The Directional Characteristics of Microphones.

In an earlier post I introduced the fundamental recording tool: The microphone. As mentioned in that post, and in a later one about a recording technique microphones have different directional characteristics. Not all microphones pick up sound equally from every direction, or as is most common, from the front.

To understand these differences it is important to understand some of the physics (Yes... everything involves physics) behind the technology. As mentioned in a previous post, sound is the movement of high and low pressure zones in air. These zones and their frequency are called waves. When two waves are added together, say by a microphone membrane into a single electrical current they can reinforce, subdue or cancel each other out entirely.

Keeping this in mind I can go ahead and introduce the four main directional characteristics found in microphones. These are: Cardioid (directional), Omni-Directional, figure-8 (or Bi-Directional) and Super-cardioid.

CARDIOID

The cardioid pattern is by far the most common in modern day microphones. Basically every dynamic microphone has a cardioid pattern. The reason the cardioid pattern works is because of the way the housing around the membrane is constructed. It is formed in such a way that the sound takes longer to reach the rear of the membrane than to reach the front, this goes for sound approaching from the back as well.

When two identical waves hit the membrane at the same time, they will cancel each other out, because the opposing forces will be equal. When the same two waves hit with a delay of half of their period (the time between two "peaks") They will accentuate each other. The delays that the construction of the microphone create means that sounds from the front accentuate each other while sounds from the rear cancel each other out.

The above picture shows a diagram of how a cardioid microphone pics up sound from different directions. Notice that higher frequencies are more directional than low frequencies. This can be very useful in a variety of recording techniques, and is a trait that is shared by all characteristics.

OMNI-directional.

As the name implies, omni directional microphones theoretically accept sound equally from all directions. This is achieved by creating a membrane which is only accessible to sound from one side, meaning that there is no possibility for opposing waves to cancel each other out as with the cardioid pattern.

Interesting results can be gained by using omni mics because they admit a large amount of room sound. They are especially good for larger ensembles such as orchestras and choirs as they do not accentuate a specific person or instrument.

As the above diagram shows, not even the omni-directional microphones are completely omni throughout the frequency spectrum.

FIGURE-8 (Bi-directional)

Figure-8 microphones are directional both to the front and to the rear. This is achieved by haveing one, freely placed membrane. This means that sounds from the front and rear work together, while sounds coming directly from the sides, or the 90-degree axis are completely nulled out. This is the one and only directional pattern which has a 100% null point. This is exceedingly useful in many aspects of recording, and also in calibration of acoustics. When talking into the null, very little of your direct speech is recorded. The only thing recorded is the resulting room noise. Recording uses are numerous (I wrote a post on this topic a few days ago) and can make many tasks much easier.

Again, the diagram clearly demonstrates the way higher frequencies are more directional than lower frequencies.

Many modern microphones use dual membranes and through the use of these can achieve almost infinite variations of these three basic patterns. This is done by summing the signals from the different membranes in different ways, and it is in this fashion that we get the fourth general pattern. This pattern is someplace between cardioid and figure 8 as is called Super, or Hyper (depending on the degree of directionality) Cardioid. This pattern is a lot more directional than cardioid and are most commonly used in broadcast and television applications in the form of Shotgun-Microphones. These microphones use the hyper cardioid pattern whilst incorporating some mechanical modifications to all but eliminate sounds coming from the sides.

As you can see from the diagram above, the influence of a bi-directional membrane is obvious, thinning out the width of the cardioid and introducing a small peak in the rear.

So thats all for tonight. I hope the post has been interesting, both to beginners and those who have been using microphones for a while. Remember, don't hesitate to comment or send me an email if you have any questions.

Microphone techniques: Guitar and vocal

If you have ever tried recording a singer who plays acoustic guitar you have probably encountered this problem. Because the guitar is in such proximity to the vocal source, seeing as they are being performed by the same person, there is alot of bleed between the two channels. Getting a good separation between the different sources is often a headache in these situations, where one wishes to process the vocal and the guitar separately.

The technique I am about to write about is extremely useful in these situations, and takes advantage of microphones with a figure of eigh directional pattern (more on this in another post). Different microphones have different directional patterns, meaning they pick up sound differently from different directions. A cardioid pattern is by far the most common and means that the microphone picks up sounds coming streight on much better than sounds coming from the back. Another pattern is the figure of eight pattern. As the name impies, this means that the microphone picks up sounds from the front and back, but rejects sounds from the sides (90 degre axis). Only the figure of eight pattern has a complete nullpoint, meaning a direction where it rejects 100% of all sounds, and this can be used very efficiently for recording an acoustic guitar played by a vocalist.

We start by using two identical microphones (ideally, though it will work with different mics) and place the first one in front of the guitar, and the second one in front of the vocalists mouth. When these microphones are angled so that their 90 degree axis' point directly at the other source they will effectively "null" it out. Now, placement is very important, as the level of separation depends on how accurately you have set up the microphones.

There will allways be some bleed from the guitar or the vocal of course, because we sadly do not live in a theoretically perfect world. The guitar radiates sound over a slight area, and to a larger degree so does the vocal. So there will always be some vocal and some guitar on the respective tracks. The separation however is much easier to work with and better, clearer results can be gained.

The figure of eight pattern is also incredibly useful in other settings as well. Lets imagine a kick drum, if a microphone is placed angling up towards thetoms and cymbals, this will hlp reduce the blled on the microphone, og a hihat mic for example, angled to eliminate the snare or the cymbals.

The possibilities are endles, playa round with them, and have fun!