Basic techniques of microphone placement for recording and amplification
1) Ambient miking
Ambient miking is a technique in which a sound source is both directly and indirectly recorded. A sensitive microphone is placed at some distance to the source. Due to the distance, not only the source is recorded but also ambient noise such as reverberation from the room is recorded. The result is a very broad and natural sound, similar to the sound a present observer would hear.
2) Close miking
Close miking is the direct opposite of ambient miking. A directional microphone is placed at a very close distance to the sound source. This serves to eliminate ambient noise and results in a very clean and dry signal. This can be of benefit during a live performance to avoid feedback, or when recording several sources simultaneously while keeping the signals separated.
3) Stereo microphone techniques
Stereo recording is a technique which allows one to “place” a sound in a stereo field. Stereo microphoning is typically used, when recording several sources at the same time. For example, stereo microphoning is often used when recording a drum kit. This way, the placement of each part in the stereo field is similar to its actual placement, resulting in a “wide” sound. There are two main methods of achieving a stereo field, intensity stereophony and time-of-arrival stereophony. The first creates a stereo field resulting from the differences in intensity between the right and left signal. Time-of-arrival stereophony works as a result of the time of arrival at each microphone. Due to the finite speed of sound (appr. 330m/s), depending on which microphone is closer to the source, the signal will reach one microphone before it reaches the second. This time difference is audible and results in a respective placement in the stereo field.
I. Intensity stereophony
• X-Y technique
In the X-Y technique two cardioid microphones are placed
together at an angle of 90° or greater (typically pointing to
the far left and far right side of the objects being recorded). Due to
the close proximity of both microphones there is little time-of-arrival
information in the stereo signal. The stereo field is depicted by the
intensity difference between the two signals. This method has the
advantage of being mono-compatible, meaning that the signal is suitable
for playback on devices, which do not support stereo (for example old
radios).• Mid-Side (M-S) technique
The M-S techniques uses an omnidirectional microphone (a
directional cardioid microphone can also be used) and a bidirectional
microphone. The bidirectional “side” microphone is
placed at a 90° angle to the omnidirectional
“mid” microphone with the bidirectional microphone
facing the source. The stereo signal is created by adding the side
signal to the mid signal on one channel, and subtracting it (reverse
phase) from the mid signal on the second channel. The M-S technique is
also a mono-compatible method. When the stereo signal is combined to a
mono signal, the side signal cancels out as a result of the reversed
phase leaving only the mid signal.II. Time-of-arrival stereophony
• A-B technique
The A-B technique takes two omnidirectional microphones at
a set distance from each other (typically somewhere between 20 cm and
one meter). With this technique the stereo field is depicted by the
differences in time which it takes for a signal to reach one microphone
and then the next. This time difference is audible and results in a
stereo impression of the source. However as a result of the time
differences signals, the phase for example of the right signal is
shifted from that of the left signal. When combined to a mono signal
these phase differences can result in the canceling out of certain
frequencies. This method therefore does not have the same
mono-compatibility as the intensity methods.III. Mixed stereophony
• ORTF technique
The ORTF (Office de Radiodiffusion
Télévision Française) stereo technique
is a mixture of intensity stereophony and time-of-arrival stereophony.
In this technique a pair (best to use the same model) of cardioid
microphones are placed at a distance of 17 cm from each other at a
110° angle. This stereo method incorporates both intensity
differences as well as time of arrival differences to create a stereo
field. Though this method has reasonable mono-compatibility, the
time-of-arrival differences still make it not as mono-compatible as
“pure” intensity stereo methods are.• NOS technique
The NOS (Nederlandse Omroep Stichting) stereo technique is very similar to the ORTF technique. The distance between the microphones is 30 cm and the angle is 90°.