Learning Event 3:
DESCRIBE THE PROPER
After cables and plugs have been eliminated as a source of trouble,
Remember that the
microphone lines must not be checked with an ohmmeter without first
disconnecting the ribbon microphone.
The line then may be checked,
(unterminated), for high resistance, shorts, or (terminated), for opens or
Hum and noise may occur in any part of the audio circuit; In the
microphone circuit this can result from ground loops or imbalance caused by
faulty or improper cable connections to the bus or preamplifiers.
Electrical machinery may induce hum into the microphone transformer or
ribbon microphones. Sometimes this can be minimized by turning, tilting, or
Microphone phasing: It is well known that correct phasing is important
This is especially true when two similar microphones are
placed in symmetrical relationship to a performer.
Polarity of a microphone, or the microphone transducer elements,
refers to in-phase or out-of-phase conditions of voltage developed at its
terminals with respect to the sound pressures of a sound wave causing
voltage. An exact in-phase relationship can be taken to mean that the phase
of the voltage is coincident with the phase of the sound-pressure wave
causing the voltage. In practical microphones this perfect relationship may
not always be obtainable.
The in-phase terminal, or a microphone, is that terminal of the
connector or conductor that is connected to the in-phase terminal of the
transducer. On microphones using a connector per E1A, the in-phase terminal
is No. 1, the out-of-phase terminal is No. 2, and the ground terminal is No.
3. On microphones with a cable but no connector, the out-of-phase terminal
The polarity of a pressure microphone (or omnidirectional) does not
vary with the direction of arrival of a sound wave.
The polarity of a
gradient microphone is reversed for sound waves toward the rear of the
microphone. There may be a substantial phase shift in the microphone at the
low and high frequency ends of the spectrum. Therefore, the definition of
polarity is generally restricted to the midpoint of the useful transmission
To check the phasing of two or more microphones, connect one
microphone to the associated amplifier input and set the volume control to
obtain the desired output. Then talk into the microphone. Now connect the
second microphone in parallel with the first, without changing the volume
control setting. Hold both microphones close together and speak into them.
If the volume decreases from the previous level, reverse the connections of
one of the microphone cables at the microphone plug. Similarly check each
additional microphone for phasing.