In the entire audio circuit, there is a negative feedback structure. The deeper the negative feedback, the better the fidelity. If negative feedback is reflected in the feedback loop, it will cause flying marks. That is, howling. This is a headache for all operators. The best way now is to use other devices for frequency correction. The most effective device is a frequency shifter. It is to move the required frequency and amplify it. This way, there will be no negative feedback reflected in the sound reproduction. Usually, an equalizer is used. It can also be easily and effectively controlled. The equalizer will change the composition of the sound, causing it to vary strangely. Because using an equalizer for correction can change the composition of the sound, there is a possibility of feedback squealing in the microphone's pickup sound system.
The harm of microphone whistling is significant, mainly manifested in the following aspects:
1. When self-excited, the power amplifier will produce a large power output, which may exceed the capacity of the sound reinforcement equipment, and burn out the power amplifier and sound producing equipment.
When the feedback coefficient approaches 1, due to the comb filtering effect, the superposition between the delayed sound field and the direct sound field will make the amplified sound field narrower in terms of sound sense than the original sound field.
2. The delayed feedback of the speaker sound field will cause a series of delayed echoes throughout the entire system, and this echo will exacerbate the comb filtering effect, resulting in significant distortion of the reverberation tail - just sound distortion.
3. When howling, the output sound pressure is very high, which seriously affects the atmosphere of various activities.
To eliminate feedback howling, one must start with the necessary conditions for generating feedback howling. As long as one of the conditions can be broken, the goal can be achieved.
1、 Adjusting distance method
One of the most effective ways to avoid whistling and increase the amplification volume is to pick up the microphone as close as possible to the sound source, while using a non directional microphone. To be clear, directional microphones (especially sharp directional microphones) have minimal pickup attenuation from distant sound sources, and adjusting the distance has little effect on increasing the amplification volume and preventing whistling. Whether the sound reinforcement system is prone to howling is not directly related to the sensitivity of the microphone. However, highly sensitive microphones are highly directional and prone to squealing. Shortening the distance between the sound equipment and the audience can actually improve the loudness of the amplification. The total gain of the system can be appropriately reduced. If accompanied by a near field speaker with wide directionality, the microphone can be kept slightly away from howling. For the direct feedback sound field of speakers, 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 in a place where the microphone cannot be very close.
2、 Frequency equalization method (broadband notch method)
Due to the frequency curve of microphone pickup and sound production equipment not being ideal flat straight lines (especially for some poor quality playback equipment), as well as the acoustic resonance effect of the hall sound field, the frequency response fluctuates greatly. A frequency equalizer can be used to compensate for the sound amplification curve, adjusting the frequency response of the system to an approximate straight line, so that the gain of each frequency band is basically consistent, and improving the sound transmission gain of the system. Equalizers with 21 or more segments should be used, and parameter equalizers should be configured in areas with higher requirements. Feedback suppressors can be used when higher requirements are required. Actually, the sound reinforcement system is experiencing a backlash
3、 Feedback suppressor method (narrowband notch method)
In high demand situations, such as some live singing places, audio feedback automatic suppression devices are commonly used. This device can automatically track the frequency of feedback points, adjust the Q value bandwidth, automatically eliminate audio feedback while maximizing the protection of sound quality. The principle is to suppress whistling by trapping waves. For example, Sabine's FBX series feedback suppressor is a 9-band narrowband automatic voltage limiting device controlled by a microcomputer, which can effectively distinguish feedback self-excited signals from music signals,
When the system is self-excited, it can react quickly, and set a very narrow digital filter at the feedback frequency point. Its notch depth will also be automatically set. The filter bandwidth is only 1/3 Octave band. Such a narrow notch frequency band will hardly affect the loudness and timbre
4、 Reverse offset method
Anti phase cancellation to prevent self-excitation is common in high-frequency amplification circuits. Two microphones of the same specification can be used in the audio amplification circuit to pick up direct sound and reflected sound, and the reflected sound signal can be phased out before entering the power amplifier through an inverting circuit, effectively preventing self excitation of whistling.
5、 Phase modulation method
The self-excited howling of the amplifying system has a positive feedback loop. If the microphone signal is phased, it will disrupt the phase condition of self-excited howling, thereby preventing the system from self-excited howling. There is data indicating that the stability is best when the phase deviation value is 140 °; Moreover, the higher the modulation frequency, the better the stability of the system. In order to ensure that the processed sound quality does not experience significant distortion, the maximum allowable value of its phase modulation frequency is 4Hz.
Finally, after adjusting various devices, it is strictly prohibited for others to move around, including DJs who are unfamiliar with the performance of the equipment and only know how to turn it on, off, and adjust the volume. Therefore, Aircross Audio has also taken certain measures on this, that is, each Effects unit can be locked. Professionals can press the lock key after debugging.
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