Microphone pickup sound systems have the possibility of feedback whistling. The microphone whistling rhyme is very harmful, mainly in the following aspects: 
  1. When the power amplifier is self-excited, it will produce a large power output, which may exceed the bearing range of the sound reinforcement equipment, burn out the power amplifier and sound equipment. 
      2. When the feedback coefficient is close to l, due to the comb filter effect, the superposition between the delayed sound field and the direct sound will make the amplified sound field narrower than the original sound field. 
      3. The delayed feedback of the sound field of the loudspeaker will make the whole system form a series of delayed echoes, and this echo will aggravate the comb filter effect, resulting in obvious distortion of the reverberation tail-just distortion. 
      4. The output sound pressure when whistling is very large, which seriously affects the atmosphere of various activities. 
      To eliminate feedback whistling from the necessary conditions for generating feedback whistling, as long as one of the conditions can be destroyed, the goal can be achieved.
  
1, adjust the distance method 
  One of the most effective ways to both avoid whistling and increase the volume of the PA is to pick up the microphone as close to the source as possible, and the microphone should be non-directional. Here it is clear that the pickup attenuation of the directional microphone (especially the sharp directional microphone) remote sound source is very small, and the adjustment of the distance has little effect on improving the amplification and preventing whistling. Whether the sound reinforcement system is easy to scream is not directly related to the sensitivity of the microphone. But highly sensitive microphones are sharp directional, easy to produce whistles. Shortening the distance between the device and the audience can actually improve the loudness of the PA. The total gain of the system can be reduced appropriately. If coupled with a wide directional near-field speaker, the microphone can be slightly farther away to avoid whistling.
  For the direct feedback sound field of the speaker, the farther the microphone is from the speaker, the better, and the closer the speaker is to the listener, the better. The microphone should be placed on the back of the speaker radiation direction, if the microphone is likely to be carried around, the speaker should be placed where the microphone cannot get very close.

2. Frequency equalization method (broadband notch method)

  Because the frequency curve of microphone pickup and sound production equipment is not an ideal flat straight line (especially some poor quality playback equipment), as well as the acoustic resonance of the hall sound field, the frequency response fluctuates greatly. The frequency equalizer can be used to compensate the sound amplification curve, and the frequency response of the system can be adjusted to an approximate straight line, so that the gain of each frequency band is basically consistent, and the sound transmission gain of the system can be improved.
  An equalizer above 21 Yin should be used, a parametric equalizer should be configured where the requirements are higher, and a feedback suppressor can be used when the requirements are higher. 
  In fact, when the feedback self-excitation occurs in the sound reinforcement system, its frequency is only a pure tone fixed at a certain point, so as long as a very narrow band notch excise this frequency, you can suppress the system whistling.

  
3, feedback suppressor method (narrow band trap method) 
  In highly demanding occasions, such as some live singing places, commonly used audio feedback automatic suppression device, this device can automatically track the feedback point frequency, automatically adjust the Q value bandwidth, automatically eliminate the acoustic feedback and maximize the protection of sound quality. The principle is to suppress whistling by trapping waves. For example, Sahine's FBX series feedback suppressor, which is a 9-segment narrow-band automatic pressure limiting device controlled by a microcomputer, can better distinguish the feedback self-excited signal from the music signal, can react quickly when the system appears self-excited, and set a very narrow digital filter at the feedback frequency point, and its dip depth will be automatically set. The filter bandwidth is only 1/3 octave, and such a narrow notch band almost has no effect on loudness and timbre.
  
4. Inverse offset method 
  Reverse phase cancellation to prevent self-excitation is common in high frequency amplifier circuits. Two microphones of the same specification can be used in the audio amplification circuit to pick up direct sound and reflected sound respectively, and the phase of the reflected sound signal can be cancelled out before entering the power amplifier through the reverse circuit, which can effectively prevent whistling from self-excitation.
  
5. Phase modulation method 
  The self-excited roar of the PA system, its feedback loop is positive feedback, if the microphone signal is phase modulation processing, it will destroy the self-excited phase conditions, so as to prevent the self-excited roar of the system. Some data show that when the phase deviation value is 1400, the stability is the best; Moreover, the higher the frequency of modulation, the better the stability of the system. In order to make the processed sound quality without too much distortion, the maximum allowable value of the phase modulation frequency is 4Hz.
  Finally, when all kinds of equipment are adjusted, you must not let others move, including some DJS who are unfamiliar with the performance of the equipment and only know how to open, shut down and adjust the volume.