一.Why does a microphone howl?
After mic picks up the sound, via consoles, nearby devices, power amps, speakers such devices output the sound. This sound enters the microphone by direct radiation or acoustic reflection which makes the whole sound reinforcement system produce positive feedback and causes self-motivation of acoustic and electrical
signals. Then the loudspeaker whistles, this phenomenon known as microphone sound feedback.
二，The conditions and reasons why howl exits
（1）Uses mics and speakers at the same time;
（2）Speakers’ sound can be send to the mics;
（3）Speakers output large volume signal and mics with higher sensitivity.
Loudspeaker system exits howl the major reason that some frequency response is too powerful. When enhancing mic gain, the powerful frequency response will reach to a specific height. The powerful is the reason causing howl.
三,What damage will microphone howl come with?
In slight case, you can’t adjust the volume, if at higher level volume, it will cause serious howl. It has bad effect on the stage performance. It also appears ringing after turn up the volume, sound exiting mixing quality and damaging the sound quality.
In series case, it may ruin in your speaker or power amplifier if signal is too powerful. Makes performance unable to go on normally, causing enormous economic losses and reputation losses
四,How to control howl
1、 Mics far away from speakers
This method is suitable in a large space environment, such as theatre or large performance venues. The microphone can be hung over the band or stage to pick up the sound. It ensures that the microphone is close enough to the sound source, also keeps the microphone away from the speaker as far as possible.
2.Takes advantage of mics and speakers’ polar pattern
Besides depart mics and speakers in a specific distance to isolate the howl, you also can take advantages of polar pattern of mics and speakers to control the howl.
If the microphone is not located in the radiation area of the speaker sound, the sound of the speaker will not be easily transmitted to the microphone.
If you uses polar pattern mic to control mic direction that ensures it not picking up the speaker sound to avoid howl.
3.Appropriately reduces the volume of microphone
Reducing the volume is actually a bad idea, because this will limit the performance. But sometimes we have to adopt this idea. According to the requirements, engineers should lower the volume if needed. When singers or actors pass through the speakers, engineers should turn down the volume to avoid howl.
Eliminating the howl can also be solved by processing the sound signal. Nowadays, electronic acoustic factories D & R various devices to eliminate the howl, such as frequency shifter, equalizer, howl suppressor, pressure limiter.
5.Acoustic Design of Room Buildings
Coloration in room is the major acoustic reason to cause howl. Indoor concave reflection can cause sound focusing, which leads to excessive internal volume of sound field. When the microphone picks up sound in the focusing area, it is likely that the phenomenon of howl will occur because of the large amount of sound energy feedback.
Therefore, design indoor structures various measures should be taken to avoid the reflection of concave sound as far as possible. If there is an insurmountable uneven surface in the room, it has a negative effect on reducing and isolating the howl.
When choosing interior decoration materials and schemes, the problem of uniform absorption (reflection) of sound absorbing materials and structures to different frequencies should be fully considered in order to reduce the inconsistency of reflection or absorption of sound at different frequencies. This is not only better to frequency response but also good at isolating the coloration.