If you don't hear this effect, check to make sure the filter is set to lowpass mode. For you to understand what you've just heard, we have to back up a step and talk a little about the nature of sound itself. If, for example, the vibration repeats at a rate of 100 cycles per second, we say it has a frequency of 100Hz. Curiously, we can also talk about the frequency spectrum of noise, but that discussion can get pretty technical, and it isn't relevant to this month's topic. The lowest sounds that human ears can perceive as sound have a frequency of about 20Hz, while the highest sounds we can hear have a frequency of about 20,000Hz.
Most sounds are composites in which a number of frequencies are blended together.
There's a lot more to the story than this, and we're not going to get into the whole business of overtones, the harmonic series, and Fourier analysis this month. If you want to know more, drop me an email, and maybe I'll devote a column to it. At this point, what you need to understand is that the sound you're feeding into a filter, even when it presents itself to your ears as a single tone, will most likely consist of a blend of frequencies from low to high. It can also boost the loudness of other frequencies. So when we say, for instance, that a filter attenuates the frequencies above 1kHz, what we're saying is that whenever a sound passes through the filter, any vibrations within the sound that have a frequency of 1,000Hz or greater are reduced in level.