选择滤波器的坡度值:12,18 还是 24 dB/八度?

在低通滤波器或者高通滤波器中常常配备有一个坡度选项,如下图。

微信截图_20160810142939

Operator 的滤波器提供 12 或 24 两档坡度选择

那么滤波器的坡度值意味着什么呢?我们又该如何选择适合的坡度值呢?

我们知道,一个低通滤波器的电路原理是让声音信号的低频部分通过,并切掉高频部分。可是信号频率的切除与通过之间又是如何过渡的呢?绝大多数的滤波器并不是在某一频率点将声音拦腰截断。那种「一刀切」或者说「砖墙」似的滤波器效果并不实用,听起来也并不好听。

事实上,滤波器中存在一个「完全通过频段」,在这一频段上声音信号完全通过而不受滤波器的影响,然后信号逐渐减弱 3 dB 后到达滤波器设定的截止频率(Cutoff Frequency),接着信号继续逐渐减弱到 0 dB。接下来的问题是,上述信号减弱到 0 dB 的过程中,信号的频率相对于截止频率改变了多少呢?

fig9

信号强度相对于频率的减小速率即「坡度」

以低通滤波器为例,如果用一个电阻和一个电容来构成一个最简单的低通滤波器模型,那么频率每升高一个八度(频率值加倍),该频率的信号强度相应减少 6 dB。即信号强度相较于频率的改变速率为 6 dB 每八度。

如果滤波器的设计更复杂一些,信号减弱的速率可以达到 12 dB/八度、18 dB/八度甚至 24 dB/八度。在电路设计中,6、12、18以及 24 之外的中间值是很难设计出来的。

问题是,这些不同的信号减弱速率即「坡度」有什么实际用处呢?

坡度值为 6 dB/八度的滤波器滤波效果更加柔和,产生的声音不那么明亮尖锐,而12 dB/八度的坡度值产生的声音更加棱角分明。24 dB/八度的坡度值在大多数情况下都有些「切太狠」,但是在合成器的滤波器单元中,24 dB/八度对因对声音塑形有着很显著的效果而常常使用。

扬声器的低中高单元间的交叉同样要用到滤波器原理和其坡度值。12 dB/八度 和 18 dB/八度的坡度值广泛应用于划分扬声器低频单元、中频单元和高频单元。而陡峭的 24 dB/八度的坡度往往只应用于 PA 系统(公共扩声系统)中。

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