Increasing Potential Acoustic Gain

A number of steps can be taken to optimize the potential acoustic gain of a sound reinforcement system. This gain is limited by the feedback condition. Some of these measures are strictly geometrical and can be modeled from a simplified amplification system. Others involve more technical approaches.

Usable amplification can be increased by the geometrical factors:

1. Moving the loudspeaker further from the microphone
2. Moving the loudspeaker closer to the listener
3. Moving the source closer to the microphone
4. Moving the source closer to the listener
   
   and by more technical means such as:
5. Using more directional microphones
6. Using more directional loudspeakers
7. Use of notch filters (feedback suppressors)
8. Equalizing the sound system

Moving Loudspeaker Farther from Microphone

The first practical step which is usually taken when a portable sound system rings from feedback is to move the loudspeaker farther from the microphone. The amount of anticipated improvement in the potential acoustic gain can be modeled for the simplified amplification system. In a real auditorium, you cannot achieve as much improvement as that modeled amount because of reverberation. In general, it does no good to move the loudspeaker out past the critical distance at which the reverberant sound field contributes as much to feedback as the direct sound field. As a practical measure, this critical distance for the speakers must be determined by experiment, moving the speakers farther out until you can no longer increase the gain before feedback.

Critical Distance for Speaker Placement

You can get more amplification of sound without the annoying ringing by moving the speakers further from the microphone. However, this has practical limits.

The direct sound field from a point source in an auditorium drops off according to the inverse square law. To the extent that the speaker can be considered to be a point source, then the feedback from that speaker to the microphone is decreased by moving the speaker further away.

The reverberant sound field, on the other hand, more or less fills the entire room and the contribution of the loudspeaker to the reverberation does not decrease as you move the speaker further out from the microphone.

The critical distance is defined as the distance at which the reverberant sound is equal in intensity to the direct sound. At distances greater than the critical distance, the reverberant sound is dominant, and you get no further increases in potential acoustic gain by moving the speakers further out.

Move Loudspeaker Closer to Listener

Moving the loudspeaker closer to the listener without changing the distance from the microphone will increase the available amplification. In most practical applications, this means adding extra speakers which are closer to the listener. However, this creates sound image problems, and the use of digital delay of the signal to those extra speakers is recommended.

Move Loudspeaker Closer to Listener

One of the obvious ways to get more amplified sound to the listener is to move the loudspeaker closer to the listener. The amount of anticipated improvement in the potential acoustic gain can be modeled for the simplified amplification system. As a practical matter in larger auditoriums, this means using additional speakers which are closer to the listener to add to the sound from a main speaker cluster. A problem which arises is that the signal from the amplifier to the distant speaker travels at the speed of light whereas the direct sound from the source travels at the speed of sound. A sound image problem results from the fact that the sound from the nearby speaker reaches the listener before the sound from the visible source in the front of the auditorium - your ear locates a sound partly by time of arrival and therefore hears it coming from the speaker. The location conflict between your ears and eyes can be disconcerting. This is typically overcome by using a digital delay for the sound signal going to the distant speakers.

Use of Digital Delay

To maintain the perception that the sound is coming from the front of the auditorium, it is necessary to use digital delay to speakers under balconies, etc., where they are much closer to the listener than the main speakers. The signal to the speaker from the microphone travels at the speed of light, and the sound to the listener would arrive first from the closest speaker. Precedence has a strong localizing influence, and all the sound would seem to be coming from the nearby speaker. With appropriate delays, the sound to all listeners seems to come from the main speaker.