Microphones for Speech Reinforcement and Two Potential Problems
Microphones for Speech Reinforcement
Figures 18.6 and 18.7 show some of the microphone types that are used in speech reinforcement. The handheld vocal microphone is used as shown at Figure 18.6A. Vocal microphones are normally designed so that they produce the desired response when positioned about 5 to 10 cm (2 to 4 in) from the performer’s mouth. At such small operating distances, the vocal microphone is fairly immune to feedback—a classic example of reducing Ds to a very low value. The best microphones for vocal use are those that have integral multiple screening surrounding the capsule to minimize the effects of inadvertent puffs of wind from the talker. Many of these microphones have a pronounced “presence peak” in the 3–5 kHz range for added brightness and improvement of articulation. Many performers feel very much at home with a vocal microphone in hand—and they often feel at a loss without it. Proper microphone etiquette must be learned; never blow on the microphone to see if it is on; always hold it slightly to the side, outside the breath stream, and maintain a consistent operating distance.
The head-worn microphone (shown at Figure 18.6B) has long been a staple in communications activities, but it was not until its adoption by singer Garth Brooks that it became a staple for onstage performance. The microphone element is an electret, normally with an omni or hypercardioid pattern, that is equalized for close use. When properly worn it is stable in its positioning and offers excellent performance. It is invariably used with wireless body packs, and for lecturers or panelists it provides complete freedom of movement. If properly positioned out of the breath path, the use of a windscreen is not required for indoor use.
For permanent podium or lectern mounting there are numerous miniature electret cardioid or Hypercardioid models mounted on flexible gooseneck extensions, as shown at Figure 18.7AandB. These can be unobtrusively located to one side and positioned at a distance of 15–30 cm(6–12 in) from the mouth of the talker. A small windscreen is recommended. It is important that the gooseneck portion be out of the range of movement of papers or notes used by the talker, and that the talker’s normal motions not be impeded.
For use on flat surfaces, such as tables used in panel discussions, or altars in houses of worship, a boundary layer microphone (shown at Figure 18.7C) can be used when system gain before feedback allows. An omni pattern often works best, but cardioid models may be necessary to minimize local noises. The cardioid, however, will be more subject to impact noises than the omni. The operating distance is normally in the range of 45–60 cm (18–24 in).
The lapel microphone, which was introduced in the previous chapter, has the advantages of a small electret and is very popular primarily because it is inconspicuous. It is important that it be fastened to the user’s lapel or tie with enough slack in the cable to avoid pulling or tugging on the cable as the performer moves around. For sedentary applications the microphone may be wired directly, but for most purposes it is used with a wireless body pack. Position the microphone as high as possible on the tie or lapel; however, be aware that in a high position normal up and down head movements may cause audible shifts in level. Make the right compromise. The lapel microphone’s response is normally shaped to minimize radiation from the chest cavity and to maintain HF response.
Two Common Problems
When microphones are positioned close to reflecting surfaces, the delayed reflection may combine with the direct sound, producing some degree of comb filtering in response. Figure 18.8A shows an omni microphone mounted on a podium in such a way that it will pick up a distinct reflection from the reading desk, producing uneven response. Moving the microphone to one side will alleviate this problem to some degree. A better solution is to use a hypercardioid microphone, whose off-axis response will attenuate the reflection, as shown at B.
Another common problem is the use of two microphones where one is sufficient. Improper usage is shown in Figure 18.9A. In broadcasting of important events, doubling of microphones is often done for transmission redundancy in case of failure of one channel, but more often than not both microphones end up operating in parallel. For a talker positioned exactly between the two there may be no problem. But talkers move around, and the combined signals from both microphones will cause peaks and dips in response as shown. The solution is shown at B, where both microphones are mounted in coincident fashion and splayed slightly if needed to increase the effective pickup angle. In this case the position of the talker will not be a problem, since the talker’s distance to the two microphones remains the same.
Excerpt from Ray A. Rayburn’s Eargle’s The Microphone Book, 3e.
Ray A. Rayburn is a Senior Consultant with K2 Audio LLC. He is a member of the AES Standards Working Group on Microphones, and Chair of the Standards sub-committee on Interconnections. He is also a recording engineer with a lifetime interest in microphone use, testing, and design. Ray is also the author of the recently published Eargle’s The Microphone Book, 3e. Visit the book’s companion site!