Sound Performance Lab 2595 Frequency Filters, Shelf Filters, Peak Filters, Bandwidth, Passeq

The Basics of Frequency Filtering

Frequency Filters

As a rule almost everyone of us has first made an aquaintance with frequency filtering through our listening to home stereos. Such elementary kinds of filters are simple ampli- tude-based filters: When one turns a bass control clockwise, one hears a general or overall increase in bass frequency energy.

But with the explanation above on the composition of a complex, natural tone, it is clear that such a low frequency control does not only influence the energy of the fundamental frequency, but also always the sound of a tone—the relationship between energy of the fundamental and harmonics frequencies is changed.

Typically amplitude-based frequency filtering boosts or cuts the energy of a specific audio frequency band. In such processes it is possible to employ filters with design and function that are very different from each other: Depending upon the technical construction, such filters may, for example, process only high or low frequencies in certain way.

Filter Types

There are two types of filters used in the Passeq: wide-band filters which are comparable to shelf-filter characteristics and bell-formed peak-filters with narrower bandwidths.

Shelf Filters

A shelf filter increases or decreases the energy of all frequencies above or below a chosen frequency. Depending upon the direction of processing one refers to high frequency (HF) or low frequency (LF) shelf filters. Beginning with the threshold frequency, the frequency band is boosted or cut much like a shelf. The maximum boost or cut achieved at the point furthest from the threshold frequency. The threshold frequency is usually about 3 dB less (with the overall increase set to maximum). This gives the typical rising form of the shelf filter’s response curve.

Peak Filters

A peak filter boosts or cuts a chosen frequency‘s energy with a maximum amplitude and a definable frequency range around this frequency with a fall off of up to 3 dB to both sides. The chosen frequency with the maximum amplitude is called center frequency—it takes place in the middle at the peak of the response curve. The response curve forms a bell, thus peak filters are also often referred to as bell filters.

Bandwidth

The width of a frequency range or band is musically defined in octaves. The technical coun- terpart to this is the “Quality” of a filter, and the abbreviated “Q” is the most common value for the bandwidth of a filter.

A high Q value means a narrow bandwidth while a smaller Q factor corresponds to a wider one:

Bandwidth 2 Octaven: 0.7 Q

Bandwidth 1 1/3 Octaven: 1 Q

Bandwidth 1 Octave: 1.4 Q

Bandwidth 1/2 Octave: 2.8 Q

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