Sound Performance Lab 2595 Basics of Frequency Filtering, Frequency and Energy, Tone and Sound

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 fundamental along with many additional higher components called harmonics. The arrangement of these pitches is called the harmonic series, which includes the entire group of frequencies from fundamental 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 multiples 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 frequency spectrum if one wishes to be able to recog- nize 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 frequency spectrum is further and decisively influenced by the acoustic 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 example, harmonics of a reflected frequency spectrum may have measurable more energy), and this can change a tone‘s perceived sound. Later, when a musician has the impression that a recording is not true to the original, that he or she has either played or sung, an important consideration to make is to examine the resul- tant frequency spectrum.

Sound Correction and Sound Design

Along with acoustic influences of a recording ambience, it should also be understood and accepted that, to say the least, there are definite technical limits to recording and playback that may strongly influence an end result. In the first decades of electronic recording, the principle influence on the quality of such recordings centered on the choice and placement of microphones. The first Equalzers were used to combat technical and acoustical problems such as insufficient frequency response from microphones and loudspeakers or even inad- equate relationships in room acoustics that needed to be corrected or brought into balance. The goal was always to bring into balance as much as possible—and maintain—the correct frequency spectrum of an originating tone source.

The introduction of multi-track recording in the 1960’s brought a fundamental change in the way recording was done. Instruments intended for one production could be recorded in sepa- rate sessions and at different times. The mix from many individual recordings—at first in only four tracks—nonetheless added a new problem of offering ways to add further sonic quality through further processing of individual tracks, because with each copy a track’s quality was reduced.

This introduced an entirely new function for EQ, for example, the emphasizing of partic- ular instruments in their tracks to prevent them from being lost in a mix by altering certain parts of their frequency ranges. Along with this ability to emphasize specific tonal qualities appeared the added capacity to indulge in even more creative work through much stronger or even exaggerrated processing of a sound and thereby lend it even more presence in mixes. Without a doubt, the increasing popularity of electronic tone production has played a large part in the further development of EQ filtering as a creative element in audio production.

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