The sound in the meeting room should be bright, clear and moderate in loudness. When setting up the meeting place, it is impossible for the speaker to come to the scene for audition. Therefore, in order to achieve a bright and clear sound effect (the loudness is just right but not the requirement of screaming) when amplifying on site, there is a certain knowledge about the selection and placement of conference microphones.

1 Selection of conference microphone

Because different microphones have different effects on different sound sources, it is necessary to deeply understand the technical characteristics, performance and applicable occasions of microphones before selecting them. In addition to the electroacoustic indexes such as the sensitivity of microphone, the matching of output impedance and input impedance of mixer, the directional characteristics, frequency response characteristics and the shape of microphone should also be considered in the conference room.

As far as directivity is concerned, supercardioid is suitable for long-distance pickup, cardioid is suitable for most cases, and undirected is not suitable for language pickup, because it is easy to cause feedback. In addition, in the conference room, the microphone with proper directional characteristics should be selected according to the layout of the speaker.

1) For centralized or semi centralized layout of sound reinforcement system, the directional characteristics of microphone shall be cardioid or supercardioid.

2) For the sound reinforcement system with distributed layout, the cardioid microphone should be selected generally. If the microphone is far away from the sound reinforcement box and the reverberation time in the hall is not too long, the directionless microphone can also be selected.

3) When the sound source is very close to the microphone, the proximity cardioid microphone with technical indicators to eliminate proximity effect can be used.

As far as the frequency response characteristics of microphones are concerned, those with lower static technical indicators and better transient characteristics are better than those with higher static technical indicators and worse transient characteristics. In addition, a microphone with uniform and smooth response from low frequency to high frequency is better than a microphone with prominent frequency band, because the prominent part is also easy to cause squealing.

In addition, blunt and heavy mouth sound is often over accentuated due to the proximity of the microphone. Of course, it can be compensated by electronic correction (in the sound transmission circuit), which has no effect on the oral sound with too low audio, but there is a limitation on the front and back movement. The distance between the speaker and the microphone should be kept constant, otherwise the low-frequency components will change, and the ratio between direct sound and reverberation (in the sound field with "active" sound state) will also change.

A large number of microphones with peaks between 6000-9000 Hz will make the hissing accent more dentate. Of course, it can also be corrected electronically, but at the cost of bringing a dead point at the summit. Therefore, it is not necessary to use a high-quality microphone in the conference room, just a microphone with smooth response.

In terms of the internal structure type of microphones, dynamic microphones were used for sound reinforcement in general meeting rooms. In recent years, with the enlargement and widening of conference table, most of them choose capacitive gooseneck microphone. In this way, when the conference table is relatively wide, it will not be pushed to the table by the speaker without picking up the sound. In addition, the microphone head of the condenser microphone is relatively small, so it is one of the reasons why it is chosen not to cover the face of the speaker.

To sum up, in addition to special requirements, it is generally recommended to select capacitance gooseneck cardioid microphone with wide frequency response, uniform frequency response transmission characteristics, smooth and low distortion.

2 placement of conference microphone

The microphone placement in the conference room is generally divided into table top placement and microphone rack placement.

When placing a microphone on the desktop, pay attention to the angle and distance between the speaker's mouth and the center of the microphone, so that the central axis of the microphone is aligned with the speaker's mouth, at this time, the frequency response characteristics of the microphone output are better; the farther the mouth deviates from the central axis, the worse the frequency response characteristics, the more severe the loss of high pitch, and the output voltage of the microphone will be reduced. Generally speaking, the angle between the mouth and the central axis is within 450.

When speaking in the conference room, in order to improve the speech intelligibility, it is better to keep the microphone 20-30cm from the mouth. If it is too close, it is easy to have a low-frequency accent, which will affect the intelligibility of the voice. If it is too far away, the microphone will pick up too many reflections and reverberations, which is easy to cause screaming. At this time, if the mixer is pushed too small, the loudness of the voice can not meet the requirements, the audience can not hear the speech clearly, and it is too large and easy to scream. Therefore, it is very important to accurately estimate the speaker's speaking distance when placing the microphone.

When more than one person speaks and more than one microphone needs to be placed, the distance between microphones should be 3 times greater than the distance between the sound source and the microphone (i.e. meeting the 3:1 principle), so as to reduce the phase interference phenomenon when the signals are added.

When a person needs to use two or more microphones for pickup (for example, to make a report), keep two or more microphones as close as possible, and keep the distance from each microphone to the sound source (speaker's mouth) as equal as possible, so as to avoid phase interference when the signals are added. At this time, pay special attention to the microphone phase to be consistent. Take two microphones as an example. If the volume of two microphones is not large, it means that the two microphones are reversed. At this time, the phase of one microphone needs to be adjusted. Generally speaking, the problem lies in the microphone line, as long as the wiring of one microphone is adjusted.

When placing the microphone stand, because many of the microphone stands are triangular supports, after installing the microphone, the center of gravity of the microphone should be projected on a certain support foot. In addition, the movable rod for installing the microphone should not be drawn too long to avoid the microphone center of gravity falling out of the range of the stand.

3 audition of conference microphone

Because of the particularity of the conference room, it is impossible for the speaker to come to the meeting room for audition in person (for example, the leader's speech) in most cases, so just adjust the volume of the normal person, try to make the speaker reach the moderate volume at the beginning (the sound size is moderate without screaming), but leave enough volume adjustment. But if there are many microphones, the mixer needs to adjust and test repeatedly between the mixer and the microphone, which is very troublesome and inefficient. Here I have a trick, that is, after the microphone is placed, turn down the mixer push of all microphones, turn down the gain knob, and then turn the power amplifier to the proper position for normal use, push the main output of the mixer to 0dB, and then push the corresponding push of a certain microphone to the high level of the channel (generally, the mixer is 10-12 DB), and then slowly adjust the gain knob of the microphone on the tuning table, and carefully * the sound, until there is a slight howl, turn the knob back 3 dB, and then pull the push back to 0dB. In this way, the microphone can not only ensure that the speaker has enough volume and loudness performance, but also not cause screaming. When the speaker's voice is small, there is enough margin (13 dB) for adjustment. Handle other microphones as well. After this adjustment, opening multiple microphones at the same time will not cause squealing.