1.Adjust the positionThe easiest way to eliminate the howling is to adjust the position of the microphone and speaker, it will be better when microphone is far away from loudspeaker, and loudspeaker is closer to the audience. As shown in the picture, the microphone should be placed on the back of the loudspeaker radiation direction and try to increase the distance between them.2.Frequency shifterThe frequency shifter is based on changing the frequency of the input signal to decrease the effect of the peak at room, destroying the factor of a sound feedback, eventually restrain the noise.The procedure of frequency shift preventing howling as follows. The frequency bandwidth of the input signal is wH, which is less than the frequency of multiplier w1, and w1 is less than w2. We can see the result in the picture 3,the frequency of input signal m(t) has moved (w2－w1).By this way,we can reach the purpose of frequency shift.3. Phase shiftAt some point in the air, when the voice of the feedback and the original voice expand or compress the air of this point at the same time, we called the feedback sound phase and the original sound phase are in-phase, the sound of this point is increased. On the contrary, If a sound compresses the air of this point and another sound expands it at the same time, we called the two sounds are phase reversal, and the sound of this point is weakened.Phase shift is to move the peak of in-phase to the direction of the reverse phase. It can be realized by installing phase shift devices in the loudspeaker, as shown in the picture 4and 5. The phase of the feedback signal is obtained from the phase of the main channel, damage the self-excitation condition of the system.4.EqualizerThe working principle of the equalizer is to remove the frequency of howling l, so as to achieve the purpose of preventing howling. Equalizer can be divided into graphic equalizer and parametric equalizer.Graphic equalizer:According to the certain octave, the graphic equalizer can divide the voice band into some small paragraphs from 20Hz to 20000Hz,15 and 31 paragraphs are common.The 15-paragraph graphic equalizer selects the center frequency point according to the 2/3 octave relationship, and the 31-paragraph graphicequalizer selects the center frequency point based on the 1/3 octave relationship. The equalizer's frequency point is completely fixed, and the sound pressure of the frequency point can be multiplied or divided 6～7 times.The correct step of using the equalizer to preventing howling as follows. First, putting the push rod at each frequency point on the central point, increasing the system volume slowly until the first noise point happening. Second, judging the frequency of howl point by your ear, finding the push rod which controls the howl point and pull down it quickly. Then, the system gain will be decreased and the whistle will be disappeared. Behind the operation and so on.Parametric equalizer: the parameter can be fine adjusted,these parameter contents include frequency band and frequency point (sweep type, can be arbitrary choice), gain and the quality factor Q.5. Feedback suppressorThe working principle of feedback suppressor as follows. First, detecting the input audio signal, if there is no acoustic feedback, the audio signal goes through directly; if there is some acoustic feedback, setting the digital filter (wave trap) to prevent the howling.The acoustic feedback signal is gradually established, the amplitude increasesfrom small to large in the process of building. As shown in figure (a) and (b), the acoustic feedback signal is a sine wave of single frequency which has the same frequency in the process of establishing. The amplitude of the acoustic feedback signal will not grow continuously after it established a certain time, and the obvious amplitude limiting maybe occur(the clipping phenomenon), as shown in figure (c). In the spectrum we can see a wealth of harmonic components.We can see the acoustic feedback signal as a sinusoidal signal which has single frequency, and the amplitude increases with times.The work process is as follows. First,amplifying the input signal and converting it to digital signal, using the detector to detect the feedback signal. Second, finding the feedback signal, the CPU transmitsthe information to set the frequency of the digital signal processor, finding the frequency point in the digital filter and cut it down, the decrement is about -40dB. The filter bandwidth can be adjusted from 1/60octave to 1/5octave.。