanced Instructional Media
2
Digital Music Production
Microphone Classification
A microphone is a transducer (2), that means itʼs a device that converts one form of energy into another(3).
A microphone converts mechanical energy (traveling waves of compression and rarefaction in a medium like air) into
electrical energy (similar waves of high or low electrical pressure.(4) It doesn't measure
pressure changes, it simply converts them. Therefore it's basically an
Diaphragm
analog device even if it's in an all digital audio system. 1 (5)
Capsule
Diaphragm
All microphones have certain parts in common,
a capsule containing a thin diaphragm that moves
with the pressure changes of a soundwave
and changes them into voltage changes,
circuits and/or wiring that either boost or
simply transfer the voltage changes to the cable port,
and a housing that encloses everything in a form
best suited to the requirements of the user.(6)
These common parts allow for many variations in
construction-- both in design and in quality.
All of these varieties lend unique colorations
to the sound they record and make specific mics
better for certain uses than others.
Capsule
Housing
Housing
Wiring
Wiring
Circuits
Dynamic mic
Condenser mic
In spite of their many varieties in construction,
we can separate microphones into two broad classes: dynamic and condenser.(7)
Each of these classes contains several different designs and numerous variations of those designs.
Dynamic Mics
Dynamic microphones work by vibrating metal in a magnetic field.
In the classic moving coil design, a thin diaphragm is attached
to a coil of wire surrounding-- or surrounded by-- a high powered
permanent magnet. Any movement of the diaphragm in response
to sound waves moves the coil in the magnetic field and this in turn
generates small changes in electrical pressure--voltage. The physical
principle behind this process is known as electromagnetic induction.(8)
Diaphragm
Wire coil
Permanent
Magnet
Dynamic Mic Capsule
1
Well, o.k., a point of clarification: The recently developed USB mic (the kind that attaches directly to your computerʼs USB port)
contains an analog to digital processor that converts the fluctuating voltage of the mic into numerical measurements. So what
comes out of the cable port actually is digital. Nevertheless the working part of any USB mic is identical to a traditional mic,
3
As you might imagine, there are many
factors that can come into play in designing
a moving coil mic. One example: The more
windings in the coil, the more current will be
generated and so the more sensitive the
mic will be. But more windings also means
more metal, which means more mass and
inertia, which means slower response to
initial sound transients (how quickly the
sound reaches full volume) and less
responsiveness to high frequency sounds.
(9)
Popular Dynamic Mics
Shure SM57
Shure SM58
Electro-Voice RE-20
Overall, moving coil mics are good all-purpose microphones. They are simple, cheap, and rugged, which makes them
particularly good for field work and for hand held applications.(10) They also make good voice mics and are great for
sound reinforcement. They usually have smooth response curves and higher headroom which means they can be
used for louder sound levels of drums and other loud instruments.(11) On the other hand they lack fast attack
response and high frequency response. Moving coil mics are very popular-- probably the first ones a person will
purchase for recording and general sound reinforcement. The most common microphone in use today is a moving coil
mic, the Shure SM58 shown above.(12)
Another design that works on the principle of electromagnetic induction is the
ribbon microphone. Less common than the moving coil type, it relies on a
very light ribbon of metal-- usually corrugated-- suspended in the powerful
magnetic field. (13) Since ribbons have a lot less mass than a coil of wire,
they usually have better transient response and are better able to reproduce
high frequencies. However the thinness of the ribbon makes them fragile-definitely not for outdoor use or even for the high SPL of drums even if in a
controlled environment. In a studio they can be very effective for voice and
most instruments. They produce a sound very close to the original-- in other
words not colored by the microphone design. This lack of sonic coloration is
often called "transparent" or "open." (14)
Magnet
Metal Foil Ribbon
Some Ribbon Mics
Nady RSM-4
Beyer M160
Nady RSM-2
RCA 44BX
4
Condenser Mics
Charged Diaphragm
Condenser microphones work on a different principle from dynamics.
Itʼs called electro-static induction. If two oppositely charged plates
are moved closer or farther apart, they will output a change in voltage.
Itʼs the same principle as that used in electronic capacitors. Condenser
mics usually have a very thin diaphragm coated with vapor deposited
metal film (sometimes "gold-sputtered"). This is held very close to an
oppositely charged metal plate. The surfaces can be anywhere from
1/4" in “small diaphragm” condensers to well over an inch in diameter
in so-called "large diaphragm" models.(16)
Charged Plate
Small Diaphragm Condenser Capsule
Popular Condenser Mics
Shure SM-81
(small diaphragm)
Shure KSM27
(large diaphragm)
Neumann KM 184
(small diaphragm)
The electrical charge on most professional level condenser mics is supplied by Phantom Power, a direct current
voltage that is applied through the same mic cable that carries the alternating current voltage of the audio signal.
Phantom power's unvarying pressure is usually 48 volts.(17) Less expensive condenser mics may get their voltage
from internal batteries. A few even have permanently charged plate and diaphragm. These are called electret mics.
The audio signalʼs voltage fluctuations coming off a condenser mic's capsule are very weak-- about a millionth of a
volt-- and thus have to be pre-amplified even before they reach the normal microphone preamplifier . In spite of the
condenser's weaker original signal, this pre-pre-amplification usually yields a hotter signal than a dynamic mic out of
the micʼs cable. Another attribute: the condenser's thin diaphragm without any attached metal coil allows the
condenser mic to respond well to fast attacks and to high frequencies.(18) Condenser mics are becoming increasingly
popular for general recording. The ones shown above are common examples of this type of mic.(19)
5
An interesting variation of the condenser
type is the boundary mic. Operating on
the same electro-static principle, it features
a small diaphragm/plate capsule held very
close to a solid plate of metal or other
heavy material. This proximity-- usually
about an eighth of an inch-- effectively
eliminates out-of-phase wave patterns and
thus produces a hotter signal through
simple acoustic principles.(20) Boundary
mics are quite sensitive to signals from any
direction in front of the plate and reproduce
high frequency signals well.(21) They are
often used as conference microphones
because of their hemispherical pickup pattern.
The original design of the boundary microphone was patented by Crown in the mid '70's. It is often referred to as PZM
for Pressure Zone Microphone.(22)
Other Mics
There are a few other types of microphones that work on different electrical principles, but they are not used much in
professional audio settings. They are the piezo-electric design in which the moving diaphragm twists a quartz crystal
producing a small voltage change, and the carbon granule design that works on varying electrical resistance. Carbon
granule mic capsules were common on early telephones.(23)
Piezo-Electric Mic
Carbon Granule Mic
6
Cables
Microphone cables are important to the overall quality of
a recording. Inexpensive "consumer level" mics often use
an unbalanced line cable like the one shown at right.
Electrically, this is the simplest 2 wire loop that is
essential for any electrical transmission to occur. This
type of line uses a 1/4" or 1/8" tip-ring Phone connector
on the far end of the cord.(24) If the length of the cord is
less than 20 feet, unbalanced cables usually work fine;
however, longer lengths risk corrupting the audio signal
with inducted noise interference from any number of
sources such as radio stations, ac power lines, and
florescent lights.(25)
The solution to this inducted noise problem is the
balanced line cable which is now common on all
professional level microphones. This cable uses 3 wires.
Two of these wires carry mirror image audio signals, one
of which is flipped over at the end of the cable. If any
interference is added through the cable's length, it
becomes a mirror image of itself when one of the audio
signals is flipped. Since mirror image signals cancel each
other out the noise is eliminated.(26) These cables
usually use a 3 prong XLR connector at their far ends
(sometimes called a cannon connector). Balanced
cables can exceed 200 feet without risking noticeable
inducted noise.(27)
Pickup Patterns
Microphones come in a variety of pickup patterns. In other
words their sensitivity varies depending on the direction of the
sound source. Common patterns are Omnidirectional,
Cardioid (sometimes mistakenly labeled unidirectional),and
bidirectional or figure 8.(28) These patterns are controlled by
the overall microphone design and sometimes by the addition
of baffles or vents at the sides of the microphone housing.
Pickup pattern often may be the most important characteristic
for matching a mic to a job.
The ElectroVoice RE20 shown at right is an excellent example
of a mic that uses baffles in its housing to help reject off-axis
sounds. This makes its pickup pattern more directional
Omnidirectional
7
Hypercardioid
Cardioid
BiPolar (figure 8)
The most common pickup pattern is the Cardioid. (29) It is most sensitive
when pointed directly at the sound source and less so when pointed off-axis.
Sound coming from in back of the mic is rejected to a large extent. Most
condenser and dynamic mics use cardioid patterns. More expensive
condenser mics often feature a switch that allows the user to select among
several different Cardioid pattern variations or an Omni pattern.
The Omni pattern is useful in situations where the recording engineer
wants to capture sounds from all directions--the ambience of the hall
perhaps, or a large number of sound sources equally. Omni mics are
also a good choice for close voice micing because they tend to eliminate
the boomy, bassy proximity effect. that cardioid mics exhibit when used
too close(30)
The AKG C 414 B-XL II
Condenser Microphone
features five selectable
polar patterns
Hypercardioid designs feature more rejection of off-axis sounds, however this side rejection usually comes at the
expense of a little more sensitivity to sounds originating from behind the mic. The pattern is a kind of an unequal
figure 8 design. If the sensitivity of both front and back becomes equal, then the pattern is bi-directional of figure 8..
(31)
The ribbon mic is a good example of a true figure-8 design because sound can enter the ribbon element equally
whether it comes from in front or in back.(32) This pattern can be useful for recording dialog or instruments placed
facing each other as in a duet.
In addition to the cardioid, omni,
and hypercardioid patterns, more
extreme patterns exist for special
purposes. Shotgun mic
extensions reject a great deal of
off axis sound through baffles, but
they do so at the expense of
frequency response and overall
sensitivity.(33)
8
Other Characteristics
1. Although pickup pattern is probably the number one characteristic for matching a mic to an application, several
other factors can also be useful in describing suitability. You should be aware that changing any single
characteristic tends to affect the others, so be wary of over-emphasizing the importance of any single factor
without considering the others. The characteristics include:
2. Sensitivity. For recording quiet sounds or sounds at a distance, a more sensitive mic is needed. In general a
sensitivity rating of -50 dB would be more sensitive than a rating of -60 dB, but this rather simplistic comparison is
confounded by the many different rating scales manufacturers use and by the fact that raising sensitivity also
tends to raise the mic's electronic noise level. Large diaphragm condenser mics tend to be more sensitive than
moving coil mics. PZM condensers add even more sensitivity because they reject interfering sound waves.
But be realistic; not every application requires the most sensitive mic.(34)
3. Frequency response. Even the cheapest Radio Shack mic often boasts a frequency response of 20 - 20,000 Hz.
However, though technically true, this kind of information is simplistic and very misleading. Truer data is shown by
a frequency response graph which indicates how much deviation in sensitivity the mic exhibits at various
frequencies. Supposedly, perfection would be indicated by a flat line from 20 to 20,000 Hz, but in reality this is
unachievable and maybe not even desirable. The industry standard for acceptability allows + or - 3 dB from this
theoretical flat line and even for high end microphones this occurs only between frequencies of 40 Hz and 17,000
Hz.(37) Often mics are chosen because of their deviation from flat line-- for example a more sensitive high end for
capturing a sound's brilliance. The typical Frequency Response graph below is for a Shure SM58.
4. Self-noise. Mics-- especially condenser mics with active pre-amplifiers built into them-- generate their own
electronic noise. This can be minimized by using high quality components and good design, but usually the more
sensitive a mic is, the more noise it will produce. In comparing mics, lower dB numbers in this categorygenerally
mean lower noise, but this like other numbers can be deceptive.(40) Since human ears are most sensitive to
sounds of between 1000 and 5000 Hz, a mic's self-noise at other frequencies is not sonically as important.
Because of this, the most accurate rating for a mic's self-noise is usually stated in dBA, a decibel scale weighted
by how the human ear responds. Think of dBA's as similar to the "wind-chill" factor often given in weather reports.
(41)
9
5. Transient response. Sounds that have fast attacks can have their sound dulled by inappropriate mic choice.
Small diaphragm condenser mics usually offer the best attack response to instruments like orchestra bells,
triangles, and xylophones.(36)
6. 3. Maximum SPL. Drums and close miking techniques for loud
instruments require a mic that doesn't distort when sound pressure
levels approach 130 to 140 dB. A moving coil mic can distort at loud
levels but often this sonic coloration is not unpleasant. Condenser mics
are more prone to unpleasant clattering artifacts at loud SPL's,
however better condenser mics often feature pad switches that give
10 to 20 dB of high SPL protection(35)
7. Low frequency response. It's important to be able to capture a full
range of sonic vibrations-- a string bass's lowest note is around 40 Hz.
Sure, you want good low end response in a mic. However mics tend to
also pick up handling noises and stage rumble as low frequency sound.
In addition cardioid pattern mics in particular are prone to "proximity
effect" coloration in which close micing emphasizes these low
frequencies too much. To allow the best of both sides of this
conundrum, many mics include a bass roll-off switch that trims as much
as 10 dB in a curve from around 100Hz down.(38) To eliminate rumble,
better mics often include an elastic banded shock mount rather than the familiar simple clamp.(39)
8. Impedance. This is a measure of a microphone's resistance to electrical fluctuation. All modern, high quality
mics-- the ones that use balanced line cables-- are classed as low-impedance output, usually around 200 ohms
(varying from 600 ohms to a very low 40 ohms). Modern quality recording equipment expects this low impedance
at its inputs. If it receives a high-impedance signal instead (between 10,000 and 30,000 ohms resistance) the
mismatch will cause dullness and sound distortion. It is important to know the output/input impedance specs of
your mics and your pre-amp, mixer or recorder. If you must mismatch them (for example using a good modern
microphone with an old classic tape recorder), you can compensate through the use of an impedance-matching
transformer.(42)
It can be deceptive to make judgements based simply on the numbers that manufacturers give for the above
factors. First, the rating scales are not standardized. Which is more sensitive, a rating of "-56 dB" or a rating of
"25mV/Pa (8mV/Pa +/-1dB?" Second, a recording may not require the highest level of a particular rating. For
example a mic recording a string bass doesn't need the best high frequency response.(43) Any audio professional
will have a good selection of different microphones to match different applications. A good starting setup would
include a pair of rugged dynamics, a pair of small diaphragm condensers. and perhaps a pair of variable pattern,
large diaphragm condensers. (44)
All microphones add their own color and personality to the sound they record. In the long run it is best to rely on
careful listening experience-- your own or that of other audio professionals you trust-- rather than just comparing
numbers. The manufacturer's reputation is important too. Shure, AKG, Audio-Technica, Neumann, Sennheiser,
BeyerDynamic, RODE, and Crown are all respected names.(45) Price can also be a guide: the sonic difference
between a $20 mic and a $500 mic is pretty easy to hear if youʼre critical. Expect to pay $100 for even the most
basic good professional mic (like a Shure SM58). Expect to pay $300 to $500 for mid-level quality.(46) If you
want to set up a high quality recording facility, remember that the first recommendation of every professional
involved with audio "invest in good microphones."
10

Basic Microphone Knowledge

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