Have you ever had this experience when using microphones in meetings or live performances? When holding the microphone at a certain angle, there will always be a sharp and continuous noise in the speaker.
In fact, this phenomenon is called “howling”. For performance, the appearance of the “howling” phenomenon is definitely not a good thing in general. So, how did this phenomenon occur, and how to prevent it?
The sound system whistle refers to the positive sound feedback generated between the pickup and the speaker. This positive feedback goes through an infinite loop to form a phenomenon of self-excitation and feedback. The microphone pickup distance is closely related to the live acoustic environment and the isolation between the microphone and the speakers. These are combined together and we call the system’s “sound gain”. Under reasonable system configuration, equipment layout and acoustic environment conditions, the sound transmission gain will increase.
A. Definition and hazards of “howling”
“Howling” is actually a kind of acoustic feedback oscillation that appears in the sound reinforcement system, and it is a common phenomenon in the operation of audio equipment.
Live speeches broadcast live nationwide, as small as performances in individual band rehearsal rooms, “howling” can be described as ubiquitous.
Once howling occurs, it will cause the microphone volume to be unable to increase. If the volume is turned up and the howling is very serious, it 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. The sound has a sense of reverberation, which destroys the sound quality, and in particular, causes the speaker or power amplifier to burn out 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 such a strong high-frequency signal, it will cause the voice coil to burn. In addition, in the howling state, if the output of the power amplifier is overloaded, it may also be burned.
B. The mechanism of “howling”
“Howling” is naturally inseparable from modern electronic audio equipment.
So, there are probably three prerequisites for howling:
①Use qualified microphones and speakers;
②Use the energized microphone and speakers at the same time;
③The sound from the speaker can be picked up by the microphone.
Satisfying the above three points is not enough. The generation of “howling” also needs to meet the corresponding conditions in terms of amplitude and phase.
First of all, in terms of amplitude, the sound system needs to satisfy the condition that the feedback signal is greater than the input signal. The simple explanation is that the sound emitted by the speaker is greater than the sound picked up by the microphone from your vocal cords. This is generally satisfied, otherwise there is no need to use a microphone.
Then, in terms of phase, it is necessary to satisfy the condition that the input signal and the feedback signal have the same phase. In other words, the sound you make with the vocal cords into the microphone is basically the same as the sound from the speaker when picked up by the microphone.
You think that the sound you make with the vocal cords will be amplified from the speakers obediently after entering the sound system. Who knows that certain components of your sound emitted by the speakers also have their own ideas. It feels that it is not loud enough, and it is just right at this time. The position of your microphone allows it to take advantage of the loopholes and zoom in again…
Do you think this is over? Too naive. The second-amplified sound will also feel that I am not loud enough, so I take advantage of the loopholes to enter the microphone and amplify it again, and then the three-time amplified sound will repeat the same trick… This is a bit like a Russian matryoshka.
It is conceivable that the original sound will become completely unrecognizable and evolve into a breathtaking “howling”.
C.Methods to prevent “howling”
In fact, it is almost impossible to completely prevent the occurrence of “howling”, but we still have many ways to reduce the occurrence of “howling” as much as possible.
As we said before, there are many conditions for the occurrence of “howling”. Even if only one of these conditions is not met, then the phenomenon of “howling” will not appear.
The few points that we can make a fuss about are still in the following conditions.
① The speaker has a certain frequency peak in the sound characteristics, the microphone will keep receiving this peak, and the sound will keep playing back this peak. It will be played back multiple times a second, and this peak will be infinitely amplified. Until the equipment is damaged.
② The input signal is too large, causing the sound to be distorted and howling. In the final analysis, it is the first reason.
D. How to solve this problem?
①You need to use good microphones and speakers. Because good speakers and microphones are tuned very smoothly in terms of sound characteristics. This smoothness does not mean that his curve is straight. But the waveform is excessively smooth and the slope is small. In this way, the makeup filter will not be generated, and there will be no narrow band peaks, and the howling will naturally occur rarely.
②The microphone should not be too close to the speaker or pointed at the speaker. In this way, even if there are certain frequencies of howling points, it can be effectively avoided.
Frequency shifter can make the sound from the speaker enter the microphone again, not equal to the original frequency. The phase shifter can make the sound from the loudspeaker enter the microphone again, and it is inconsistent with the original phase.
Now some advanced audio systems also come with some feedback suppressors. This equipment can automatically find the “howling” frequency point and automatically generate a narrow-band notch filter. So as to reduce the narrow band gain centered on this frequency, to achieve the purpose of destroying the conditions for “howling”.