The Inhibition Method of Acoustic Feedback

- Sep 17, 2018-

The sound reinforcement system usually uses the microphone gain to indicate the degree of acoustic feedback. Ideally, the volume should be turned to the maximum, and the howling should still not occur. The difference between the sound level of the speaker received by the listener and the direct sound received by the microphone is dB represents the level of the sound gain. Eliminate howling to take the necessary measures from all aspects of the cause, as long as one of the conditions can be destroyed, the howling can be suppressed and eliminated.

1. Directional selection

The whistle of the sound reinforcement system is not directly related to the sensitivity of the microphone, but the high-sensitivity microphones are often sharply directional, which is prone to whistling. For this reason, the non-directional form should be used as much as possible.

When using a strong directional microphone, it can only be used to receive sound from the main surface of the sound source. When using a directional speaker, do not point it toward the microphone, preferably using a sound column if possible.

2. The method of adjusting the distance

Keep the microphone as close as possible to the active pickup, shortening the distance between the sound source and the sounding device of the microphone and the listener, which can enhance the direct sound, improve the loudness of the sound, and maintain the total gain of the system, avoiding whistling and increasing the volume of the sound. If it is supplemented by a near-field speaker with a wide directivity, it is better to avoid howling from a far point.

For the direct feedback sound field of the speaker, the farther the microphone is from the speaker, the closer the speaker is to the listener, the better the microphone should be placed on the back of the speaker's radiation direction. If the microphone is likely to be moved around, the speaker should be placed on the microphone. Very close.

3. Frequency equalization method

Since the frequency curve of the microphone pickup and sounding equipment is not an ideal flat straight line, especially for some low-quality sound-emitting equipment, the acoustic response of the auditorium sound field often has a resonance effect, which makes the frequency response fluctuate greatly. The frequency equalizer can be used to compensate the sound reinforcement curve, and the frequency response of the system is adjusted to an approximate straight line, so that the frequency response of each frequency band is basically the same and the sound transmission gain of the system is improved. This method is also called broadband notch method.

Generally, an equalizer of 21 or more segments should be used. In the case of higher requirements, a parametric equalizer should also be configured. When the requirement is higher, a feedback suppressor can be used. In fact, when the feedback system is self-excited, the frequency is only a pure tone fixed at a certain point. As long as the frequency is cut off by a narrow-band notch, the effective suppression can be performed.

Select a microphone and speaker with a flat frequency characteristic to reduce the self-excitation caused by the peak.

4. Feedback suppressor method

In high-demand occasions, such as some live singing places, the microphone feedback suppressor is commonly used, which is a device that automatically suppresses howling, which automatically tracks the feedback point frequency, automatically adjusts the Q-value bandwidth, and automatically provides acoustic feedback. Eliminate and maximize the sound quality. The principle is to control howling through the notch.

5. Inverse cancellation method

The inversion cancellation method is a common method for preventing self-excitation in the high-frequency amplifying circuit. In the audio amplifying circuit, two microphones of the same specification can be used to respectively pick up the direct sound and the reflected sound, and the reflected sound signal is made by the inverter circuit. The phases cancel each other before entering the power amplifier to effectively prevent the howling from self-excitation.

6. Phase modulation method

The self-excited howling of the sound reinforcement system is generally positive feedback. If the microphone signal is phase-modulated, the self-excited phase condition will be destroyed, thereby preventing the self-excited howling of the system. When the phase deviation value is at 140 degrees, the stability is the best; and, the higher the frequency of modulation, the better the stability of the system. In order to prevent the processed sound quality from being too distorted, the maximum allowable value of the phase modulation frequency is 4 Hz.

7. Shielding method

The howling caused by electromagnetic radiation can be solved by the shielding method. For example, in a computer speaker, the sound card can be moved away from the slot of the display card, or a metal plate can be added between the two cards. Increase the sound absorption process near the microphone to reduce the reflected sound to the microphone while minimizing the number of microphones.

8. Strengthen the filter

Try to use a narrow-band filter to attenuate the most severe frequency band to increase the sound gain. Due to the whistling caused by poor filtering, the filtering should be enhanced. Usually, the most effective method is to connect a 1000-2000pF capacitor in parallel with the power supply filter capacitor.

 

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