Most of the professional stereos used in outdoor performances and dance halls are imported equipment, which should be said to be highly reliable. The main problem is that the professional quality of operators is not complete, and there are few units that really have qualified mixers. In this paper, the operating points of audio equipment in small and medium-sized dance halls are explained, which can be used as a reference for formulating operating procedures. In addition, the self-excited howling phenomenon caused by microphone sound feedback in small and medium-sized dance halls is a common headache for users, because the frequent howling will disappoint guests, leaving no sound effect to speak of, and even damage the equipment. Therefore, the phenomenon of self-excited howling is an important problem in the use of music and dance hall acoustics, which is described separately below.

1、 On/off sequence of audio equipment

Start the machine from front to back, that is, from sound source equipment (CD player, LD player, DVD player, recorder, video recorder), audio processing equipment (voltage limiter, exciter, effector, frequency divider, equalizer, etc.) to audio power amplifier to TV, projector, and monitor. Turn off the power amplifier in reverse order. This operation can prevent the impact of power on and off on the equipment, and prevent the power amplifier and loudspeaker from burning.

2、 Preparation before singing - debugging

1. The volume control potentiometer of the power amplifier shall be set to the maximum position; The accompanying music and microphone shunting pusher on the mixer shall be set at 0dB; The input gain of each shunt GAIN on the console is placed at the adjusted position; The total volume pusher of the mixer shall be placed at the minimum position (lower end); The sound quality compensation knob of the mixer is placed in the middle position.

2. Test the accompaniment channel, that is to say, use CD or LD to play song music, and turn the tuning platform into a place near - 6dB. At this time, the singing voice and accompaniment music are roughly the volume of normal work; However, it should be noted that the volume should be moderate and pleasant. Too loud a sound can easily make people tired and unbearable. The tuner should go to different places in the hall to listen to the effect. Such as stereo audio and video, music quality, etc. The played tracks should be familiar ones, and the volume can be adjusted repeatedly (adjust the shunt GAIN gain) and the shunt sound quality compensation until the sound effect is satisfactory. The requirements for the music effect should be strong and aesthetic, the high pitched voice should not be harsh, the low pitched voice should not be muddy, and the singing voice should be clear, such as the female voice's dental voice is clear and audible, but not too heavy. Set the shunt pusher to 0dB and the total volume pusher to 0dB. Adjust the shunt GAIN input gain knob to make the AU meter indicate 0dB or so. At this time, the system reaches the rated output power. However, in normal operation, the total volume fader is generally adjusted below - 6dB or - 10dB, which is less than the rated output power.

3. Test microphone channel. Generally speaking, at least two microphone channels should be prepared. First test the sensitivity and dynamic performance of the microphone, and then add reverberation and accompaniment music to sing. After reverberation, the singing sound should be more mellow, plump, layered and live than the original song. Adjustment of microphone volume: set the shunt pusher to 0dB, adjust the shunt GAIN input gain button of microphone volume, and it is better that the shunt peak level indicator light flashes occasionally. The measurement of total output power is indicated by the AU meter.

4. Adjust the sound test for small bands, that is, perform sound test for microphone lift and electrical signals of various instruments, and balance the sound proportion according to the music style.

5. Adjustment of video images, that is, projectors and color TVs should make their images clear and colorful by adjusting their brightness, contrast, color saturation and other knobs. The sound engineer shall be able to skillfully use the DVD player and the music recorder, and be familiar with the dials on the music order list. Note that you should press the D/A button on the DVD player during the official singing. Eliminate the original singing voice.

3、 Adjustment of audio processing equipment

1. Room equalizer. The room equalizer has two functions: one is to adjust the sound quality to compensate for the frequency response imbalance caused by the reverberation time in the hall; Another important role is to reduce a certain frequency band and suppress the howling caused by acoustic feedback. The room equalizer shall be kept at the position set during audio engineering commissioning.

2. Pressure limiter. Medium voltage limiter is also an important equipment in audio engineering. Its functions include: first, compressing or limiting the dynamic range of programs, preventing overload or distortion, and protecting power amplifiers and loudspeakers; Another important role is to improve the loudness of the program (which can be clearly felt by hearing).

The adjustment data of the pressure limiter are as follows:

(1) Noise gate GATE: When the indicator light is on, the noise gate is closed and the sound is low. It plays a role of noise suppression. When the input signal drops to the threshold level, it starts to close, and the noise gate is generally set between 0PEN and - 20dB.

(2) Compression threshold level THRESHOLD: determines the level at which compression starts. It is generally set to - 10 to 0dB. When compression starts, the gain decreases, and the GAIN REDUOTION (dB) indicator starts to light up.

Compression ratio RATT0: set to 2:1;

Action time ATTACK: 10ms;

RELEASE: set to O.3s.

3. Reverber. At present, digital reverberators are widely used. There are many different reverberation effects solidified inside these machines for selection. The tuner shall audition each reverberation effect of the used reverberators one by one, record the available programs, and use the keyboard in the machine at any time during tuning.

4、 Key points of tuning (mainly operating the console)

1. The concert hall tuner works in the control room. When tuning, he/she should use monitoring and monitoring speakers and headphones to monitor the main channel and the return channel respectively. The tuner shall be familiar with the relationship between the monitoring sound and the on-site sound, and the adjustment of the sound quality largely depends on the personal hearing.

2. Use pressure limiter and exciter to increase the loudness and beauty of sound. The adjustment of the exciter mainly depends on hearing, and the sound should be adjusted to be full and pleasant according to the equipment instructions.

3. Use reverberation to beautify singing. For non professional singers, the reverberation should be increased appropriately to cover up the noise and the defects in vocal production.

4. Pay attention to raising low frequency and high frequency when the volume is low; When the volume is high, increase the mid-range appropriately to enhance the brightness of the sound.

5. Tuning is mainly about singing. When the singing voice appears, the accompaniment is gradually lowered to highlight the singing voice. The low frequency shall be attenuated by 3-5 dB, and the high frequency shall be attenuated by 3dB above 7kHz. The increase near 200Hz in the middle and low frequency can be strengthened, and the increase of 3-6dB in 2-4kHz can obviously feel the singing sound bright. For disco or rock music, we should pay attention to raising the low frequency (40-100Hz) and high frequency (7-20KHz).

6. Do not turn the compensation button violently when lifting the bass to avoid damaging the amplifier and loudspeaker due to excessive power output. The same is required for the low-frequency adjustment of the equalizer.

7. In case of audible feedback whistling, pull down the total volume pusher of the loudspeaker rapidly to remove the whistling, find out the cause and then push it up gradually.

8. When the main channel fails to play, the rotation angle of the echo speaker can temporarily replace the main channel, so that the singing can continue. Microphones used for singing should be backed up, which can be replaced when the microphone is silent. The DVD player should also have a backup, which can be replaced when the DVD player fails.

5、 Suppression of acoustic feedback (howling)

1. Self excited howling caused by microphone acoustic feedback is a common phenomenon in concert halls and karaoke halls. Due to the presence of acoustic feedback, the gain of public address systems cannot be very large. The causes of acoustic feedback howling are:

(1) The microphone is too close to the speaker, and the microphone points to the speaker in the positive direction;

(2) Excessive reverberation adjustment on the mixing console;

(3) The microphone volume is adjusted too much;

(4) The pressure limiter is not connected;

(5) Acoustic design defects in the hall.

2. The following measures can be taken for the above reasons:

(1) Define a general scope for the singer's stage, within which no howling should occur. In other words, the singer should not be too close to the main speaker, which should be called on both sides of the stage; The standing position of the singer should not make the microphone point to the speaker.

(2) The stage of the singing hall shall be acoustically treated, and the walls and both sides shall be equipped with sound absorbing materials.

(3) When the pressure limiter is switched on, its compression ratio should be set to<=2:1, the action time should be 10ms, and the release time should be 0.3s

(4) The reverberation adjustment and volume on the mixing console should not be turned too high.

(5) If the above measures do not work, the equalizer can be adjusted to attenuate the frequency that is prone to howling. The specific operation method is as follows:

Record the positions of each frequency point of the equalizer; Then, demonstration performance. Increase the volume (adjust with the main fader of the mixer), and reach the position where the system just generates self-excited sound. Turn down the adjusting buttons on the equalizer one by one from the low frequency to effectively eliminate self-excited howling frequency points. According to experience, there is generally only one self-excited resonant frequency (such as 250Hz), which can be pulled down 3-5 dB near this frequency, and the remaining frequency points should still remain the original recorded positions.