Dinyaar and Anil Kumar has given you some good answers.
Unfortunately my fingers itch too much to keep away from the keyboard. Sorry Dinyaar and Anil. I just have to add my two bit to this. No offense meant.
At the outset, there is a bit of contusion on the terminology.
The term ??equalizer?? is often erroneously used, Audio components and mixing equipment include the so-called graphic equalizers, or simply, equalizers. These are actually general all-purpose filters. Divided into a number of frequencies, these filters produce the effect of high-pass, low-pass, bandpass and band-stop filters. These filters function as equalizers only when their mode of arrangement is such that they reverse the effects of the internal circuitry on the output.
The control of frequency loss or gain is only a small part of an equalizers. The main function is to counter balance the unequal frequency response of an external signal processing system. Using an equalizer, you can adjust one or more parameters that are determinants to the overall shape of the filter's functioning. Equalization filters are generally used to enhance sound fidelity, eliminate undesired components in the signal, emphasize certain instruments, and develop completely new sounds.
Equalizer filters are of multiple type - namely peaking filters, shelving filters, low-pass and high-pass, and band-pass filters.
Peaking filters work on frequencies, 'Q. and Boost/Cut. The peaking filter uses a bell curve and functions smoothly over a wide frequency range. The center frequency, occurring at the crest of the bell, is the one affected by the equalization. Frequency, as you know, is measured in Hertz. 'Q' is a quality factor that refers to the width of the bell curve. Q is inversely proportional to the bandwidth - a higher Q denotes that only a few frequencies are affected and vice versa. Q is generally measured in Octaves. Boost/Cut, also called, gain, determines the amount of selected frequencies that should be present. A boost enhances the presence of the frequencies, while a cut softens the frequencies. A gain of +10dB indicates doubling of the amplitude of the original sound.
Shelving Filters are used to cut or boost a determined frequency till a preset level is reached. The same level is then applied to the rest of the frequency spectrum. Shelving filters are common on the treble and bass controls of home audio systems, and are also employed in audio mastering.
High-pass and Low-pass filters are used to boost or cut frequencies above or below a set frequency termed as the cutoff frequency. High pass filters allow the frequencies above the cutoff frequency to pass through unaffected. Frequencies that fall below the cutoff are attenuated at a constant rate per octave. Low-pass filters, on the contrary, allow frequencies below the cutoff to pass through unaffected.
High-pass and low-pass filters are used to eliminate pops, reduce noise, and remove rumble.
A bandpass filter is simply a combination of one low-pass filter and one high-pass filter. Band-pass filters allow frequencies that fall within a certain range and reject all others. An RLC circuit such as those used in AM/FM radio to tune to a particular radio frequency, uses bandpass filter extensively.
A parametric equalizer is used in live sound reproduction, in public address systems and, in sound recording. Parametric equalizers enable audio engineers to control the parameters such as the center frequency, amplitude and bandwidth of the internal bandpass filters. A parametric equalizer allows the amplitude of each band to be controlled, and the center frequency to be widened, narrowed or shifted.
A parametric equaliser
1. removes excessive unwanted resonance, which can cause a ??boomy?? sound found in musical instruments such as double basses and acoustic guitars.
2. eliminates undesirable elements such as foreign noises.
3. eliminates feedback that causes an excessive resonance (or what we hear as shriek). This usually happens in live performances when amplified sound is captured by a microphone.
4. can be used for tonal enhancements. For, example a tampura player usually sits behind the main artists and does not have a separate microphone. At the same time, the sound of the tampura is important for the rhythm of the music. The equaliser can boost the sound of the tampura before it is cut into a CD or tape.
Graphic equalisers use slide potentiometers to gain control. In addition they may have a display that shows the frequency curve in real time.
Applications of Equalizers
One of the most important use of equalizers is in live performances where speakers and microphone operate together. Here the equalizer is used to eliminate feedback causing frequency bands.
Equalizers are frequently used in public address systems and in places such as churches, temples, stadiums, sports arenas. etc. Here equalizers are used to sharpen the sound. Temporary structures will need equalizers as it is impossible to build a good system in such places. Studios invariably use equalizers to coordinate the various microphones and sound inputs into the system, and to reduce and eliminate external noises such as a hum. .
As Dinyaar and Anil have said, at present times, the equalizers as we know them do not have much use in a home environment. Speakers have full bandwidth capabilities and excellent crossover inside eliminating the need to boost or cut a frequency. Similarly, amplifiers have reached level of sophistication where their internal circuitry takes care of smoothening across frequencies, if required. Modern AVR have a set-up system (such as from Audessey) that give a feedback to the system about the room, so that the amplifier can handle the room acoustics.
There may yet be a small set of people who demand amplified low frequencies. For such people using a filter to boost low frequencies when you record for them may make sense. But for those who value high fidelity, we always listen to music without any artificial attenuation. Whenever I audition equipment, I ensure that the demo equipment are all set to flat so that I can hear the natural capabilities of the source, the amplifier, the connectors/cables, and the speakers.
In your business Muthusami, whether to have a equalizer or not depends upon the kind of customers you deal with. If there is a high percentage of people who are happy with artificial frequency boosting, then I suppose that is what you have to give them.