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The Basics of Frequency Filtering
Frequency and Energy
In general, a frequency prescribes a number of events in a time
interval. The per-second cycle of a wave form is given in Hertz (Hz).
Lower tones produce longer waves and higher, waves of shorter
length, and the higher the frequency, the higher the tone. The
higher the amplitude of a wave, the higher its energy level and in
turn, the louder it is perceived.
Tone and Sound
In the area of music, a sound event is referred to as a tone. Such a
tone is complex: it is comprised of different frequencies, each at a
different energy level.
In analyzing the components of a naturally produced tone (such as
those created by a real instrument or voice), we see the following
ingredients: A natural tone is comprised of a lowest pitch or funda-
mental along with many additional higher components called har-
monics. The arrangement of these pitches is called the harmonic
series, which includes the entire group of frequencies from funda-
mental to higher harmonics and is called the frequency spectrum
of a tone.
As the lowest pitch, a fundamental determines the basic frequency
and its perceived pitch. The frequencies of the harmonics are mul-
tiples of the fundamental frequency and determine the specific
sound of a tone (that is, whether it sounds like an instrument,
voice, etc.).
Should one wish by electronic means to record, process and play
back a given tone, it is crucial to maintain the accuracy of the fre-
quency spectrum if one wishes to be able to recognize it later as
the original. Just as important is the aspect of maintaining the
original energy levels of all frequencies it is composed of.
In producing a tone, the distribution of energy within the fre-
quency spectrum is further and decisively influenced by the acous-
tic environment through the mixing of direct and reflected sound.
The energy relationship between fundamentals and harmonics
is different between direct sound and that which is reflected (for
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