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produce hum on their own. Unwanted noise (hum) can only come from an external source.
Fortunately, there is a simple procedure that can be used to identify the source of the
noise and eliminate it. Repositioning a dynamic and ribbon microphone is the number-one
way to get rid of unwanted noise. If hum is detected, it means that the microphone is in
the proximity of an alternating magnetic field. While listening (preferably with
headphones) to the mic, move it around. The mic will “find” the noise source quite easily.
If you are miking a guitar amplifier and suspect the amplifier’s power transformer may be
the source of unwanted noise, move the mic around the amp. You will probably find that
the noise is louder as you approach the amplifier’s power supply and quieter when you
move it away. Eliminating the noise can be fairly simple and requires finding the “null”
point of the noise. The “null” point is the position that places the microphone’s magnetic
components away from the lines of radiation. Simply rotating the microphone slightly is
many times all that is required to cancel out the noise. This is similar to the procedure
guitarists use to eliminate noise from single coil guitar pickups. There will be a “magic
position” where the noise disappears completely. Finding the exact position that also
provides acceptable acoustic placement of the mic requires a little practice, but the results
are well worth the time.
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Finding and Working with the Sweet Spot
Good engineers know the importance and benefits of finding and working with the “sweet
spot” of a given microphone. The sweet spot will be defined as the optimum placement
(working distance and angular position) of any microphone relative to the sound source.
Each microphone has its own sweet spot whether it is a ribbon, dynamic or condenser
type. The sweet spot will vary with the type of sound source and its volume intensity,
the polar pattern of the microphone and how consistent it is with frequency and the
acoustic environment.
This condition is called the sweet spot because the microphone and the sound source are
in a harmony of sorts; the acoustic information is exciting the microphone in such a
fashion that the resuling reproduction is very desirable, usually without the need for
additional equalization or electronic manipulation.
There are only general rules as to where the sweet spot may be found for any given
microphone, and usually experimentation reveals it. The sweet spot can be extremely
variable since it depends on the quirks of a given microphone and a given room. Once the
sweet spot is discovered, this placement can become a rule of thumb starting point for
future microphone placement with similar sound sources. Remember this:
If it sounds
good, it’s probably right
. If it doesn’t, move the microphone. It’s often more effective to
reposition the microphone than to start fiddling with knobs. Knob twisting can affect
headroom and phase coherency and add unwanted noise.
The following is a list of variables that contribute to “sweet spot” effect
1. Frequency response variations due to proximity effect.
