【 Weak current College 】 acoustic feedback and eliminate method

1, introduction
Whistler is sound reinforcement system often appear in a normal phenomenon, the majority of professional audio workers in order to eliminate it, did a lot of work but still it is not possible to completely eliminate acoustic feedback. The author believes that the Elimination of acoustic feedback should adopt a comprehensive prevention approach, from studies on acoustic feedback mechanism to explore ways to eliminate acoustic feedback in order to gradually increase the level of acoustic feedback.
2. causes of acoustic feedback
Acoustic feedback is a part of the speaker sound energy through sound propagation by way of transfer to the microphone and the phenomenon of Whistler, Whistler before a critical state, vibrating ring tone (i.e. sound stop after the end of the high-frequency), when the General believes is acoustic feedback. The volume attenuation 6dB, defined as the highest available gain, silent feedback.
2.1 acoustic feedback resulting conditions
(1) microphone and speaker at the same time;
(2) sound broadcasting of sound transmission through space; the microphone
(3) speaker sound energy is large enough, the microphone pick-up sensitivity is high enough.
In sound reinforcement systems, when using a microphone pickup, because the microphone pickup area and speaker for playback is not possible to take measures, the speaker sound through space to the microphone, because the amplifier gain too high and result in acoustic feedback (feedback). In General, only in sound reinforcement system issue whistling only exists in recording and sound reproduction systems simply do not have the conditions producing howl. If recording system only listen in with speaker, studio microphone with regional and za really good playback region are mutually isolated and do not have the sound feedback; and in the cinema sound reproduction system almost do not use a microphone, the microphone even occasional use is also made in the Afar language nearly stresses pickup, show speaker is far away from the microphone, so it's not possible acoustic feedback.
Sound reinforcement system has the screech of the main reasons is the system some frequencies of sound (signal) is too strong, when upgrading the microphone channel gain, since these too strong frequency was the first arrival of acoustic feedback needed strength conditions if the frequency of a feedback type just as positive feedback, it will appear on the frequency of self-excited Oscillation phenomenon, since the oscillation frequency of the screech of the sound tone.
2.2 causes acoustic feedback
(1) the room shape and acoustic conditions
Any room can be considered as an acoustic resonance cavity resonance will make certain that the frequency of sound particularly strengthening the divisor. According to the principle, different acoustical shape and volume of the room its resonance frequency is different, through normal resonance room, you can figure out the formula one-room resonance frequency; on the other hand, sound-absorbing material on different frequencies of reverse and absorption is different, different materials on different frequencies of sound absorption coefficient vary considerably, the different sound-absorbing structure also results in absorption of different frequency. So the room acoustics (mainly acoustic dyeing conditions) for the role of acoustic feedback should not be underestimated.
(2) speaker frequency response fluctuations and ring mode
Speaker's pronunciation unit for the speakers, since the materials and structures, and many other reasons, any one of the speakers can guarantee absolute flat frequency response curve, there will definitely be some frequency peaks. Thus, in the sound playback, the speakers sound appears certain frequency sounds too strong, the frequency of the sound too is likely to result in Whistler. Speaker is mounted on the loudspeaker, speaker of the cavity of mechanical resonance and acoustic resonance cavity produces a ringing modal RM (RingMode), the ring mode can cause the incidence of acoustic colouring, the speaker sound in certain frequency components too strong, this frequency is also possible acoustic feedback.
(3) microphone on certain frequency too high pick-up sensitivity
Microphone frequency response is to determine the scope of application of the timbre and microphone. And speaker, microphone frequency response curve nor is it possible to guarantee absolute flat, with some frequency of pick-up sensitivity too high is inevitable, that is to say, the microphone for each frequency sound sensitivity of ten different, resulting in the sound of certain frequency output is very strong, the result may appear in these frequency acoustic feedback. In General, the microphone in the high frequency of certain frequency sensitivity is high, so it is easier to produce a whistling at high frequencies.
3, the harm of acoustic feedback
Acoustic feedback phenomena happen, light will cause the microphone channel volume cannot be adjusted, adjusted after the screech of a very serious, to live performances will cause adverse effects, or the microphone it up after the ringing sound (that is, when the microphone sound feedback critical voice sounding muddy phenomenon), sound sense of failure exists reverb sound quality; to lead speakers or amplifier because the signal is too strong and burned.
Possibility of Whistler full suppression, is a sound reinforcement professional audio master tuning skills. Because screech suppression, singing and playing with microphone, adjustable volume space, singing and playing a satisfactory result.
Due to the acoustic feedback while burning sound treble unit are numerous, because of the situation in the State of acoustic feedback signal strong, will make amplification occurs clipping (CREST) distortion, clipping distortion will produce a lot of high-frequency harmonics, these high-frequency wave into sound treble unit, treble unit cannot afford such a powerful high-frequency signal, it will cause the voice coil burn; Moreover, in acoustic feedback mode, the output amplifier is too large and it is very likely overload was burnt.
To sum up, acoustic feedback for sound reinforcement system not only makes sound effects, also cause serious damage to the device, affect the site performance, resulting in a very bad influence.
4, acoustic feedback suppression method
In public practice, audio masters summarized many sound feedback inhibition of audio systems, acoustical experts have taken effective measures to eliminate acoustic feedback, electro-acoustic experts research and development of a variety of electro-acoustic devices to reduce theSmall acoustic feedback phenomenon occurs, the application of these practical initiatives, achieved good results, the following brief introduction of suppression, reduce and eliminate acoustic feedback.
The voice of the speaker 4.1 difficult to transfer to the microphone in
To make the speaker sound is transmitted to the microphone to reduce acoustic feedback phenomenon of the possibility to take the following measures.
(1) microphone away from the speakers. This method is very difficult to achieve in song and dance halls, dance halls of area generally. But on the theatre or concert venue, this approach will have a role, for example, you can consider microphone hanging above the stage in a band or pickup, so that both can guarantee the microphone close enough from the sound source, or you can make as much as possible away from the microphone and speaker.
(2) the use of speaker and microphone directivity. Using the speaker and microphone to the characteristic to suppress acoustic feedback whistling, often also played the role of a non-scheduled, if the microphone is not used in the speaker sound radiation area, the speaker's voice is not easy to microphone; on the other hand, in the use of directional sensing devices don'tlet its being on the speaker, try to avoid whistling. In practice, you can adjust the angle of the sound box, make the speaker sound radiation area try not to sound playback area singer, and not just on speaker microphone.
(3) appropriate to reduce the volume of the microphone channel. Doing so will make the song or the band pick-up is restricted, the performance results are affected, but sometimes the sound engineers have to take this to reduce the volume of an unwise move. If the performance of real-time control volume size, when the ring phenomenon occurs, the tuner to promptly turned the volume down to avoid whistling. Actor handheld or wear microphone through the speakers before, but also to control the volume, otherwise will result in Whistler.
(4) If a mixer of parametric equalizer band Q value adjustment, the compensation measures can be taken, on the volume of sound properly compensated by combining Equalizer bands to microphone pickup sources sound frequency bands will be improving this band, a halt immediately before the whistle.
4.2 use can inhibit acoustic feedback sound-processing equipment
Eliminate acoustic feedback, can also be accessed via the sound signal processing, at present the electroacoustic equipment manufacturers developed a variety of acoustic feedback can be suppressed, which inhibits the device sound feedback have very good results, the following describes some of the equipment.
(1) frequency divider. This is a way to change the sound frequency equipment, works similar to the tone, it will increase the frequency of a sound signal, failure to produce 5Hz acoustic feedback, thus inhibiting the acoustic feedback. However, the use of the equipment have certain limitations, in the language you use sound reinforcement equipment allows to effect good, sound damage is very small, but in concert and instrumental music in use will have clear voice tone. This is because the language of the lower frequency range is between 130 ~ 350Hz 5Hz tone changes alone will not make it clear tones varying high feeling, but in vocal and instrumental music of lower frequency is about 20Hz 5Hz tone change obviously ear has been aware of it.
(2) equalizer and feedback suppressor. These two devices are effectively attenuate feedback frequency gain (fed), sound reinforcement system is producing acoustic feedback, because the system some frequency signal is too strong, too strong attenuation of these frequencies will be able to suppress acoustic feedback. Equalizer and feedback suppressor of difference is that the equalizer need according to Whistler frequency manually fed point down; and feedback suppressor can automatically discover acoustic feedback frequencies and decay, attenuation of band width and attenuation of the machine according to the actual situation to decide, almost does not have any impact on sound quality, but also makes the microphone picks up sound become good.
(3) compression. Compression is a strength under input signal automatically change output signal on a large number (gain) for the device. Used to suppress acoustic feedback when the compression ratio can be tuned to ∞: 1 (at which point it to the limiter), set the threshold to feedback the critical point. Then, when the volume to be producing acoustic feedback, voice signal strength exceeds the threshold, strength nor will continue to increase, and therefore does not result in acoustic feedback. But with compression inhibits acoustic feedback will bring sound dynamic losses and should use this method.
4.3 doing room acoustic design of buildings
Room acoustic colouring occurs is the most important sound feedback acoustic reasons. As mentioned earlier, as a Chamber room, produce acoustic resonance is inevitable, and will make the sound resonance in certain frequencies be strengthened on this frequency produces acoustic feedback whistling for granted. Eliminate the main dyed article sound is to minimize normal resonance phenomena occur.
Indoor exist concave reflector can cause sound focused phenomenon from occurring, but sound focus causes the sound volume is too strong Dairy Council, when the microphone in the acoustic pickup spotlight, because of the sound energy grant-bound, very likely Whistler phenomena. Doing indoor structure design, to take various measures to avoid concave reflector, if the room in insurmountable rugged surface, which reduces and suppression of acoustic feedback positive.
In the selection of interior decoration materials and programmes should take full account of the sound-absorbing materials and structure of individual frequency absorption (reflection) uniform in order to reduce the frequency of sound reflection or absorption of inconsistency, not only for the room of the frequency response of improved benefits, but also to further eliminate the phenomenon of acoustic colouring.
4.4 reasonable selection speaker
Speaker's point to properties and frequency response to acoustic feedback has an important role, its index will determine the speaker used the severity of the acoustic feedback, speaker well-selected, possibility of acoustic feedback will be greatly reduced.
Speaker's directivity is speaker on various directions of radiation sound capabilities, pointing to the viewing angle of the speaker, speaker sound easy to directly send to the microphone, the possibility of increasing sound feedback, point to point of small speakers reduces the speaker sound directly back to theThe possibility of awarding to the microphone, acoustic feedback is not likely to occur. Therefore, to meet the listening area sound coverage, select point to point of small speakers for reducing acoustic feedback last name would be more beneficial.
As previously described, the impact of speaker frequency response characteristics of the main causes of frequency characteristics of speakers and the speaker of the ring mode (RM). Buy speakers, sound quality and ensuring the tone, you can consider the selection of the frequency response curve is flat and no obvious spikes of speaker, because too strong frequency peaks very likely induced screech. In 1998, the United States EV company successfully developed the RMD technologies with sound, vibration damping and acoustic absorption method, aims to eliminate resonance Chamber acoustic resonance and box of mechanical resonance, reduce acoustic colouring, not only the sound quality has been markedly improved, acoustic feedback phenomenon has been a certain degree of restraint.
4.5 reasonable choice of microphone
Microphone is picking up sound information, pickup is possible to pick up the sound, you need to avoid picking up to unwanted sound, not only for improving the sound quality is great pickup, but also can reduce the incidence of acoustic feedback.
Concern is that 2 frequency response and the same directivity microphone, and not because the microphone sensitivity is high and it is easy for produces acoustic feedback. In sound reinforcement system meet the desired output sound pressure level, the high sensitivity of the microphone, relative to the low sensitivity of the microphone, without the need to gain down too large to avoid too much gain from acoustic feedback, such as the capacitor microphone sensitivity up to 20mV/Pa, but moves ring microphone sensitivity proximity 1Mv/Pa, both related some 10 times, the former than the latter amplifier gain decreases (i.e. 20dB). If due to improper operation, the gain adjustment too high and screech, this should not be attributed to the high sensitivity of the microphone. If mistaken for microphone sensitivity higher than low easy to produce screech, while the low sensitivity of the microphone, you can take advantage of high-quality microphone (such as the signal to noise ratio high, you can pick up faint sounds) erase.
Directivity angle is the microphone on the different sound reception, the point to point, pick-up area, but also easyto pick up sound reinforcement-field sound down cause whistling, such as pressure zone microphone (PZM) due to the pick-up point of large (up to 120 °) is apt to produce a whistling.
Microphone angle from the point of regard from small to large order with strong links to, bi-directional (90 °), Rui-shaped (105 °), heart (115 °), heart (131 °) and non-directivity (360 °), etc. From suppression of acoustic feedback point of view, point to point as small as possible, some microphone instructions indicated on the outside with axis, that exclusion characteristic is its sharp direct comparison to the point of view, has the good performance of inhibition of acoustic feedback. Select the characteristics of the microphone must be reasonable, since the point to point of view is too small will cause pick-up area is reduced, so select the microphone to directivity, don't lose another.
Microphone frequency response curve peak also appears likely to lead to acoustic feedback, because of the frequency characteristics of the microphone is not smooth, microphone output signal is certainly not in exactly the same as the frequency, output signal in certain frequencies have strong, this is an acoustic feedback. In selecting a microphone when its frequency response, as do not have a noticeable spike, brand microphone at the factory to go through the frequency response curve, even the same model of microphone, its frequency curve is necessarily different.
With the continuous development of audio technology, elimination and suppression of acoustic feedback tools will be more and more, but in theory, sound reinforcement system simply eliminate acoustic feedback is not very realistic.