Preparation for using the audio system and specific methods to solve the problem of microphone self-excitation

Most of the professional sound equipment used in outdoor performances and dance halls is imported equipment, which should be said to be highly reliable. The main problem is that the professional quality of operators is uneven, and there are few units that are truly equipped with qualified tuners. This article explains the key points of operating sound equipment in medium and small dance halls, which can be used as a reference for formulating operating procedures. In addition, the self-excited howling phenomenon caused by microphone feedback in small and medium-sized dance halls is a common problem that causes headaches for users, because frequent howling will spoil the guests' fun, and the sound effect is out of the question. In severe cases, it will cause damage to the equipment. Therefore, the self-excited howling phenomenon is an important problem in the use of dance hall audio, which is described below.

1. The order of turning on and off audio equipment

The power should be turned on from front to back, that is, from the audio source device (CD player, LD player, DVD player, recorder, video recorder), audio processing equipment (compressor, exciter, effector, crossover, equalizer, etc.) to the audio power amplifier to the TV, projector, and monitor. When shutting down, the order is reversed, and the power amplifier should be turned off first. This operation can prevent the impact of turning on and off the device and prevent the power amplifier and speakers from being burned.

2. Preparation before singing - debugging

1. The volume control potentiometer of the power amplifier should be adjusted to the maximum position; the music accompaniment and microphone branch faders on the mixing console should be set to 0dB; the GAIN input gain of each branch on the mixing console should be set to the adjusted position; the total volume fader of the mixing console should be set to the minimum position (lower end); the sound quality compensation knob of the mixing console should be set to the middle position.

2. Test the accompaniment channel, that is, use a CD or LD disc to play songs and music, and turn the mixer up and down to -6dB. At this time, the volume of the singing and accompaniment music is roughly the same as that of normal work; but it should be noted that the volume should be moderate and pleasant, and too loud will easily make people tired and unbearable. The tuner should listen to the effect at different positions in the hall. For example, stereo sound image, music quality, etc. The songs played should be familiar to oneself. The volume (adjust the branch GAIN gain) and the branch sound quality compensation can be adjusted repeatedly until the sound effect is satisfactory. The requirements for the music effect should be powerful and beautiful, the treble should not be harsh, the bass should not be turbid, and the singing should be clear, such as the sibilance of female voices can be clearly heard. But it should not be too heavy. The branch fader is set to 0dB, the total volume fader is set to 0dB, and the branch GAIN input gain knob is adjusted to make the AU meter indicate about 0dB. At this time, the system reaches the rated output power. But during normal operation, the total volume fader is generally adjusted below -6dB or -10dB, which is less than the rated output power.

3. Test the microphone channel. Generally speaking, at least two microphone channels should be prepared. First test the microphone sensitivity and dynamic performance, then add reverberation and accompaniment music to sing. The singing voice after reverberation should be more rounded, full and layered than the original voice, and full of live feeling. Adjust the microphone volume: set the branch fader to 0dB, adjust the microphone volume with the branch GAIN input gain knob, and it is better to have the branch peak level indicator flash occasionally. The total output power is measured by the AU meter.

4. Test and adjust the small band, that is, test the microphone lift and electrical signals of various instruments, and balance the sound proportions according to the style of the music.

5. Adjustment of video images, i.e. the projector and color TV should adjust their brightness, contrast, color saturation and other knobs to make the image clear and colorful. The sound engineer should be able to skillfully use the DVD player and song request machine and be familiar with the disk positions on the song request list. Note that during the formal singing, the D/A button on the DVD player should be pressed. 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 and make up for the frequency response imbalance caused by the reverberation time in the hall; the other important function is to lower a certain frequency band and suppress the howling sound caused by acoustic feedback. The room equalizer should usually maintain the position set during the sound engineering debugging.

2. Compressor. Compressor is also an important device in audio engineering. Its functions are: first, compressing or limiting the dynamic range of the program to prevent overload or distortion, and protecting the amplifier and speaker; another important function is to increase the loudness of the program (which can be clearly felt by hearing).

The adjustment data of the compressor is 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 in squelching. When the input signal drops to the threshold level, it starts to close. The noise gate is generally set between 0PEN and -20dB.

(2) Compression threshold level THRESHOLD: determines the level at which compression starts, usually set between -10 and 0 dB. 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: set to 10ms;

Release time RELEASE: set to 0.3s.

3. Reverberator. Digital reverberators are widely used nowadays. This type of machine has many different reverberation effects built in for selection. The tuner should try out the various reverberation effects of the reverberator used one by one, record the available programs, and call them up at any time using the keyboard in the machine when tuning.

4. Tuning points (mainly operating the mixing console)

1. The karaoke bar tuner works in the control room. When tuning, he should use monitoring speakers and monitoring headphones to monitor the main channel and the return channel respectively. The tuner should be familiar with the relationship between monitoring sound and live sound. Sound quality adjustment depends largely on personal hearing.

2. Use compressors and exciters to increase the loudness and beauty of the 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 manual.

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

4. When the volume is low, pay attention to improving the low and high frequencies; when the volume is high, appropriately improve the mid-frequency to enhance the brightness of the sound.

5. Tuning is mainly based on the singing voice. Before the singing voice appears, gradually lower the accompaniment to highlight the singing voice. The low frequency should be attenuated by 3-5dB, and the high frequency above 7kHz should be attenuated by 3dB. The mid-low frequency around 200Hz can be increased to increase the intensity. 2-4kHz can be increased by 3-6dB to clearly feel the singing voice. For disco or rock music, pay attention to significantly increase the low frequency (40-100Hz) and high frequency (7-20KHz).

6. When increasing the bass, do not turn the compensation knob violently to avoid damaging the amplifier and speakers due to excessive power output. The same applies to the low-frequency adjustment of the equalizer.

7. If there is a feedback howling sound, you should quickly pull down the total volume fader of the audio console to eliminate the howling sound, and then gradually push it up after finding the cause.

8. When the main channel fails and cannot be broadcast, the feedback speaker can be rotated to temporarily replace the main channel so that the performance can continue. The microphone used for the performance should have a backup, which can be used as a substitute when the microphone is silent. The DVD player should also have a backup, which can be used as a substitute when the DVD player fails.

5. Suppression of acoustic feedback (howling)

1. The self-excited howling sound caused by microphone sound feedback is a common phenomenon in karaoke halls and karaoke halls. Due to the existence of sound feedback, the gain of the general sound reinforcement system cannot be very large. The reasons for the sound feedback howling are:

(1) The microphone is too close to the speaker and is pointing towards the speaker;

(2) The reverberation on the mixing console is adjusted too high;

(3) The microphone volume is adjusted too high;

(4) The compressor is not connected;

(5) Acoustic design defects in the hall.

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

(1) Define a rough range for the singer's activity stage, and no howling should occur within this range. In other words, the singer should not be too close to the main speakers, and the main speakers should be symmetrical on both sides of the stage; the singer's position should not make the microphone point directly at the speakers.

(2) The stage of the karaoke hall should be acoustically treated, and the walls and sides should be installed with sound-absorbing materials.

(3) Turn on the compressor, and set the compression ratio to <= 2:1, the action time to 10ms, and the release time to 0.3s.

(4) Do not turn up the reverberation and volume on the mixing console too high.

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

Make a record of the frequency positions of the equalizer first; then, perform a demonstration. Increase the volume (adjust with the mixer master fader) until the system just produces self-excitation, and turn down the adjustment knobs on the equalizer from the low frequency one by one, which can effectively eliminate the frequency points of self-excited howling. According to experience, there is generally only one self-excited resonance frequency (such as 250Hz), and the frequency can be pulled down 3-5dB near this frequency. The other frequency points should still maintain the previously recorded positions.