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.