This month we will discuss the terminology associated with equalizers. As we stated last month, an equalizer consists of a series of filters that are used to to isolate and boost or reduce a given frequency range.
One type of filter is the shelving filter or shelf filter. A shelving filter applies a constant amount of boost or cut above or below a wide frequency range. It produces a frequency response curve that resembles a shelf and is called a shelving curve. A shelving filter can be either highpass or losspass, sometimes called a low shelf or high shelf filter, respectively. A low shelf is used to cut or boost only the lower frequencies. This is the type of control found on most hi-fi amplifiers and is called the bass control. Similarly, a high shelf is used to cut or boost only the higher frequencies, like the treble control on such equipment. The frequency where the shelf levels off is called the shelving frequency. The frequency where a filter begins to take effect is known as the cutoff frequency or corner frequency. In the case of shelving filters, it is sometimes called a hinge frequency or inflection point.
A highpass filter passes signals above the cutoff frequency and reduces the signal of frequencies below that. Because it filters out the lower frequencies, it is sometimes called a lowcut filter, which is more intuitive. A lowpass filter does just the opposite and is likewise called a highcut filter. A bandpass filter passes frequencies of a certain range while reducing or rejecting the frequencies outside that range. This is probably the most common filter used with equalizers. The opposite of a bandpass filter is a stopband filter, better known as the notch filter because it takes a notch out of the signal.
The passband is the frequency range of signals that a filter allows to pass through. It is the opposite of stopband, which are the frequencies attenuated by a filter. The range of frequencies between a pass band and a stop band is the transition band. Within the transition band, the amount of change is indicated by the cutoff rate, the slope in decibels (dB) per octave that a filter reduces the level of the signal. It is sometimes called the rolloff rate, rolloff slope, or cutoff slope.
The center frequency, also known as a mid-band frequency or middle frequency, is the midpoint between the upper and lower cutoff frequencies of a bandpass filter. It is usually defined as either the arithmetic mean or the geometric mean of the lower and upper cutoff frequencies.
Another term you are likely to encounter with equalizers is the mysterious Q. It stands for quality factor and is a dimensionless parameter that designates the sharpness of a filter peak. Sometimes called a Q factor, it is defined as the center frequency divided by the half power bandwidth. A small Q has a sharp peak, while a large
Q refers to a wide, broad peak. Q is also used to describe characteristics in other components, electronic circuits, microphones, and speakers, but that is a topic for a future discussion.
Next month in Part 3, we will take a look at the various types of equalizers.
• “The Stars and Stripes Forever” by John Philip Sousa has been performed at Carnegie Hall on the average about once a year. Sousa himself conducted the march three times at Carnegie Hall. The first time was in 1905 and then twice on the same day in 1917.
• The contract making Brian Epstein the manager of the Beatles was not valid. At the time, both Paul and George were under 21 and should have had a legal guardian sign for them. Epstein never even signed the contract himself.
• In July 1954, Elvis Presley, along with Scotty Moore on guitar and Bill Black on bass, recorded “That's All Right (Mama)” and “Blue Moon of Kentucky” at Sun Studio in Memphis. The success of that record, which became a regional hit throughout the South, resulted in increased airplay for several of Sun's other recording stars, including Carl Perkins, Johnny Cash, Roy Orbison, Jerry Lee Lewis, and Charlie Rich.
• Ronald White of the Miracles wrote the tune, while Smokey Robinson wrote the lyrics to the Temptations' 1964 classic “My Girl.” Robinson said it was inspired by his wife, Claudette. They divorced in 1986.