Howling (that is, the tail phenomenon of the microphone sound at the critical point of acoustic howling) is an abnormal phenomenon that often occurs in the sound reinforcement system. In the sound reinforcement system, when the microphone is used to pick up the sound, due to the sound pickup of the microphone It is impossible to take sound isolation measures between the area and the sound box of the speaker. Excessive sound at certain frequencies causes self-excited oscillation of the acoustic and electrical signals, resulting in howling.
Causes and hazards of howling
Simultaneous use of microphone and speakers
The sound from the speaker can be transmitted to the microphone through the space
The sound energy emitted by the speaker is large enough, and the pickup sensitivity of the microphone is high enough.
Once the howling occurs, the weak one will cause the microphone volume to be unable to be increased, and the howling is very serious after it is turned up, which will have a bad impact on the live performance. When the critical point of howling is reached, the sound of the microphone will ring when the microphone is turned on loudly, and the sound will have a sense of reverberation, which will damage the sound quality. In the worst case, the speaker or power amplifier will be burned due to excessive signal.
It is not uncommon for the tweeter of the speaker to be burnt due to howling, because in the howling state, a strong signal will cause the power amplifier to appear clipping (topping) distortion and generate a large number of high-frequency harmonics. If the tweeter cannot withstand this A strong high-frequency signal will cause the voice coil to burn out. In addition, in the howling state, if the output of the power amplifier is overloaded, it may also be burned.
Ways to suppress howling
In the practice of sound reinforcement, sound engineers have summarized many methods to suppress howling (crazy screaming) in the sound system. Sound experts have taken some effective measures to eliminate howling (crazy screaming). Acoustic experts have developed and developed a variety of electro-acoustic equipment to reduce the occurrence of acoustic howling (crazy barking). After these practical measures are applied, good results have been achieved. The following briefly introduces suppression, reduction and elimination Howling method.
01 Make the sound of the speaker not easy to pass into the microphone
1) Keep the microphone away from the speakers
This method is difficult to implement in song and dance halls, because the area of song and dance halls is generally small. But in theaters or large-scale performance venues, this method will have a certain effect. For example, consider hanging the microphone above the band or stage to pick up the sound. This can ensure that the microphone is close enough to the sound source, and it can also be as far away from the speaker as possible.
2) Decrease the volume of the microphone
If the microphone volume is too loud, the probability of ringing will increase, and the volume must be reduced in time to avoid howling. In addition, if you want to hold or wear a microphone through the speaker, you must also pay attention to controlling the volume, otherwise it will cause severe howling due to too close.
3) Reasonable use of the directional characteristics of speakers and microphones
Both the microphone and speakers have directivity. If the location of the microphone is not in the sound radiation area of the speaker, the sound of the speaker will not be easily transmitted to the microphone. Similarly, if the speaker is not in the pickup area of the microphone, it will be difficult for the microphone to pick up the sound of the speaker. Therefore, by appropriately adjusting the angle of the speaker, when using the microphone, avoid the sound area of the speaker, or make the sound radiation area of the speaker not overlap with the use area of the microphone.
02 Use electro-acoustic equipment to suppress howling
1) Frequency shifter
Changing the sound frequency through a frequency shifter can destroy the conditions for howling. However, this device has certain limitations and is not suitable for use in singing and musical instruments. It is enough to increase the sound signal by 5Hz, and it is very effective when used in speech sound reinforcement, because the frequency range of the language is between 130 and 350Hz, and the change of 5Hz frequency will not make people have a clear sense of pitch increase; while vocal and The lower limit frequency of instrumental music is about 20 Hz, and the pitch change of 5 Hz has a clear sense of tone shift when people hear it.
2) Equalizer and feedback suppressor
The reason for howling is that the signal of some frequencies in the system is too strong. Weakening this part of the signal can suppress the howling. Both the equalizer and the feedback suppressor can effectively weaken the gain of the feedback frequency point (pull the feed point). Suppress howling. The difference is that the equalizer requires the sound engineer to manually pull down the feed point according to the frequency of howling. The feedback suppressor can automatically find the howling frequency and attenuate it, hardly any influence on the music, but also make the sound picked up by the microphone become nice.
The pressure limiter can automatically change the output signal amplification (gain) according to the strength of the input signal, so when the volume is so high that howling is about to occur, the pressure limiter will cause the sound signal strength to exceed the threshold and no howling will be generated. However, the use of a compressor to suppress howling will bring about a loss of sound dynamics.
03 Do a good job in the architectural acoustic design of the room
Sound staining in the room is one of the main causes of howling. The sound condition of the room is not good. For example, the acoustic resonance of the room enhances certain frequencies in the sound, which will lead to howling. To eliminate room sound staining, it is necessary to reduce the occurrence of resonance as much as possible. In addition, the sound focusing caused by the reflection of the concave surface in the room causes the local volume in the sound field to be too strong. When the microphone is in the sound focusing area to pick up the sound, the feedback amount of sound energy will be large, and howling will also be generated.
In the interior design, the concave surface should be avoided as much as possible or the concave surface should be made into an uneven diffuse reflection structure. At the same time, sound-absorbing materials and sound-absorbing structures are used, which not only improves the frequency response characteristics of the room, but also effectively suppresses howling.
The acoustic characteristics such as the directional characteristics and frequency response characteristics of the speakers also have an impact on the generation of howling. For example, the sound from a speaker with a large pointing angle is easily sent directly to the microphone, causing howling. Speakers with prominent frequency peaks may also induce howling. Therefore, you should try to choose a speaker with a small pointing angle, a flat frequency response curve and no obvious peaks.
05 Reasonable use of microphones
The main function of the microphone is to pick up sound information. When picking up the sound information, avoid picking up the sound that does not need to be picked up and try to pick up the sound you want to pick up as much as possible. It can not only bring better on-site effects, but also reduce howling. happened. Generally speaking, the higher the sensitivity of the microphone, the easier it is to produce howling, so you should try to use a microphone with low sensitivity.
The directional characteristics and frequency response characteristics of the microphone can also cause howling. To suppress howling, a microphone with a flat directional angle and a flat frequency characteristic curve should be used.