The Audio Problems Caused by Multiple Open Microphones
High quality audio becomes progressively more difficult to achieve as the number of open microphones increases. All audio systems face the same problems whenever multiple open microphones are needed. These problems are:
- Build-up of background noise and reverberation
- Reduced gain before feedback
- Comb filtering
All of the problems above can plague boardrooms, city council chambers, conference centers, churches, teleconferencing rooms, talk shows, anywhere multiple microphones are used. Since audio quality rapidly deteriorates as the number of open microphones increases, the solution is to keep the minimum number of microphones open that will handle the audio. A Shure Automatic Mixer keeps all unused microphones turned off (attenuated), and activates any microphone spoken into within 4/1000ths of a second.
Build-Up of Background Noise and Reverberation
The first problem of multiple open microphones is the build-up of background noise and reverberation. This build-up can adversely affect the quality of recordings or broadcasts originating from the audio system. Let’s consider the case of a city council with eight members and eight microphones. For this example, only one member is talking. If all eight microphones are open, when only one microphone is needed, the audio output will contain the background noise and reverberation of all eight microphones. This means the audio signal will contain substantially more background noise and reverberation than if only the talker’s microphone were open.
This build-up of background noise and reverberation greatly deteriorates the audio quality. Speech clarity and intelligibility always suffer as background noise and reverberation increase.
As the number of open microphones increases, the background noise and reverberation in the audio output also increases. In our city council example, the audio output from eight open microphones would contain 9 dB more background noise and reverberation than a single open microphone. To the human ear, the noise would sound almost twice as loud when all eight microphones were open.
To minimize background noise and reverberation build-up, a Shure Automatic Mixer activates only the microphone(s) being addressed and employs a NOMA circuit. NOMA is an acronym for Number of Open Microphones Attenuator. NOMA systematically decreases the audio output whenever the number of open microphones increases. Without NOMA, the audio system would produce objectionable noise modulation (“pumping and breathing”) as background noise and reverberation increase and decrease with the number of open microphones. With a Shure Automatic Mixer, background noise and reverberation remain constant no matter how many or few microphones are activated.
Reduced Gain Before Feedback
The second problem of multiple open microphones is reduced gain before feedback. Acoustic feedback (“howling”) can be a problem anytime a sound reinforcement (PA) system is used. To avoid feedback, PA systems are operated below the point where the system becomes unstable and starts to “howl”. However, this feedback safety margin is reduced each time another microphone is opened. Have one too many open microphones and the result is feedback.
The Shure Automatic Mixer solution is to keep unused microphones turned off and utilize NOMA. As more microphones are activated, the overall gain will remain constant thanks to the NOMA circuit. With a Shure Automatic Mixer, you are assured that if the audio system does not feedback when any one microphone is open, the system will remain feedback-free even if all the microphones are open.
The third problem of multiple open microphones is comb filtering. Comb filtering occurs when open microphones at different distances from a talker are mixed together. (See Figure 1.) Since sound travels at a finite speed, the talker’s voice arrives at the microphones at different times. When combined in a mixer, these “out-of-step” microphone signals produce a combined frequency response very different from the frequency response of a single microphone. The aural result of comb filtering is an audio signal that sounds hollow, diffuse, and thin.
Not surprisingly, the solution to comb filtering also is keeping the number of open microphones to an absolute minimum. By automatically turning off unused microphones, a Shure Automatic Mixer reduces comb filtering and the resultant poor audio.
Keeping the number of open microphones to a minimum always improves overall audio quality. The primary function of a Shure Automatic Mixer is to keep unused microphones attenuated (turned off) and to instantaneously activate microphones when needed. Build-up of background and reverberant noise, reduced gain before feedback, and comb filtering can be all controlled by using a Shure Automatic Mixer.