I. Frequency Response

Any signal can be broken down into an infinite number of amplitude levels for an infinite number of frequencies. Many devices will tend not to equally affect all frequencies - this effect on the signal can be called its frequency response. Speakers often have unique responses, and will often display a frequency response graph - a visual representation of how it affects frequencies input into it. All devices have their own responses, but many (most obviously amplifiers) include EQ controls to vary this.

ii. Equalization/EQ

Refers to the technique of altering the amplitude of certain frequency ranges of a signal. Despite being called "equalization", EQ controls are often purposefully used to create a frequency response that features unequal amplitudes across the frequency spectrum.

iii. Filter

All analog EQ's work by reducing the amplitude of some frequencies vs. others. Thus, they are commonly called filters as they "remove" some frequencies but not others. EQ's that claim to boost some frequencies are actually filtering out all the other frequencies, then amplifying the entire signal.

iv. Band-Stop

A filter designed to reduce frequencies around a specific target frequency.

v. Band-Pass

Basically the opposite of a band-stop - reduces all frequencies except for those around a specific target frequency.

vi. Low-Pass/High-Pass

A low-pass filter allows low frequencies to pass, but filters out frequencies above a certain point. It can be rather gradual so that as frequencies get higher and higher they get slightly softer and softer until they are inaudible, or it can be rather abrupt, where frequencies lower than the cutoff frequency are nearly completely unaffected, but frequencies above the cutoff are completely inaudible. A high-pass filter is the reverse - high frequencies are allowed to pass but low frequencies are filtered more and more powerfully the lower the frequency.

VII. Shelf

A Shelf is similar to a low-pass or high-pass filter; however, instead of the signal getting softer and softer as the frequencies get higher and higher (or lower and lower), the effect only occurs over a range of frequencies. Any frequencies higher (or lower) than that point are not reduced any further. The effect's result on the frequency spectrum looks like two parallel shelves. IE: ---- becomes --__. Often the filtered frequencies are called the low shelf and the unfiltered the high shelf.

VIII. Peak/Valley

Just another term for band-pass or band-stop filters. Their resulting effect on the frequency spectrum looks similar to a mountain peak or a valley.

ix. Q

This is the "width" or steepness of a filter's effect on the frequency spectrum. A high Q on a band-stop indicates a very narrow range of affected frequencies, often used to "notch" out a small offensive frequency. Whereas a low Q may affect nearly the entire audible frequency spectrum. For a high-pass or low-pass filter, a high Q will be a steep, abrupt drop-off in frequencies at the cutoff frequency, while a low Q is a more gradual roll-off. Similar a Shelf with a low Q will have a gradual, smooth transition from the high shelf to the low shelf, while a high Q implies a dramatic, steep jump.

x. Cutoff

The cutoff frequency on a low/high pass or shelf EQ is the frequency where the EQ acheives 3 db signal reduction - basically the frequency when you start to notice the effect.

xi. Parametric EQ

A parametric EQ is a band-pass/band-stop filter with adjustable cutoff frequency, Q, and gain (or mix).

xii. Graphic EQ

A graphic EQ consists of a number of band-pass/band-stop filters at fixed frequencies throughout the frequencies spectrum to provide a near continuous effect across the spectrum, usually controlled by vertical sliders to approximate the visual appearance of the filters' effect on the spectrum.

xiii. Notch EQ

Usually has multiple bands, but is designed to provide very high Q band-stop filters to create narrow "notches" in the frequency response to filter out offensive frequencies - can be useful to remove an unwanted fixed frequency fizz or hiss or hum.

C. Tone-Based Terms

i. Tone

Musicians often use this term to cover any feature the sound of any particular instrument or device contributes to. Some common usages refer to frequency balance - how much high frequencies are present compared to lower frequencies or vice versa...or a particular characteristic of the frequency range (ex. dark tone, bright tone, midsy tone, scooped tone, etc.). Another usage describes the fidelity of the signal, often referring to tone as though it is a quantified amount. For example, someone might call some device a tone-suck, which means the device increased the noise relative to the signal. Or he might say some device has a huge tone, meaning the output is high-fidelity - low noise and low unwanted distortion. It can also refer to the amount of distortion the device produces. While those are the typical usages of the term, it can even refer to more quirky features, like being swirly, or shimmering, or ambient.