Recording - May 2016 - 57

might get cheated out of knowing the
capsule noise. Self-noise of a dynamic
mic, which results almost entirely from
thermal sources, is rarely stated on a
spec sheet-no cheating here since
there's only the capsule and transformer.
Active dynamic or active ribbon mics
have both thermal and electronic noise
and should be measured and specified
to include both sources.
Noise is measured as voltage, but for
a mic, it's often expressed as the Aweighted SPL at the capsule that would
produce that voltage. This is a calculated figure based on the measured noise
and sensitivity. The noise floor of a modern condenser mic is in the 15-20 dBA
ballpark (that's an impressively low 5.6
microvolts!). A technique used to
achieve a design goal of extremely low
noise is to under-damp the diaphragm.
This increases sensitivity at the cost of
some frequency response irregularities,
which are corrected by equalization in
the electronics.
Like any other noise specification, a
single number only tells part of the story.
A spectrum plot of the noise will tell you
if the noise is distributed fairly evenly
over the audio bandwidth or if it trades
off peaks at the ends (rumble and hiss)
for lower mid-band noise. Many of
today's microphones have sufficiently
low self-noise that, in actual use, the mic
preamp is the major contributor to system noise. You need a really quiet preamp to take full advantage of a truly
low-noise mic.
Impedance is specified in two ways.
Today most mic data sheets state the
mic's recommended or minimum load
impedance, typically around 1500Ω.
Others state the mic's nominal source,
generally in the range of 150 to 200Ω.
Both will work fine when connected to a
preamp with 2 to 2.5 kΩ input impedance.
Source impedance is usually measured at 1 kHz, though it's not always
constant over the audio frequency
range. A transformerless condenser mic
can have nearly constant source impedance, but the impedance of a mic with
an output transformer can vary, often
widely, due to electrical and mechanical
resonances. For some mics, optimizing
the load impedance through experimentation can improve smoothness by damping unpleasant resonant peaks, but this
is never a specification.
Nearly all microphones have a balanced output. A high quality output
transformer is an excellent balanced
source, but many transformerless condenser mics use a single-ended balanced output configuration ("impedance
balanced"). One lead carries the signal
while the other lead is terminated internally by a resistor equal in value to the

source impedance of the "hot" lead. This
configuration works well if done right,
eliminating the cost and distortion of a
poor transformer, though at the cost of a
small loss of external noise immunity.
Output configuration is rarely specified.
Power requirements used to be simple.
A tube mic has an outboard mainspowered power supply, while most solidstate condenser mics are phantom powered from the mic preamp. Dynamic
mics (including ribbons) require no
power, though they can sometimes be
damaged if connected to phantom powered input. Some mics are powered
from a self-contained battery, and miniature mics designed for portable
recorders and mobile devices often have
their own version of powering through
an unbalanced cable.
"Active" ribbon or other dynamic mics
are relatively new on the scene. These
incorporate a phantom powered "prepreamp" (also called a head amp) that
provides enough gain to bring the sensitivity up to about that of a modern condenser mic. The internal preamp may
also present the optimum load impedance to the mic element.
Standard phantom power is either 12,
24, and 48 volts, with 48 volts being
most common. Some mics specified for
lower voltages can be damaged by connecting them to a 48 volt supply, while a
mic specified for 48 V phantom power
may have reduced headroom or not
work at all at a lower voltage.
While a mic's phantom voltage
requirement is usually specified, its
current requirement often is not. Firstgeneration phantom powered condenser
mics used a very simple circuit drawing
just a couple of milliamps. Modern mics
with more complex circuitry can draw 5
mA or more from the phantom power
source.
IEC standard 61938 requires that a
minimum of 10 mA of current be available to the microphone, and most professional single- and dual-input mic preamps comply. Multiple-input devices
such as mixers or multi-channel digital
interfaces, however, may skimp on phantom power, providing, for example, a
maximum of 100 mA shared among 16
or 24 inputs. Connecting too many mics
with high current requirements could bog
down that power supply, starving some
or all of the mics.
Acoustic characteristics
Frequency response is usually the first
spec we look at, and it nearly always
looks better than it actually is. A single
frequency response specification is
always for on-axis response, and when
stated in numbers, should always have a
tolerance. Data sheets often include frequency response graphs that have been
RECORDING MAY 2016

57


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Table of Contents for the Digital Edition of Recording - May 2016

Contents
Recording - May 2016 - Cover1
Recording - May 2016 - Cover2
Recording - May 2016 - 1
Recording - May 2016 - 2
Recording - May 2016 - 3
Recording - May 2016 - 4
Recording - May 2016 - 5
Recording - May 2016 - Contents
Recording - May 2016 - 7
Recording - May 2016 - 8
Recording - May 2016 - 9
Recording - May 2016 - 10
Recording - May 2016 - 11
Recording - May 2016 - 12
Recording - May 2016 - 13
Recording - May 2016 - 14
Recording - May 2016 - 15
Recording - May 2016 - 16
Recording - May 2016 - 17
Recording - May 2016 - 18
Recording - May 2016 - 19
Recording - May 2016 - 20
Recording - May 2016 - 21
Recording - May 2016 - 22
Recording - May 2016 - 23
Recording - May 2016 - 24
Recording - May 2016 - 25
Recording - May 2016 - 26
Recording - May 2016 - 27
Recording - May 2016 - 28
Recording - May 2016 - 29
Recording - May 2016 - 30
Recording - May 2016 - 31
Recording - May 2016 - 32
Recording - May 2016 - 33
Recording - May 2016 - 34
Recording - May 2016 - 35
Recording - May 2016 - 36
Recording - May 2016 - 37
Recording - May 2016 - 38
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Recording - May 2016 - 40
Recording - May 2016 - 41
Recording - May 2016 - 42
Recording - May 2016 - 43
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Recording - May 2016 - 45
Recording - May 2016 - 46
Recording - May 2016 - 47
Recording - May 2016 - 48
Recording - May 2016 - 49
Recording - May 2016 - 50
Recording - May 2016 - 51
Recording - May 2016 - 52
Recording - May 2016 - 53
Recording - May 2016 - 54
Recording - May 2016 - 55
Recording - May 2016 - 56
Recording - May 2016 - 57
Recording - May 2016 - 58
Recording - May 2016 - 59
Recording - May 2016 - 60
Recording - May 2016 - 61
Recording - May 2016 - 62
Recording - May 2016 - 63
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Recording - May 2016 - 65
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Recording - May 2016 - 68
Recording - May 2016 - 69
Recording - May 2016 - 70
Recording - May 2016 - 71
Recording - May 2016 - 72
Recording - May 2016 - Cover3
Recording - May 2016 - Cover4
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