The most important factor affecting sound quality in sound reinforcement systems is acoustic feedback, also known as acoustic feedback. The suppression of it is an important issue that should be paid attention to when designing and using sound reinforcement systems. When using the sound reinforcement system, you will suddenly hear some trembling or continuous whistling. This is because the amplification of the sound reinforcement system is too high, and the sound energy radiated by the speaker is fed back to the microphone beyond a certain limit. The existence of howling phenomenon can make people unable to hear the sound, but the sound reinforcement system can not work normally. It can only return to normal after reducing the amplification of the sound reinforcement system. This situation indicates that the acoustic feedback limits the system. A lot of use. In fact, the sound reinforcement system is distorted before the howling is generated. Severe acoustic feedback makes the amplification of the sound reinforcement system not fully utilized. The sound reinforcement equipment cannot be used at full load. The required sound pressure level cannot be obtained in the audience area, the transmission response is also distorted, and a kind of frequency can be felt at some frequencies. Similar to the reverberation in the room, which reduces the language intelligibility of the audience area and the sound quality of the music.
When the loudspeaker is used, since the sound source and the loudspeaker are in the same area, the sound from the microphone is radiated by the electroacoustic system and then reflected by the indoor surface and fed back to the microphone. This is the acoustic feedback. The simplest acoustic feedback system includes a microphone, a volume adjuster, an amplifier, and a speaker.
The acoustic wave propagation path from the speaker to the microphone constitutes an acoustic feedback amplifier regulator loop. If the sound reinforcement system is a linear amplification path, the sound feedback sound pressure generated by the sound source of Figure 1-17 is applied to the microphone, converted into an electrical signal, and re-transmitted by the speaker after being amplified by the system, and the sound wave radiated by the speaker passes through the path. r feedback to the microphone. In general, the phase relationship between the sound pressure of the sound source acting on the microphone and the sound pressure p produced by the speaker at the microphone can form positive feedback and negative feedback. The basic signal and the feedback signal are in phase, and the amplitude of the oscillation is gradually increased. The frequency at which the self-oscillation occurs is the frequency at which P and po are in phase. Generally, the sound reinforcement system has a wide frequency range of use, and therefore, often both positive feedback and negative feedback exist. If the amplification of the system is large enough, it always contains some frequencies that satisfy the positive feedback condition, and the sound reinforcement system produces howling. The use limit of the actual sound reinforcement system is not determined by the feedback howling point. Before the sound reinforcement system starts to generate self-oscillation, it will cause frequency distortion and regenerative reverberation interference. Therefore, the sound reinforcement system must work away from the vibration point. The degree to which it is necessary to stay away from the self-vibration point is described by stability, which is defined as the number of decibels that reduces the ratio of the output voltage of the path.
Acoustic feedbackAcoustic feedback is a howling phenomenon caused by a part of the sound energy being transmitted to the microphone by means of sound propagation. In the critical state where there is no howling, a ringing ring will appear. At this time, it is generally considered that there is an acoustic feedback phenomenon. The reasons for the acoustic feedback are:
1. The acoustic characteristics of the buildings in the site are poor. For example, there is a resonance point.
2. The placement of the microphone and the speaker is incorrect. For example, the microphone is placed in front of the speaker to form an opposite shot and is close.
3. Excessively increase the gain of the input or output signal in the sound reinforcement system. For example, the input gain and balance of the mixer.
4. The output gain of the device, the input and output gain of the voltage limiter and the electronic crossover.
5. Too much boost the gain of a point in the mixer's tone circuit. For example, IF gain or low frequency gain.
6. Excessively increase some frequency points in the system equalizer. For example, the frequency point in 200 Hz to 3 kHz.
7. Too much increase the reverberation component of the microphone.
8. Simultaneous use of a variety of microphones with different frequency characteristics and different directivity, especially omnidirectional, high sensitivity condenser microphones.
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