In the entire sound loop, there is a negative feedback structure. The deeper the negative feedback, the better the fidelity.
If negative feedback is embodied in the reverberation loop, it will cause flying marks. That is howling. This is a headache for all operators. The appropriate method now is to use other equipment to perform frequency correction. An effective device is a frequency shifter. It is to move the required frequency and amplify it. In this way, there will be no reflection of negative feedback in the sound reproduction. An equalizer is usually used. It can also be controlled simply and effectively. The equalizer will change the composition of the sound, making the sound strange. If a good quality DJ can be achieved, but a bad quality DJ will feel more and more chaotic. Because the equalizer is used for correction, it will change the composition of the sound, and the sound system that the microphone picks up has the possibility of feedback. Microphone whistling is very harmful, mainly in the following aspects:
1. During self-excitation, the power amplifier will produce a large power output, which may exceed the endurance range of the sound reinforcement equipment, and burn out the power amplifier and sound equipment.
2. When the feedback coefficient is close to 1, due to the comb filtering effect, the superimposition 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 loudspeaker's sound field will cause the entire system to form a series of delayed echoes, and this echo will aggravate the comb filtering effect, resulting in a noticeably distorted reverbe
ration tail-rigid sound distortion.
4. The output sound pressure during howling is very large, which seriously affects the atmosphere of various activities.
Elimination of feedback whistle starts with the necessary conditions for generating feedback whistle. As long as one of the conditions can be destroyed, the goal can be achieved.
1. Adjust the distance method
One of the effective ways to avoid howling and increase the amplification volume is to place the microphone as close as possible to the sound source to pick up the sound. At the same time, the microphone should be omnidirectional. To be clear here, directional microphones (especially sharp directional microphones) have very little pickup attenuation from long-distance sound sources, and adjusting the distance has little effect on increasing the amplification volume and preventing howling. Whether the sound reinforcement system is prone to howling is not directly related to the sensitivity of the microphone. It's just that high-sensitivity microphones are all sharp, and they are prone to howling. Shortening the distance between the sound generating device and the audience can actually increase the loudness of the sound reinforcement. The total gain of the system can be appropriately reduced. If supplemented by a near-field speaker with wide directivity at the same time, the microphone can be a little farther away to avoid howling.
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 audience, the better. The microphone should be placed on the back of the speaker's radiation direction. If the microphone is likely to be carried around, the speaker should be placed where the microphone cannot be close.
2. Frequency equalization method (wideband notch method)
Because the frequency curve of microphone pickup and sound equipment is not an ideal flat straight line (especially some poor quality playback equipment), and the acoustic resonance effect of the hall sound field, the frequency response fluctuates greatly. The frequency equalizer can be used to compensate the sound reinforcement 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 the same, and the sound transmission gain of the system is improved.
Equalizers with more than 21 bands should be used. Parametric equalizers should be configured where the requirements are higher, and feedback suppressors can be used when the requirements are higher. In fact, when the sound reinforcement system has feedback self-excitation, its frequency is only a pure tone fixed at a certain point. Therefore, as long as a trap with a very narrow frequency band is used to cut this frequency, the system's howling can be suppressed.
3. Feedback suppressor method (narrow-band notch method)
In high-demand occasions, such as some live concerts, audio feedback automatic suppression devices are commonly used. This device can automatically track the feedback point frequency, automatically adjust the Q bandwidth, automatically eliminate the acoustic feedback, and protect it to a large extent. Up the sound quality. The principle is to suppress howling through notching. For example, Sabine’s FBX series feedback suppressor, which is a 9-segment narrow-band automatic compression device controlled by a microcomputer, can better distinguish the feedback self-excited signal and the music signal, and can react quickly when the system is self-excited. And set a very narrow digital filter at the feedback frequency point, and its notch depth will be automatically set. The filter bandwidth is only 1/3 octave. Such a narrow notch frequency band will hardly affect the loudness and The timbre has an impact.
Four, anti-phase cancellation method
Anti-phase cancellation to prevent self-excitation is more common in high-frequency amplifier circuits. Two microphones of the same specification can be used in the audio amplifier circuit to pick up the direct sound and the reflected sound respectively. The phase of the reflected sound signal is cancelled out before entering the power amplifier through the inverting circuit, which can effectively prevent howling self-excitation.
Five, phase modulation method
The feedback loop of the self-excited whistling of the PA system is positive feedback. If the microphone signal is phase-modulated, the self-excited phase condition will be destroyed, thereby preventing the self-excited whistling of the system. Some data show that when the phase deviation value is 140°, the stability is appropriate; and, the higher the modulation frequency, the better the stability of the system. In order to prevent too much distortion of the processed sound quality, the maximum allowable value of the phase modulation frequency is 4 Hz.
At the end, when the various equipment is adjusted, you must not let other people move, including some DJs that are not familiar with the performance of the equipment and only know how to turn on, turn off, and adjust the volume.