(1) Do a good job in acoustic design, try to avoid acoustic defects. Audio equipment can only beautify the sound by improving the loudness, frequency response characteristics and adding reverberation. The acoustic defects of buildings cannot be overcome by circuit design. Acoustic staining in the room is the most important acoustic cause of the acoustic feedback whistling. Poor sound conditions in the room can lead to serious acoustic feedback. The main way to eliminate room acoustic staining is to minimize the occurrence of normal resonance. In addition, the presence of concave reflection in the room is also the main cause of acoustic feedback. Concave reflection causes the occurrence of acoustic focusing, while acoustic focusing causes the local volume in the sound field to be too strong. When the microphone is picked up in the area where the sound is focused, the sound is due to sound. The amount of energy feedback is very large, and it is highly probable that acoustic feedback whistling occurs. Therefore, in interior design and decoration, acoustic design must be carried out to reduce acoustic defects. For example, the acoustic ratio of length, width and height should be satisfied as much as possible, the sound absorbing material should be added appropriately, the sound field should be kept uniform, and the concave reflection should be avoided as much as possible, thereby improving the sound transmission gain of the system. The questions about acoustic design are not detailed here. The following discussion focuses on issues that should be noted in work practice.
(2) Reasonable arrangement of the sound reproduction system is an effective way to reduce the acoustic feedback. The indoor sound gain is related to the following factors: the distance between the speaker and the listening area, the distance between the sound source and the microphone, the total indoor surface area, and the indoor average sound absorption. Coefficient, indoor transmission response, directivity of the speaker, microphone directivity, angle between the microphone pointing and the speaker axis. Pay attention to the following points in the work: a. Reasonably use the microphone, pick up the sounder as close as possible to the sound source, shorten the distance between the sound source and the microphone, and improve the sound pressure level of the audience area. Although this does not affect the feedback amount of the system, since the sound pressure level of the audience area is increased, it is equivalent to the improvement of the system power gain. b. Use directional speakers and directional microphones, and arrange them reasonably so that the sound from the speakers is placed in the insensitive direction of the microphone, or the microphone is insensitive to the speaker, or the directional microphone is placed behind the speaker. At the same time, a unidirectional or bidirectional microphone with a relatively flat frequency characteristic should be selected. Practice has proved that for indoor sound reinforcement, the use of a heart-shaped or figure-shaped microphone can improve the stability of the system by about 5 dB compared with the omnidirectional microphone. For outdoor sound reinforcement, the use of a heart-shaped directional microphone and an omnidirectional microphone is compared. , can improve the stability of the sound reinforcement system by about 6dB. However, it should be noted that the microphone pointing point characteristic angle is too small, which will cause the pickup area to be correspondingly reduced. Therefore, when selecting the directionality of the microphone, it is necessary to consider it comprehensively. c. When using the same type of microphone on the same occasion, the smaller the number of simultaneous use, the better, so that it is easy to control the number of feedback points. d. Reasonable control of the reverberation ratio of the microphone not only improves the clarity of the sound, but also reduces the chance of feedback. e. Reasonably use the microphone input gain of the mixer and the pitch gain (3dB is 1 times) in each channel of the mixer. Do not raise a certain frequency too much, otherwise the balance of the tone will be destroyed, resulting in acoustic feedback. f. Reduce the volume of the microphone path. Although this will bring the loss of the singing or the volume of the band, the volume of the microphone channel is limited, and the effect of the performance is affected. However, sometimes we have to take the next step of reducing the volume and control the volume in real time according to the performance. For example, when ringing occurs, the volume should be pulled down in time to avoid howling. Actors should also pay attention to controlling the volume when holding or wearing the microphone before passing through the speaker, otherwise it may cause severe acoustic feedback whistling. (3) Using the frequency shift method to reduce the acoustic feedback The basic idea of ​​using the frequency shift method to reduce the acoustic feedback is to use the method of offset frequency to destroy the in-phase condition of the feedback sound and the original signal, and suppress the self-oscillation of the system. In the sound reinforcement system, the frequency shifter is inserted so that the output signal of the speaker is offset by an amount relative to all frequencies of the microphone signal. This method can effectively suppress the acoustic feedback and reduce the frequency distortion and reverberation interference. When the sound reinforcement system has no frequency shift, the loop gain maximum value exceeds 0 dB and the system will generate self-excitation. Therefore, the maximum allowable value of the system gain depends on the maximum value of the transmission response, and the maximum value corresponding to the loop gain must be lower than oDB, otherwise the system is unstable. After inserting the frequency shifter, the stability of the system is no longer dependent on the maximum value of the loop gain, but is determined by the average gain of the transmission response. As long as the average gain is less than 0 dB, the system is stable. Therefore, the frequency shifting method allows the increased gain of the sound reinforcement system to be equal to the difference between the maximum gain and the average gain on the frequency response. The optimum amount of frequency shift is equal to the average distance between the peaks and adjacent valleys on the transmission response, because the excess energy generated by the gain peaks at this time is rapidly absorbed by the crystal at the valley. . Practice has proved that the optimal frequency shift is related to the reverberation time Ts0 of the hall, which is about 1/Tc0. A larger frequency shift can also increase the gain of the sound reinforcement system, but when the frequency shift exceeds 7 Hz, the sound quality will be affected. The use of frequency shifters has certain limitations. It works well in language sound reinforcement and has little damage to sound. However, it is obviously used in singing and musical instruments. This is because the frequency range of the language is between 130 and 35011z. Only the change of 5Hz frequency will not make people have a clear tone change feeling, but there will be a feeling of change when vocal and instrumental sound reinforcement, because vocal music and instrumental music The lower limit frequency is about 20 Hz, and the 5 Hz tone change can be clearly felt. (4) Using feedback suppressors In the early 1990s, engineers at the Sabine Music Center in north-central Florida began developing a new automatic feedback suppressor. This new device uses a digital filter technology with a narrower bandwidth (1/10 octave), adjustable center frequency, and very accurate, adjustable absorption depth. Using DSP (Digital Signal Processing) technology, it can automatically measure and accurately set to the feedback frequency within 1s, without any professional operation, the effect is obvious. The automatic feedback suppressor has the following advantages: a. The bandwidth of the absorption filter is not only extremely narrow, but also can be changed according to the content of the program source, which greatly reduces the loss of the spectrum component of the useful signal, and it has more than any other feedback suppression method. Small signal distortion. b. The center frequency of the absorption filter can be automatically adjusted to the feedback frequency point accurately, which improves the feedback suppression effect. c. The absorption depth of the absorption filter can be set according to the need, the maximum absorption depth can reach -50dB.d. It can automatically and accurately measure the feedback frequency point within 1s, and can automatically lock the filter to the feedback frequency point. on. e. The filter locked at the fixed feedback frequency point is called a fixed filter, and the filter that continuously tracks the backlash feedback frequency point continuously is called a dynamic filter. The number of two filters can be set by the site. The feedback suppressor can increase the sound gain of any sound reinforcement system by 6~9dB, which means that the power output of the power amplifier can be increased by 2~3 times, or the sound pressure level of the speaker output can be increased by 6~9dB. Automatic feedback suppression The device has made significant progress in effectively suppressing the acoustic feedback of the sound reinforcement system and minimizing the timbre loss. However, how to properly connect in the sound reinforcement system can achieve the best effect? ​​The feedback suppressor is to deal with the amplitude and frequency characteristics. To work, it is not advisable to use it directly in the main output channel of the sound reinforcement system, otherwise it will inevitably destroy the frequency characteristics of the whole system. The feedback suppressor should be used for the microphone. The reasonable connection method is to insert it separately into the plug-in population of the output channel of a grouping channel of the mixer, and use the group as a dedicated channel of the microphone, and then use the channel output. The hybrid operator groups the terminal signals and the human main output channel. This clarifies the relevance of the acoustic feedback suppressor and avoids the effects of other sound source signals and the overall frequency characteristics of the system.
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