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In the practice of including sound, the sound engineers have summed up many methods to suppress the sound feedback of the sound system, the architectural acoustics experts have taken some effective measures to eliminate the sound feedback, the electroacoustic technology experts have developed and developed a variety of electroacoustic equipment to reduce the occurrence of the sound feedback phenomenon. After these practical measures are applied, good results have been achieved. Here is a brief introduction Methods of suppressing, reducing and eliminating feedback squealing.

1、 Make the sound from the speaker difficult to transmit to the microphone

The reason why the sound of the speaker is not easy to transmit to the microphone is that the sound from the speaker is transmitted to the microphone. If we find a way to make the sound from the speaker not easy to transmit to the microphone, we can reduce the possibility of the sound feedback phenomenon. We can take the following measures:

(1) Keep the microphone away from the speaker

This method is very difficult to achieve in the dance hall, because the area of the dance hall is generally small, if this is done, it will be limited by the conditions. However, in the theater or large-scale performance place, this method can play a certain role. For example, it can be considered that the microphone can be hoisted above the band or stage to pick up the sound, so as to ensure that the microphone is close to the sound source and keep the microphone as far away from the speaker as possible.

(2) Reduce the volume of the microphone circuit

This will cause the loss of singing or band pick-up volume. The microphone path is limited in shade, and the performance effect is affected. But sometimes the sound engineer has to take the following measures to reduce the volume. According to the performance situation, real-time control the volume. When there is ringing, timely pull down the volume to avoid screaming. When an actor passes through the speaker with a microphone in his hand or with a microphone on his / her belt, he / she should also pay attention to controlling the volume, otherwise it may cause serious sound feedback scream.

If the parameter equalizer of the mixer has Q value adjustment, the volume compensation measures can be taken to properly compensate the microphone sound. The method is: adjust the frequency band of the equalizer to the frequency band of the microphone pickup source sound, lift the frequency band, and stop immediately before squealing.

(3) Using the directional characteristics of speakers and microphones

This one often plays a role. We know that both microphone and volume are directional. If the microphone is not used in the radiation area of the sound of the speaker, the sound of the speaker will not be easily transmitted to the microphone. Similarly, if the speaker is not located in the pickup area of the microphone, it is difficult for the microphone to pick up the sound of the speaker. Therefore, when using a microphone, it is better not to make it face to face with the speaker; or the microphone should try to avoid the playback area of the speaker. In practical application, the angle of the speaker can be adjusted properly, so that the sound radiation area of the speaker does not cover the use area of the microphone as much as possible, and the singer can also be required to avoid the sound playing area of the speaker.

2、 Using sound processing equipment that can suppress feedback

At present, various electroacoustic equipment manufacturers have developed a variety of equipment that can suppress the acoustic feedback

(1) Frequency shifter

This is a kind of equipment that can change the frequency of sound. Its working principle is similar to that of a tone changer. It can increase the sound signal by 5Hz, destroy the conditions for generating sound feedback, and thus suppress the sound feedback. However, the use of the device has certain limitations. It works well in language amplification and has little damage to the sound. However, in singing and musical instruments, it will be very obvious in the sense of tone change. This is because the frequency range of language is between 130 and 350 Hz, and the change of 5Hz frequency alone will not make people feel the obvious pitch increase, but there will be the sense of tone change when vocal music and instrumental music amplify, because the lower limit frequency of vocal music and instrumental music is about 20Hz, and the change of 5Hz tone has been clearly felt by the human ear.

(2) Equalizer and feedback suppressor

These two devices can effectively attenuate the gain (pull feed point) of the feedback frequency point. The reason why the amplifying system generates the acoustic feedback is that the signal of some frequencies in the system is too strong. If these frequencies are attenuated, the live feedback can be suppressed. The difference between the equalizer and the feedback suppressor is that the equalizer needs the sound engineer to manually pull down the feedback point according to the frequency of the scream; while the feedback suppressor does not need the sound engineer to do the work, it can automatically discover the sound feedback frequency and attenuate it. The attenuated frequency band width and attenuation are determined by the machine according to the actual situation, which will hardly have any impact on the music It will make the sound picked up by the microphone better.

(3) Pressure limiter

The voltage limiter is a kind of equipment which can automatically change the output signal and amplify a large amount of (gain) according to the strength of the input signal. When used to suppress the acoustic feedback, the compression ratio can be adjusted to. : 1 (it is the limiter at this time), adjust the threshold value to the feedback critical point. In this way, when the volume is high enough to generate sound feedback, the sound signal strength exceeds the 7I threshold, and the intensity will not continue to increase, so it is impossible to generate sound feedback. However, using a pressure limiter to suppress acoustic feedback will bring about dynamic loss of sound, so this method should be used as less as possible.

3、 Do well in acoustic design of room building

The most important acoustic reason for the squealing of acoustic feedback is the presence of acoustic staining in the room. The poor sound conditions in the room will lead to serious acoustic feedback. As mentioned before, as a room with a cavity, it is inevitable to generate acoustic resonance, which will strengthen some frequencies in the sound. It is natural to generate acoustic feedback scream at this frequency. The main method to eliminate room acoustic staining is to reduce the occurrence of normal resonance as much as possible. In architectural acoustics design, there are many ways to reduce and eliminate the indoor normal resonance. If you are interested, you can read books about architectural acoustics, which will not be covered in this paper.

Concave reflection in the room is also the main cause of acoustic feedback. Concave reflection will cause acoustic focusing phenomenon, and acoustic focusing will cause local volume in the sound field to be too strong. When the microphone is located in the area of acoustic focusing, due to the large amount of sound energy feedback, it is likely to cause acoustic feedback screaming phenomenon. In interior design, various measures should be taken to avoid concave reflection as much as possible. If there is an insurmountable concave in the room, the concave can be made into a diffuse reflection structure, which is composed of concave convex non plane, which has a positive significance for reducing and suppressing acoustic feedback.

When choosing interior decoration materials and schemes, we should fully consider the absorption (reflection) uniformity of sound-absorbing materials and sound-absorbing structures to each frequency, so as to reduce the reflection or non absorption of different frequencies, which is not only beneficial to the improvement of frequency response characteristics of the room, but also can further eliminate the phenomenon of acoustic staining.

4、 Reasonable selection of speakers

Some acoustic characteristics of the speaker play an important role in the acoustic feedback. These characteristics include the directional characteristics and frequency response characteristics of the speaker. Their indicators will determine the severity of the acoustic feedback when the speaker is in use. If the speaker is selected properly, the possibility of the occurrence of the acoustic feedback scream will be greatly reduced. The directional characteristic of the speaker is the ability of sound radiation in all directions of the speaker. For a speaker with a large angle, the sound emitted from the speaker is easy to be sent directly to the microphone, and the possibility of sound feedback squealing is increased; for a speaker with a small angle, the possibility of direct feedback from the speaker to the microphone is reduced, and the sound feedback is not easy to occur. Therefore, on the premise of satisfying the sound coverage of the listening area, it is more advantageous to choose a speaker with a small pointing angle to reduce the occurrence of sound feedback.

5、 Sound selection

The main function of microphone is to pick up sound information. When picking up sound information, you should pick up the sound you want to pick up as much as possible to avoid picking up the sound you don't need to pick up. This is not only beneficial to improve the quality of the picked up sound, but also can reduce the occurrence of sound feedback.

The sensitivity of microphone has the greatest influence on the acoustic feedback squeal. The microphone with high sensitivity (such as condenser microphone) is easy to generate acoustic feedback squeal, while the microphone with low sensitivity (such as dynamic coil microphone) is not easy to generate acoustic feedback squeal. So if conditions permit, try to use a microphone with low sensitivity for pickup. However, sometimes when picking up weak sound and far pickup, you have to use multiple microphones with high sensitivity for pickup. At this time, the sound engineer must be prepared and take various measures, otherwise the sound feedback screaming may have a very adverse impact on the performance.

Directivity is the ability of microphone to receive sound from different angles. It has a large directivity and a wide pickup area, but it is also easy to pick up the sound from the amplifying field, resulting in sound feedback squealing. For example, a pressure area microphone (pzbt) is easy to generate squealing due to a large pickup angle (up to 120 ℃). The directivity of microphone is arranged from a small to a large order, with strong directivity Two way (90 degrees), acute cardioid (105 degrees), supercardioid (115 degrees), cardioid (131 degrees) and non directivity (360 degrees), etc. From the point of view of suppressing acoustic feedback, the smaller the pointing angle, the better. Some microphone instructions indicate that it has the characteristics of off-axis rejection, that is, its pointing angle is sharp and straight, which has good performance of suppressing acoustic feedback. The selection of microphone directivity should not be biased. Because the direction angle is too small, the polar region will be correspondingly reduced. Therefore, when selecting microphone directivity, we should consider comprehensively, and do not consider one thing or the other.

The peak of frequency response characteristic curve of microphone pickup will also cause sound feedback whistling. The frequency response of microphone is that the output signal of microphone changes with the frequency. If the sound picked up by microphone has the same volume at each frequency, the output signal of microphone will not be exactly the same at each frequency due to the uneven frequency characteristic curve of microphone, which will cause the output signal to be too strong at some frequencies, This provides the condition for the sound feedback howling. When selecting microphones, pay attention to their frequency response characteristics, and try not to have obvious peak convex. Many good brands of microphones need to pass the frequency response curve test at the factory and be signed by the responsible engineer, because even the same type of microphone, its frequency response curve is also different.

It is the responsibility of sound workers to fight against the sound feedback. With the continuous development of sound technology, there will be more and more sections to eliminate and suppress the sound feedback. However, theoretically, it is not very realistic for the sound reinforcement system to eliminate the sound feedback phenomenon at all. In my opinion, we should be practical and realistic, not demanding the absolute effect. As long as we can reduce the sound feedback to the extent that it is not enough to affect the performance quality through efforts, we can say that the sound feedback suppression is successful and achieves the expected purpose.