The M-S Microphone - A Failsafe Technique
There are many different ways of arranging microphones to
produce a stereophonic image using 2 or more microphones; few are as simple or
straightforward to work with as M-S. (Mid and Side)
We frequently use a Sennheiser
MKH30 and an MKH40
microphone to provide us with the means of producing our M-S Microphone. The
microphones have different pick up patterns, these are known as polar patterns
and they
describe how the microphones respond to sounds from different directions
relative to the front of the microphone.
M (or MID)
The MKH40 has a 'cardioid' response pattern, (this is heart shaped) and has
greatest sensitivity to the sound arriving at the front face of the microphone
capsule, lesser sensitivity to the sides and even lower sensitivity to the rear
of the microphone capsule.
S (or SIDE)
The MKH30 has a 'figure of eight' response and this means that the capsule
picks up sounds from the two 'faces' but far less signal is picked up from the
sides of the capsule. When viewed from above, with the capsule facing to
the sides (or east-west) you may imagine a figure '8' placed sideways on the
capsule describing the response pattern. The signal on one face of the capsule
is of opposite phase to that on the other face, ie., the signal on the right
hand face of the capsule is 180 degrees out of phase with that on the left.
The cardioid microphone in an M-S setup will face
directly forward, (or north when viewed from above) whilst the figure of eight
microphone will be placed as close to the cardioid capsule as is physically
practical so that the left (in phase) face of the capsule points to the left or
west whilst the right (out of phase) face of the capsules points to the right or
east. I have no intention of going into the maths involved here in producing the
stereo soundstage but you will find some useful links below; suffice it to say
that when the signals from the two capsules are fed into a matrix mixer, a good
solid stereo image can be produced. Not only is the signal thus produced totally
compatible if summed to mono, the soundstage is extremely forgiving in the case
of non ideal placement of the microphones when an off-centre position is
dictated for other reasons. The soundstage provided by this means allows for
control of width and to a certain extent may be used to 'focus in' on the direct
sound or 'pull back' and use more of the ambience of the performing space. By
varying the amount of signal from the figure of eight microphone, more or less
of the ambience is mixed in with the front facing direct signal. The total
removal of the figure of eight microphone's signal results in a mono pickup with
little of the ambience from the sides and rear of the performance space.
I chose this particular microphone combination because:
The microphones are very high
quality devices with little self noise, they have a high immunity to radio
frequency noise pickup and they are moderately flat frequency response in that they
pick up a wide frequency range with very little variation throughout. The
microphones have a respectable output level which means there is less chance of
picking up audible unwanted electrical noise (such as lighting circuits in theatres) on
longer cable runs. The microphones are fairly small and lightweight, in a black
satin finish that is easy to blend in with a concert environment. These
microphones also have a very useful clip that allows for the two barrels to be
mounted very neatly on one sprung mount where necessary thus providing a very
tidy array for my main stereo pair. They are fairly expensive items, but then
most good quality microphones are!
This page last updated
09/05/08