There are many reasons why a choir or large vocal group would need to use microphones. The event or concert may be recorded or videotaped. Maybe the room is dead acoustically, and the choir needs a little extra help to produce a big choral sound. You need to add some microphones. But you wonder, what type? How many do I need? And where should I place them? These are the important questions to answer.

What Type of Microphone?

The choir microphone must be able to pick up voices at 6 to 10 feet away from the microphone. In other words, it must be very sensitive. A condenser microphone can have a 10 to 15 dB higher output level over a dynamic microphone. This means your sound system electronics will not have to provide as much gain or amplification in order to obtain a sufficient volume. The result is a higher signal-to-noise ratio (a lower hiss or noise) and a better dynamic range.

Figure 1
Close Proximity Microphone with Bass Roll Off.

That frequency (250 Hz) is about the first "A" below "middle C." The fundamental frequencies of many male voices are below this frequency. When this microphone is used at a distance, it will sound quite thin or tinny because of the low bass pick-up. However, when it is used as designed (within 6 inches) it will sound much better due to the bass boost which is caused by the proximity effect (Figure 2).

Figure 2
Close Proximity Microphone Frequency Response Curve Showing Bass Boost.

As previously stated, a microphone designed for distance will have a flat response and will reproduce a natural sound from a greater distance. See Figure 3 for a good example.

Figure 3
Distance Microphone with a Flat Frequency Response Curve.

Another important aspect is the off-axis frequency response (the response from the sides and the rear of the microphone). The gain-before-feedback and the overall sound is greatly affected by the smoothness or evenness of the off-axis response.

Figure 4
Microphone with a consistent cardioid pickup pattern.

Gain-before-feedback is the maximum amplification of sound before the sound system goes into feedback and starts squealing. The simplified polar graphs of the microphone will give a good idea of its response. Figure 4 shows a microphone which has an even response. The sound this microphone picks up at the sides is very much like the sound it receives on-axis (the front) except it is lower in volume. The frequency response curve at all angles are nearly the same. It is said to have very little off-axis frequency coloration. On the other hand, Figure 5 shows another type of microphone which has off-axis frequency coloration.

Figure 5
Microphone with a pickup pattern with a lot of coloration.

Cardioid Pattern Microphone

In most cases, a cardioid pattern (directional) will produce more gain-before-feedback than an omni-directional microphone. The cardioid pattern can also be used to reduce the pick-up of an orchestra or audience noises.

This is not an easy question to answer. Many factors must be taken into account, such as choir size and arrangement. One important rule to follow is the "3 to 1 Rule" (Figure 6). That is, the distance from one microphone to the next must be at least three times the distance the first microphone is from its source of sound. This will assure minimum interaction and cancellation. If the choir is to be amplified in the room, then gain-before-feedback will be a concern to address. Often, the fewer microphones that are used, the greater the chance of success.

Figure 6
Top view of choir showing 3 to 1 rule.

Every time the number of microphones that are on doubles, the maximum volume before feedback will drop 3 dB. Simply adding microphones will not necessarily mean you will increase the maximum available volume. You must first move the microphones closer to the choir so the sound level reaching the microphones is louder and so the "3 to 1 Rule" is not violated. Of course, you may now start picking up more of the individual voices, so the choir members may have to be shifted to provide a better balance. A good Rule of Thumb is to employ one high quality, high output microphone (in other words, a very sensitive microphone) for about 25 people, so a choir of up to 50 would use two microphones. Don't forget - in audio, less is often better.

What position?

Figure 7 shows a formula that is very effective for up to three rows of singers. The microphones are one foot in front of the first row and 24 to 36 inches over the heads of people in the first row. The microphones are then pointed at the heads of people in the back row. This technique takes advantage of the cardioid pattern. People in the back row sing directly into the microphone's most sensitive side (front, on-axis). Now as we move forward and down to the front row we get closer to the microphone. However, we also get farther off-axis so the microphone is less sensitive. Therefore, the microphone picks up the front and back row voices at about the same volume.

Figure 7
Correct alignment of a microphone for a choir.

You can also use this method to help balance a small vocal group. Place the weaker singers in the middle (on-axis) and the stronger singers on the outside (off-axis).

Choir microphones can be set up on microphone stands with booms or installed to hang from the ceiling. The method you choose will depend on your visual and portability needs.

As you can see, many factors must be considered in selecting a microphone for each particular choir application.