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
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.
thing to look for in a choir microphone is a flat frequency response. This
means the microphone should pick up all the audio frequencies at the same
volume. Many microphones have their low frequencies intentionally rolled-off
(less sensitive). A typical example, as shown in Figure 1, is designed to be
used close to the mouth. Notice in Figure 1 how the bass is rolled off starting
at about 250 Hz.
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).
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.
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.
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.
Microphone with a pickup pattern with a lot of
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
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.
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.
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.
by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This
article is for personal use only. Any commercial reproduction is not permitted
without permission. To obtain permission, contact ISI at email@example.com