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Equalizer
In today's world of sound systems, the equalizer has become a common sight. It is used in almost every sound system found in churches, schools, businesses, homes, and even cars. But what exactly is an equalizer? What can it do? What can't it do? What is an equalizer? Simply, an equalizer is a number of electronic filters which allow you to control (or adjust) the frequency response (or tone) of a sound system. The name comes from the telephone industry. The frequency response of the phone line needed to be adjusted to make the frequency response "equal" at all the frequencies of interest. There are several different types of equalizers available today and they are becoming increasingly sophisticated. Types of Equalizers... The Shelving Filter Equalizer Figure 1 
The first and most basic type of equalizer is the shelving filter. This is the conventional tone control most used on home stereo systems and basic sound system mixer/amplifiers (Figure 1). It got its name because of how its response looks on a graph. Typically, the bass control affects frequencies around 100 Hertz (Hz) and below, and the treble control affects frequencies around 10,000 Hz and above. However, as you can see, the control affects frequencies far above or below 100 Hz or 10,000 Hz, depending on how much you turn the knob. The Parametric Equalizer Figure 2   The second type of equalizer is the parametric equalizer (Figure 2). This is a sophisticated tool which allows you to adjust not only the amount of cut or boost, but you can also control what frequency is most affected. To make it more complicated, you can also adjust the bandwidth (or the amount of frequencies which are affected by the equalizer). The Graphic Equalizer Figure 3  The third type of equalizer is called the graphic equalizer. It got its name from how it physically looks. The controls are constructed so you can "graphically" see how the individual filters are set (see Figure 3). This article will concentrate on the graphic equalizer. Graphic Equalizers Figure 4  There are three basic types of graphic equalizers. The difference is mainly in the number of filters in the equalizer. The smallest is the octave band equalizer, which is usually nine to ten filters with their center frequencies set one octave apart. Figure 4 shows the response curves of a typical octave band equalizer. This graph shows the response of each filter individually adjusted and then their responses are overlaid onto one graph. The two-third octave band equalizer has more filters which are spaced 2/3 of an octave apart. There are usually 15 controls on this equalizer. The 1/3 octave band equalizer provides the highest degree of control, where the center frequencies are spaced only 1/3 of an octave apart. This device usually has 27 to 30 controls to help you accurately shape and control the sound of your system. What Do I Do with All of These Knobs? So what can you do with this sound tool? The equalizer will let you regulate, control, or entirely change the frequency response of a sound system. Here is a partial list of uses. . . Equalization of the Loudspeaker System The loudspeakers may not reproduce an even (or flat) frequency response. In other words, certain frequencies are louder or softer than others. This can produce sound which is unnatural, harsh, or doesn't have enough bass. The equalizer can be used to electronically smooth out the response and make the loudspeaker sound better. Caution: You can't turn a piece of glass into a diamond and you can't make a bad loudspeaker sound like the best. Compensation for Loudspeaker Placement The mounting position of the loudspeaker can greatly affect the sound you hear. Positioning the loudspeaker in a corner or by a ceiling will increase the bass response of the loudspeaker. The equalizer can be adjusted to provide a more even response from the loudspeaker/room system. Compensation for Room Acoustics This is a topic that has caused much confusion and debate. Let's start by looking at what the equalizer can't do. Equalizers cannot alter a room's reverberation time characteristics or remove the unwanted echoes that can destroy clarity and intelligibility. The difference you hear between an empty church and one full of people is the result of changing reverberation times due to the increased absorption from the congregation. The equalizer can't fix the acoustical problems caused by the physical construction and geometry of a room, but it can help relieve some of the symptoms. For instance, if the worship area is very boomy, the equalizer will probably improve the clarity of the sound if the bass frequencies are reduced. However, you must beextremely careful not to destroy sound quality in your attempt to compensate for a difficult room. Always listen to each adjustment. To Reduce Feedback The 1/3 octave band or parametric equalizer is one tool that can be used to minimize feedback. (The octave band equalizer does not have fine enough control to be accurate so it is not appropriate for this application.) In use, the individual filters (controls) are turned down small amounts at the same frequencies which are feeding back (squealing). For instance, turn up the volume of the pulpit microphone until you start hearing a ringing at a certain frequency. Let's use 500 Hz, which is about B above middle A. The system could feed back at that frequency because one of the sound system components has an uneven response at 500 Hz. Perhaps the microphone picks up 500 Hz a little stronger, or the loudspeaker puts out a little more volume at 500 Hz. Whatever the cause, you can electronically turn down that frequency with the 500 Hz control on your 1/3 octave band equalizer. The system is no longer as sensitive at 500 Hz and the feedback will stop. You can now increase the volume and look for the next problem spot. CAUTION: At some point you will not realize any further improvement. And again, don't forget to listen to the overall sound of the system as you are adjusting. It is too easy to destroy the sound quality when trying to get a little more volume. Additional Applications Some other applications of the equalizer would include compensation for the microphone characteristics and positioning, to improve speech clarity and intelligibility, and even create special effects. How to Adjust Equalizers An equalizer is typically adjusted by a professional with the use of a test instrument such as a real time spectrum analyzer (RTA). Test equipment is expensive, running between $800.00 and $5000.00 for a 1/3 octave band analyzer. In addition, care must be taken when interpreting the results of the RTA. It is highly recommended that you call a professional who has the right test equipment. He has the experience to get the most out of your sound system. We have seen too many "poor" systems where the main problem was a misadjusted graphic equalizer. Figure 5
 Another relatively new type of equalizer is the digital automatic feedback controller (Figure 5). These units sense feedback and insert a very narrow filter on the offending frequency. It is really a parametric equalizer that is set automatically. Some units are now available that have both a computer based multi-channel parametric or 1/3 octave band equalizer in addition to the automatic feedback filters. These are powerful tools when used correctly. So Why Use an Equalizer? The two fundamental reasons for using an equalizer are: 1. To increase the naturalness or intelligibility of the sound system. 2. To increase the gain or volume of the system before feedback occurs. Graphic equalizers are best used to touch up the frequency response of a sound system. The basic system should be good by itself. The equalizer is the final tuning measure - the icing on the cake. One final bit of advice. Be sure you put a security cover over the equalizer and also write down the final settings. Equalizers, when used competently, can do wonders for your system, but when used badly . . . . by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com
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