Why does a microphone sound like a speaker?

It is common in various Sound Amplifier systems. I remember the time when I was in my undergraduate course. Many public courses, such as advanced mathematics, would have several major classes in a large trapezoid classroom. At this time, the teacher would have to give lectures with MIC loudspeakers,
 

 
Sometimes there is a scream, and the sound is very loud (so is Xie Xun's lion roar ). In order to avoid this phenomenon, it is often solved by hardware or usage.
 

 

In practice, there are many kinds of ECHO. The most common "scream" is also a kind. But what we are looking for is the kind of echo that can control the volume, time, pitch, and even tone.
 

 
"Scream" is uncontrollable. It is a long journey from an uncontrollable call to a controllable return.
First, start with "scream. When analyzing the cause of "scream", everyone will have this experience. Generally, "scream" appears when it is distorted.
 

 
(Sometimes it may also occur when a delayed reverb is used .) This is the first cause of overload of the input signal. We also have this experience: Sometimes, as long as you stand a little farther away from the speaker, "scream" will be gone.
 

 
The distance from the speaker is too close, which is the second reason. The sound of "scream" in different positions is different. The speaker and the string are not in the correct angle, which is the third reason.
 

 
Different sounds have different "cuts", that is, the frequency of the sound is incorrect. This is the fourth reason.
 

 

I understand the four reasons for "scream,Overload volume, distance, angle, frequency. We need to change the "nickname" to "authorize" from them.
 

The sound system for microphone sound pickup has the possibility of giving feedback to the call. The microphone is very harmful and mainly manifested in the following aspects:
1. The power amplifier will generate a large power output when the self-excitation occurs, which may be out of the limit of the Sound Amplifier device, and burn out the power amplifier and sound generator.
2. When the feedback coefficient is close to 1, the overlapping of the delayed sound field and the direct sound field will narrow the sound field of the sound field to the sound field of the sound source.
3. The delayed feedback of the sound field of the speaker will lead to a series of delayed echoes in the entire system. This echo will aggravate the comb filter effect and generate a significantly distorted reverb tail-Rigid-ring distortion.
4. The sound pressure output during the call is great, seriously affecting the atmosphere of various activities.
To eliminate feedback, we need to start with the necessary conditions for generating feedback. As long as one of the conditions can be broken, we can achieve our goal.
 

 

I. Adjust the distance
One of the most effective ways to avoid calling and increase the sound volume is to pick up the microphone as close as possible to the sound source, while the microphone should be non-directional. It is clear here that the sound pickup attenuation of long-distance sound sources is very small,
 

 
Adjusting the distance does not have much effect on increasing the sound volume and preventing the call. Whether the sound amplifier system is easy to scream is not directly related to the sensitivity of the microphone. However, high-sensitivity microphones are sharp and easy to scream. Shorten the distance between the voice equipment and the audience,
 

 
In fact, it can improve the sound reinforcement. The total gain of the system can be appropriately reduced. If the microphone is supplemented with a near-field speaker with a direction and width, the microphone will be slightly farther away from the speaker to avoid screaming.
For the direct feedback sound field of the speaker, the farther the microphone is from the speaker, the closer the speaker is from the audience, the better. The microphone should be placed on the back of the speaker radiation direction. If the microphone may be held around, the speaker should be placed in a place where the microphone cannot be very close.
 

 

Ii. Frequency Modulation (broadband ripple)
The frequency curves of microphone sound pickup and sending devices are not ideal straight lines (especially for sound-releasing devices with poor quality), and acoustic resonance of the sound field in the hall causes great fluctuations in the frequency response. The frequency balancer can be used to compensate the sound reinforcement curve,
 

The system's frequency response is adjusted to an approximate straight line, so that the gain of each band is basically the same, and the sound transfer gain of the system is improved.
A parameter balancer with more than 21 segments should be used. A parameter balancer should be configured where the requirements are relatively high. When the requirements are higher, feedback suppression can be used. In fact, in the case of feedback self-excitation, the frequency of the Sound Reinforcement System is fixed at a certain point,
 

 
Therefore, as long as a trap filter with a narrow band is used to remove this frequency, the system call can be suppressed.
 

 

Iii. Feedback suppression method (narrowband trap method)
In scenarios with high requirements, for example, in some places where on-site singing is performed, the automatic Suppression Device of Audio Frequency Feedback is generally used. This device can automatically track the frequency of feedback points and adjust the bandwidth of Q, the sound quality is automatically eliminated and maximized.
 

 
The principle is to suppress howling through traps. For example, Sabine's fbx series feedback eliminator is a nine-section narrowband automatic pressure limiting device controlled by a microcomputer. It can better distinguish self-excitation signal from music signal, when the system is self-excited,
 

 
Quickly respond and set a very narrow digital filter on the feedback frequency. The trap depth is also automatically set, and the filter bandwidth is only 1/3 times the frequency range, it has almost no effect on the sound and tone.
 

 

Iv. Anti-offset Elimination Method
Reverse offset protection is common in high-frequency amplification circuits.
Two microphones of the same specifications can be used in the audio amplification circuit to pick up the direct sound and the reflected sound respectively. Through the reverse circuit, the reflected sound signal is offset before entering the power amplifier, it can effectively prevent the call from being excited.
 

 

V. Phase Adjustment Method
The feedback loop of the Sound Amplifier System is positive feedback. If the microphone signal is adjusted to phase, the phase conditions of the Self-excitation will be destroyed to prevent the system from self-excitation.
 

 
Data shows that the stability is the best when the phase deviation is 140 °. Moreover, the higher the modulation frequency, the better the system stability. In order to ensure that the processed audio quality does not have much distortion, the maximum phase adjustment frequency is 4Hz.
Finally, when the various devices are adjusted, they cannot be moved by others, including some dj who are not familiar with the performance of the equipment and only know how to turn on, shut down, and adjust the volume.