Choir Tech Know-How Part 4 - Understanding microphones

In my last article, I discussed the pros and cons of wired or wireless microphones. This time, I'll be looking at the different types of mic available and the circumstances in which you can use them.

Dynamic microphone

The microphones most often used for live vocals are dynamic microphones. These work on the principle of magnetic induction by converting acoustic energy (sound waves or sound pressure) into an electronic signal by moving a coil of wire though a magnetic field. A small diaphragm in the head of the microphone moves back and forth. The mechanism is almost exactly the reverse of a speaker, which converts electronic signals into sound.

Dynamic microphones do not require power to use, but will need amplification to boost the signal, which is usually done in the mixer.  Dynamic microphones are the most rugged, and are most suitable for live performing as they can tolerate more abuse than the other types of microphone.

Condenser microphone

Condenser microphones have a much flatter frequency response than dynamic microphones, ie they reproduce the sound passing into them more accurately. They are usually more expensive. They require an extra (internal or external) power supply. This can be a battery inserted into the microphone, a permanent charge on the diaphragm or back plate, or more commonly, phantom power from the mixing board.

The signal from a condenser microphone is very weak compared with that of a dynamic microphone, so it must be amplified before it gets to the mixing board or studio console. Condenser microphones contain an amplifier that boosts the signal before it leaves the microphone. They are most often used for acoustic instruments and studio vocals.

Condenser microphones are much more delicate and sensitive microphones than dynamics and can be easily broken, so they are not considered good candidates for travelling or live singing.

Microphone directionality or polar pattern

Once you have chosen the type of microphone that suits you best, you need to choose what is called the polar pattern or directionality. This measures how sensitive the microphone is to sounds that arrive at different angles about its central axis. In other words, the microphone’s pattern is the way in which it picks up sound from various directions.

Microphones are generally made with certain applications in mind (e.g., stage use, studio use or field recording use). For that reason, they are not always expected to pick up sound universally, from all directions and in all frequencies. There are a few standard pick-up patterns. These are generally split in to directional and omnidirectional.

Omnidirectional microphones

Omnidirectional microphones pick up sound from all directions equally. For this reason, they may not be the right choice for many live applications since they are prone feedback and picking up unwanted background noise.

Although an omnidirectional microphone's response is, theoretically, a perfect sphere in three dimensions, this is generally not the case. The body of the microphone tends to block sounds arriving from the rear, causing a slight flattening.

Omnidirectional microphones do not have resonant cavities, which can cause a delay in the sound, so they are often considered the "purest" microphones in terms of how the reproduce the sound. In other words, they add very little to the original sound.

Directional microphones

A directional microphone is sensitive to sounds from a particular direction. There are three main patterns you are likely to come across. These are cardioid, hypercardioid and supercardioid. All these are commonly used as vocal or speech microphones as they are good at ignoring sounds from other directions.

The most common unidirectional microphone is cardioid, so named because its sensitivity pattern is heart-shaped. The indented top of the heart is positioned at the back of the microphone. This means that it will block the sound from the rear. This is useful as it gives the microphone the ability to amplify a specific area while ignoring another.

The other patterns, hypercardioid and supercardioid, have similar shapes to cardioid microphones but are even more sensitive to noises from the front. The pay-off is they are slightly more sensitive to sound from the rear. When used to amplify sounds coming from sources close to each other, such as a small group of singers, these patterns can help to isolate each individual sound source.

Most vocal microphones have either cardioid or hypercardioid pick-up patterns. These are preferable for live performances because they are good at rejecting extraneous sounds around them.

We use cardioids dynamic microphones for our live choir performances. We use them both as vocal mics and for picking up the choir from the front of the stage. If you are performing indoors with the speakers well in front of the stage, a couple of condenser mics above the choir can work very well, but in my experience they are prone to feedback and are very sensitive to wind noise if you are outside.

As you've gathered from my last two articles, choosing a microphone depends on several different factors. I hope you find this information useful, and I'll be happy to answer your questions if I can.