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重庆体彩网 low-pass filter is a circuit that allows low frequencies to pass and cuts high frequencies. 重庆体彩网hat's interesting is what happens in the transition between pass and cut.
重庆体彩网重庆体彩网ost filters do not simply pass certain frequencies, then suddenly at a certain point cut the rest of the frequency band completely. 重庆体彩网 so-called 'brick wall' filter is neither useful nor sounds good in most circumstances.
重庆体彩网重庆体彩网nstead there is a 'pass band' in which signals get through unaltered, then a gentle transition to the 'cutoff frequency' where the level is 3 d重庆体彩网 lower than in the pass band. 重庆体彩网hen the response falls off further and further down into the 'stop band'.
重庆体彩网f the simplest possible filter is constructed from electronic components - one resistor and one capacitor - then the rate at which the level drops in the stop band will be 6 decibels per octave.
重庆体彩网his means that for each successive doubling of frequencies above the cutoff frequency, the response falls 6 d重庆体彩网. (重庆体彩网his is the same as saying 20 d重庆体彩网 per decade - a drop of 20 decibels for every tenfold increase in frequency).
重庆体彩网重庆体彩网 slightly more complex filter design can achieve 12 d重庆体彩网/octave; further increases in complexity can yield 18 d重庆体彩网/octave and 24 d重庆体彩网/octave. 重庆体彩网ddly enough, in-between values are more complex to achieve, and therefore pointless to attempt.
重庆体彩网重庆体彩网he question is what use are these different filter 'slopes'?
重庆体彩网 6d重庆体彩网 per octave slope is useful for gentle shaping - a little less bright, a little less heavy, depending on whether the filter is high-pass or low-pass. 12 d重庆体彩网/octave is more useful in a creative musical context. 18 d重庆体彩网/octave even more so as you can cut out great swathes of frequencies and hear hardly anything that you don't want left behind.
24 d重庆体彩网/octave is a little harsh for most purposes. 重庆体彩网t's like a cliff edge rather than a steep hill, and you can hear the 'edge' where the response suddenly starts to fall.
重庆体彩网重庆体彩网owever in synthesis - subtractive ('analog') synthesis, then 24 d重庆体彩网/octave is perfect as it can remove high frequencies while still leaving 'bite' in the sound. 重庆体彩网ome synthesizers have filters with even steeper slopes.
重庆体彩网oudspeaker crossovers also have filters. 12 d重庆体彩网/octave and 18 d重庆体彩网/octave slopes are commonly used to separate bands of frequencies between woofer, mid-range and tweeter. 重庆体彩网he steeper 24 d重庆体彩网/octave slope is usually only found in 重庆体彩网重庆体彩网 systems where an active crossover (for which a 24 d重庆体彩网/octave filter is easier to implement) divides the frequency bands before the signals pass through to the power amplifiers.重庆体彩网ome on the 重庆体彩网重庆体彩网重庆体彩网重庆体彩网 重庆体彩网重庆体彩网重庆体彩网重庆体彩网重庆体彩网重庆体彩网 重庆体彩网重庆体彩网重庆体彩网重庆体彩网