Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 44
to render its frequency response flat. With continuous sinusoidal signals it functions perfectly, but
when a signal appears suddenly, the resonating device needs a certain time to move. When a
sound disappears suddenly, the resonator continues to produce a signal. The result is that the
transient signal (e.g. a percussive sound) will be colored by the inherent resonance of the
microphones. This explains the difference noted by the ear between microphones with seemingly
identical characteristics.
In general, condenser microphones use resonators only in the extreme high frequencies, where the
coloration phenomenon has little importance. As a result, their fidelity is excellent. Ribbon
microphones can colour the low frequencies. Moving coil dynamic microphones colour to the
greatest extent, this coloration is not always undesirable. They can improve certain voices, and the
experienced engineer will not hesitate to use them under certain conditions. He can also use any
defects in the frequency response for filtering, etc.
Use at High Sound Levels
Ribbon microphones and bi-directional condenser microphones can be damaged by a large air
displacement.
To record an explosion, a moving coil microphone, or better still, an omni-directional condenser
microphone is recommended. A switchable microphone (uni-, bi- or omni-directional) risks the same
damage as an ordinary bi-directional microphone. A microphone can be damaged under these
conditions whether it is being used or not. It is advisable to place bi-directional and cardioid
microphones in sealed boxes if an explosion is likely.
Independent of the risk of damage, it is possible that a microphone will not reproduce well at levels
greater than a certain value, above which the signal would become distorted. In general, moving
coil microphones support the highest levels. Certain condenser microphones are designed so that
an attenuator can be placed between the microphone capsule and the preamplifier.
Signal-to-noise Ratio
The recording of low level sounds can be disturbed by the combination of the microphone and its
preamplifier. The word "combination" is used because the background noise does not come only
from the amplifier. Take the case of a dynamic microphone whose impedance is 200 Ohm. As it
does not have a temperature of absolute zero (- 273
?
C) the electron movement in this impedance
will produce a noise signal called the thermic noise. The preamplifier adds to the thermic noise its
own inherent noise, but in a recorder such as the Nagra 4.2, the thermic noise is by far the most
important.
The acoustic noise is measured in phons. The phons are decibels whose reference zero has been
fixed by convention at 0.0002 µbars. The measuring device is not linear, but has a frequency
response similar to that of the ear. For low levels, this frequency response is called the ASA "A". It
is possible to find out the equivalent acoustic noise level of a microphone and its preamplifier. Take
for example a microphone of 200 Ohm having a high sensitivity (0.25 mV/µbar). Its noise level
referred to the input will be -126 dBm ASA "A" (the dBm are decibels whose reference zero has
been fixed at 1 mW). Now, 0.0002 µbars is equivalent to 0.005 µV (139 dBm). Therefore the
equivalent noise of this microphone will be 139 - 126 = 13 phons.
This figure is correct only if the impedance of the microphone is 200 Ohm. Often, certain
microphones whose nominal impedance is 200 Ohm have higher impedance's, at least in certain
parts of frequency spectrum. The effect of this is to increase the equivalent noise.
A condenser microphone can also be characterized by an equivalent noise level, thereby making it
possible to compare the performance of these microphones with that of dynamic ones.
Directional Characteristics
Often, when recording sound it is desirable to attenuate certain unwanted sounds, such as echoes
coming form the studio walls. Certain microphones have a sensitivity, which varies greatly
according to the direction from which the sounds come. In effect, these combine a pressure
characteristic with a velocity characteristic. Taking into consideration the air pressure at any given
point, a microphone acting as a manometer is called a pressure microphone. The direction from