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Loud and clear

作者:百里虎燧    发布时间:2019-03-08 05:05:07    

By Barry Fox PUBLIC address systems have a fundamental problem: turn them up too loud and you get a dreadful howling because of feedback. But the Dutch electronics company Philips has now found a way to quadruple the volume of PA systems without producing any feedback. Audio systems howl when microphones pick up the sound from speakers, which is then amplified and sent back to the speakers. The sound goes around in circles, getting louder and louder, and finally resonating at whatever frequency the system is most efficient at reproducing. Early feedback suppressors worked by filtering out the resonant frequency. This kills the howl but makes the sound very unnatural. More modern systems use “decorrelation”: they shift the frequency of the sound coming out of the speakers by about 5 hertz compared to the input, preventing the reinforcement of identical frequencies that cause feedback. However, at sound levels that would normally cause feedback, the decorrelation process adds myriad faint echoes to the sound, all with a slightly different pitch. A single voice can start to sound like a choir. If the sound level is increased by more than 3 decibels over the level at which feedback starts, the echo effect makes speech unintelligible. Because the human ear and brain need an increase of 10 decibels to perceive a doubling of loudness, a 3-decibel improvement is barely noticeable. So even in courts and parliaments with expensive sound systems, nervous witnesses and quietly spoken politicians still cannot always be heard. Philips’s new system also avoids feedback by shifting the frequency of sound. Unlike other systems, however, its Profecta Acoustic Feedback Suppressor prevents echoes by predicting exactly which sounds the microphone will pick up from the loudspeakers and subtracting these sounds from the input before it is amplified. This allows the sound levels to be increased by up to 20 decibels—four times louder than would normally cause feedback—without the choir effect. Philips first tried the Profecta system at a children’s theatre near its lab at Eindhoven in the Netherlands. The theatre had to sling its microphones high to stop children hitting their heads when they skipped and jumped around the stage. But the high microphones were unable to pick up performers’ voices without feedback. Since the new system was installed, however,

 

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