The Reasons for Acoustic Feedback

- Sep 19, 2018-

Acoustic feedback howling is an abnormal phenomenon that often occurs in a sound reinforcement system (P.A.), and is an acoustic problem unique to a sound reinforcement system.

Acoustic feedback is a howling phenomenon caused by a part of sound energy transmitted to the microphone through sound propagation. In the critical state where there is no howling, a ringing tone will occur. At this time, it is generally considered that there is an acoustic feedback phenomenon, and the volume is attenuated by 6 dB. After that, it is defined as the phenomenon of silent feedback.

The sound feedback system of the sound reinforcement system must have three conditions at the same time:

(1) The microphone and the speaker are used at the same time;

(2) The sound of the audio transmission can be transmitted to the microphone through the space;

(3) The sound energy emitted by the speaker is large enough, and the pickup sensitivity of the microphone is high enough.

When using the microphone to pick up the sound in the sound reinforcement system, since the sound pickup area of the microphone and the sound emitting area of the sound box cannot be acoustically isolated, the sound emitted by the sound box can easily be transmitted to the microphone through the space to cause acoustic feedback. In general, the acoustic feedback howling problem exists only in the sound reinforcement system, and there is no condition for generating an acoustic feedback howling in the recording and restoration system. For example, if there is only a monitor speaker in the recording system, the use area of the flower tube in the recording studio is isolated from the sound playback area of the monitor speaker, and there is no condition for sound feedback; in the movie sound system, almost no microphone is used, even if it is encountered With the microphone, the voice is also heard in the projection room, and the speaker is far away from the microphone, so it is impossible to have acoustic feedback.

The main reason for the sound feedback whistling in the sound reinforcement system is that the sound of some frequencies in the system is too strong. When the microphone recording volume is increased, the intensity conditions required for the sound feedback are first obtained due to the excessive frequency. If the frequency is fed back, the type is just positive feedback, and then the self-oscillation phenomenon will appear at this frequency. The frequency of the self-excited oscillation is different from the pitch of the howling sound. In general, the cause of some frequencies in the system is too strong. There are three main aspects:

1. The room is too frequent due to resonance and acoustic reflection.

Any room can be thought of as an acoustic resonant cavity, and resonance can make the sound of certain frequencies extraordinarily enhanced. Architectural Acoustics tells us that the room's simple resonance formula for different body types and volumes can calculate the resonance frequency of a room. Also, the sound absorption material has different reflection and absorption for different frequencies. Different materials absorb sound at different frequencies. The coefficients vary widely, and the difference in sound absorption structure can also result in different absorptions of different frequencies. Therefore, the acoustic condition of the room cannot be underestimated for the acoustic feedback howling.

2. Fluctuation and ringing mode of speaker frequency response

The speaker's pronunciation unit is a speaker. Due to various reasons such as material and structure, it is impossible for any one speaker to ensure that the frequency response curve is absolutely straight. There will definitely be some cases where the frequency is peak, and the speaker whose frequency response is absolutely straight is also not necessarily very nice. Based on this, when the speaker is playing, the sound from the speaker will be too strong at some frequencies, and the sound of this excessive frequency may cause the acoustic feedback to whistle. The speaker is installed in the speaker. The mechanical resonance of the speaker cavity and the acoustic resonance of the cavity will produce a ringing mode--RM. The ringing mode of the speaker will cause the sounding to occur, that is, the sound of the speaker. Some frequency components are too strong, and it is not impossible to generate acoustic feedback whistling at these frequencies.

3. The sensitivity of the microphone to the pickup of certain frequencies is too high.

The frequency response characteristic of the microphone is an important condition for determining the sound style and range of the microphone. Like the speaker, the frequency response curve of the microphone is not likely to be absolutely straight, and the sensitivity of the pickup of some frequencies is too high. That is to say, the microphone cannot be treated equally for the pickup of each frequency, which causes a problem that the sound output of some frequencies is too strong, and as a result, acoustic feedback of these frequencies may occur. In general, the high frequency sensitivity of the microphone is high, so it is easier to generate howling at high frequencies.

 

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