There is nothing better than capturing a well rehearsed band banging out an awesome performance live off the floor. The energy, interaction and vibe just feels great but what can be a challenge is minimizing the bleed while capturing that performance and the smaller the space is that you are recording in, the more difficult it becomes.
Bleed is, for example, when the sound of the guitar is heard in the drum overheads. You would then say that the guitar is bleeding into the overheads.
Dealing with this problem is actually a lot easier than you would think as long as you keep these 4 things in mind.
1. Distance between instruments
2. Microphone polar pattern
3. Microphone proximity
You already know that the further you are away from something the quieter it becomes. In the audio world this is described in the “inverse square law” (http://bit.ly/ahWVVo). It basically says that a sound will decrease by 6db every time you double the distance to the source. In other words, a mic that is 2m or 6′ away from a source will be 6db quieter than a mic that is 1m or 3′ away from the same source.
What it boils down to is that the further you put two instruments apart the less bleed you will get.
The thing to watch out for when increasing the distance between sound sources is that if they are too far apart, whatever little bit of bleed that ends up in the other microphones can be off time and can be off enough to give an unwanted delay effect.
Microphone polar pattern
A microphone’s polar pattern describes the directionality of the microphone. I generally like to keep it simple and say that there are three main polar patterns.
Cardioid, also called unidirectional picks up sounds coming from in front of it and rejects sounds from behind it.
Bidirectional, also called figure of 8 picks up sounds that come from the front and the rear but rejects sounds from the sides.
Omnidirectional picks up sounds from all directions.
You can improve the separation greatly just by taking the pickup and rejection areas of the microphone into consideration. For example, a very common set up for recording a singer playing an acoustic guitar is to use two bidirectional microphones where one is closer to and pointing at the singer’ mouth and the rejection point is pointing at the guitar. The second mic is closer to the guitar with the rejection point pointing at the singer’ mouth.
This one is pretty much a given but a valid rule non the less. The closer a microphone is to an instrument or a sound source the more prominent that instrument or sound will be in that microphone.
Placing a barrier between a mic and an instrument will obviously have an affect on the intensity level of that instrument in the mic. The question is, where to put the barrier? Answering this question becomes more difficult the more instruments you add to the recording. I tend to think of this in terms of how easy is it to box an instrument in. It is much easier to box a guitar cab in than a drum kit. I also think of it in terms of the spreading of high and low frequencies in a room. (That will be covered in a later blog)
Some instruments have characteristics that can serve as natural barriers that may not immediately come to mind. Trumpets, trombones and closed backed electric guitar amps are quite directional. I typically do not have any separation problems when close micing a horn section and I would often place two guitar cabs back to back and close mic them.
By keeping these 4 things in mind you will be well on your way to great instrument separation and if non of this works, you could always use an electric drum kit and DI everything.
Enjoy and have fun,