Capturing Sound Pt. III – Microphone Techniques

For the final part of Capturing Sound I would like to utilize the information shown in parts one (microphone types) and two (polar patterns) and give them some practical application – microphone placement. Where you place the microphone is fundamental to the quality and texture of the source – with most instruments there will be a sweet spot. This sweet spot can be found by getting your musician to play and take a walk around until you find somewhere that represents the best tonality of the instrument (this is not an exact science and often comes down to personal taste of the engineer).

“Mics cannot effectively be placed by sight, which is mistake that is all too easy to make. The best mic position cannot be predicted, it must be found.” – Bobby Owsinski

There are two basic ways to record an instrument: mono and stereo. The former uses one microphone and the latter two – stereo can give a more realistic replication of audio as we naturally hear in stereo anyway. The advantages recording with stereo over mono are as follows:

  • Great representation of the left and right sound fields.
  • It can capture the depth and distance between instruments.
  • Provides the sense of a spatial environment (hall ambience/reverberation).
  • Can create a feeling of distance between the listener and the performer.

This does not necessarily mean you record everything in stereo as it usually used in recording drum kits (overheards), pianos and ensembles. With this in mind, lets take a look at the 4 main stereo miking techniques:

 

I. XY (Coincident Pair)

A coincident pair means two directional microphones that are placed very closely together (not touching though as the microphones are likely to knock of each other during recording). This is done so that the diaphragms can be in almost the same position (meaning the sound reaches both at the same time) and still be angled left and right for a stereo image. The XY pair a commonly used technique and uses two identical directional microphones, placed as close to each other as possible, sometimes just above each other to align diaphragms (see diagram below). The angle between diaphragms is normally around 90 degrees but can vary up to 135 degrees to widen the stereo image.

 

II.  Spaced Pair

This technique is great for capturing ensembles and as the name suggests, a spaced pair is a pair of identical microphones spaced apart (felt silly typing that). For this technique it does not really matter what type of microphone you use, but it is common for a pair of omnidirectional microphones to be used. The microphones are placed a few feet in front of the source, facing directly forward: the greater the space between the microphones, the wider the stereo image. However, too far apart and the stereo image will sound excessive, and too close will nullify the stereo image, so balance is key. As with most principles in recording it comes down to the engineer’s choice, there is no real right and wrong but as a guide, try spacing them around 4 feet away from the source and 12 feet apart (diagram below).

 Spaced

 

III. Near-Coincident Pair (ORTF)

This technique is similar to the XY technique but as the name suggests the microphones are spaced a little further apart. Once again use two identical cardioid microphones and spaces them 17cm (7 inches) apart with an angle of 110 degrees. The spacing is to provide an accurate representation of sounds as the microphones are placed similarly to that of our own ears (however this does not compare to accurate representation of the Baffled-Omni Pair).

In case you were wondering, ORTF stands for the Office of French Radio and Television Broadcasting (English translation)

IV.  Middle-Side (Coincident Pair)

The last of the four main stereo microphone techniques is the most complicated but has great uses. For this you will need one directional (or omni) and one figure of 8 microphone. Place the directional mic in front of the source pointing at it (making it the Middle) and the figure-8 mic perpendicular to it so faces the sides, remembering to keep those diaphragms as close as possible. M/S has little phasing problems and has a great stereo image but this technique requires an additional step in the mixing stage to make it work. In order to have a middle and two sides, the figure-8 channel must be duplicated and one of the channels to be put out of phase. If both channels are panned hard left and right, they will not cancel each other out with phasing.

 

What is phasing?

As I just mentioned, phasing is something that you will almost never want to occur in the recording process and by understanding it, you can learn to recognise and avoid it. With stereo miking, the sound can reach each microphone at different times if they are not placed correctly (fractions of seconds are more than enough). This causes the recorded audio to be ‘out of phase’ which results in a thin and weak sounding mix, as opposed to a full ‘in phase’ one. It is important to listen out for this when recording so you do not cause problems later on. A good example of this is when you are recording drums: listen to the overheads together and then flip them into mono and see if they lose anything. After this gradually bring in the other elements of the kit, remembering to check the phase and if you have problems, change your microphone placement. Although you cannot completely rule out the possibility of phasing, you can certainly ensure it does not have a big impact on your mix.

 

The 3 to 1 Principle

When discussing the spaced pair I recommended that you spaced them 4 feet away and 12 feet apart – this is the 3 to 1 principle. This principle is great at eliminating problems with phasing and states that the distance between the microphones should be 3 times greater than the distance they are from the source. Although it is not an exact science, it is a great guideline for microphone placement.

  

Some final tips

Here are some points to consider when choosing microphones:

  • Close miking is not always necessary (normally used to prevent other instruments bleeding in) so try sitting the microphone further back to capture the sound more naturally
  • Correct microphone choice for instrument (think of frequency response and their characteristics)
  • Select one that won’t be overloaded by the source (ribbon microphones are very sensitive and fragile)
  • Pick the right polar pattern for the job

“I’ve been doing this long enough to know that a change in microphones or positions is worth a lot more than tweaking EQs. I have a tendency to think that if you start tweeking EQs too soon, you’re going to miss some obvious things, so the first thing I do is get the session sounding great flat”. – Frank Filipetti.

 

Further Reading

Stereo Miking Techniques by Bruce Barlett (ProSoundWeb)

Stereo Miking Part One and Part Two by Hugh Robjohns (SoundOnSound)

Middle-Side by Bryan Heller (eMusician)

Blumlein Pair (eMusician)

The Decca Tree by George Petersen (HdvideoPro)

The Decca Tree & Surround Sound by Ron Streicher (online pdf)

 

 

I hope you have found this blogging trilogy helpful and if you have any questions, please ask :]

Part I on Microphone Types found here.

Part II on Polar Patterns found here.

 

Neil.

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