Creating the perfect sound quality between mics and speakers is a science. You want and are paying for quality sound at your event. At the same time, you are also balancing the aesthetics of a room with tall black stands and speaker boxes that are obviously there.
To create quality sound for a room, audio engineers have to evaluate both the size of the room and the number of people in attendance, then strategically place the speakers so everyone can hear equally as well without any feedback (that high pitched squealing sound).
To do that, speakers generally need to be placed so the microphones are in back of the speakers (on the stage, for instance) and in the front or along the sides of the first row of the audience. The height of the speakers should be at least at the height of the audience heads. This placement prevents the feedback from occurring, and unfortunately, makes the speakers very noticeable.
Because the front speakers may also be angled towards the back of the room, the folks in the first few rows of the audience may not hear as well. In that case, a center speaker in front of the stage and on the floor will do the trick.
For a large audience, more speakers are placed down the sides of the audience so everyone can hear at the same sound level while the presenter is speaking at a normal conversation tone. If you only had speakers up front, the sound would be blasting to the audience up front in order to reach the back of the room.
Of course, every event and room layout is different. That is where working with your audio engineer will result in the best balance between finding the quality sound you desire, while locating the equipment to keep it as unobtrusive as possible. This is best done on site during set up, when the equipment can be moved around to test the sound, and feedback.
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