Associate Professor of Voice, Shenandoah Conservatory Artistic Director of the CCM Vocal Pedagogy Institute
Last week I discussed the types of microphones you are most likely to encounter. This week I am going to discuss sensitivity and amplitude response.
Providing an in-depth explanation of sensitivity is difficult without going into additional discussions of electricity and electrical terminology. However, a simplified explanation should suffice for our purposes. In the simplest terms, sensitivity describes how responsive a microphone is to the amplitude of the source. For instance, compare a dog’s ears to a child’s ears. A dog can hear a squirrel running through the back yard while a child cannot. We therefore describe a dog’s ears as more sensitive than a child’s. Microphones with higher sensitivity can be placed farther from the sound source without adding excessive noise to the signal. Microphones with lower sensitivity will need to be placed closer to the sound source in order to keep excess noise at a minimum. When shopping for a microphone, you should audition several options plugged into the same soundboard with the same volume level settings. As you sing on each microphone, you will notice that some microphones replicate your voice louder than others do. This change in output level is due to differences in each microphone’s sensitivity. If your voice is naturally loud, you may prefer a microphone with lower sensitivity (one that requires more acoustic energy to respond). If you have a lighter voice, you may prefer a microphone with higher sensitivity (one that responds well to softer signals).
The amplitude response of a microphone describes how sensitive it is to sound arriving from various angles. The amplitude response will vary depending on the angle at which the singer is positioned in relation to the axis of the microphone. Microphone manufacturers publish polar pattern diagrams (also sometimes called a directional pattern or a pickup pattern), to help consumers visualize the amplitude response of a microphone at various angles. Polar pattern diagrams consist of six concentric circles divided into twelve equal sections. The center point of the microphone’s diaphragm is labeled “00” and is referred to as “on-axis” while the opposite side of the diagram is labeled “1800” and is described as “off-axis.”
Although polar pattern diagrams are printed in two-dimensions (see figure 1), they actually represent a three-dimensional response to acoustic energy. Think of a round balloon as a real life polar pattern diagram. Position the tied end away from your mouth and the inflated end directly in front of your lips. In this position, you are singing on-axis at 00 with the tied end of the balloon being 1800, or off-axis. If you were to split the balloon in half vertically and horizontally (in relationship to your lips), the point at which those lines intersect would be the center point of the balloon. That imaginary center represents the diaphragm of the microphone. If you were to extend a 450 angle in any direction from the imaginary center and then drew a circle around the inside of the balloon following that angle, you would have a visualization of the three-dimensional application of the two-dimensional polar pattern drawing.
The outermost circle of the diagram indicates that the amplitude of the instrument or voice is transferred without any amplitude reduction, indicated in decibels (dB). Each of the inner-circles represents a -5dB reduction in the amplitude of the signal up to -25 dB. For example, look at figure 6-6. If the microphone’s response curve crossed point A on this diagram, we would know that the strength of the signal received by the microphone at that point would be reduced by 10 dB. The examples below (figure 2, 3, and 4) show the most common polar patterns that you will encounter.
Implications for the Singer
Picking the right polar pattern can help alleviate problems with feedback and excess room noise. If you are performing with a floor monitor, a cardioid microphone is the best option. Since cardioid microphones have virtually no amplitude response from the posterior of the diaphragm, they will have only minimal response to the monitor. In some situations, such as recording a classical singer in a recital hall, you may want the microphone to respond to acoustic energy from more than one direction in order to capture the natural reverberation of the room. In that situation, a bi-directional or omni-directional microphone is the better choice.
Next week I will discuss frequency response and offer tips for how to practice with a microphone. Check back next Thursday, or “Follow” the blog to receive a message when there are new posts.