It is virtually impossible to reproduce the complex timbre of music in exactly the way it was created in the recording studio. Every loudspeaker is a unique instrument of reproduction that is inherently incapable of reproducing every part of the sound spectrum with equal intensity. In multiple speaker systems, things are even more complex because different drivers interact with each other.
Equalizers offer the capability of both compensating for defects and fine tuning the system. With an equalizer, certain frequency ranges can be either increased or cut, to shape the overall sound spectrum to compensate for defects in the original soundscape, or to bring out certain instruments, or simply as an expression of the listener's preference. While the accuracy of high fidelity remains the goal, many listeners strive for more accurate reproduction, and then add bass for a fuller, more "dynamic" sound.
Electronically, the equalizer is situated on the line feed between the receiver and the amplifier. Because it modifies low level high impedance signals only, it is less vulnerable to electrical overloads than the amplifier, and is overall less likely than either the receiver, or the amp to create problems. Of course, as with any piece of electronic circuitry, a fault may develop; it's just less likely.
The number of bands in an equalizer control how finely the frequency pattern can be amplified or attenuated. Three to 30 bands or more, can be provided by available equipment.
The Q of an equalizer describes how wide a range each adjustment has. If a specific band is labeled as 100 Hz. a high Q equalizer will only boost or cut frequencies right around 100 Hz and not really affect signals at 80 Hz. A low Q equalizer generally affects a wide range of frequencies even though it may be centered at one specific one. Typically, the more bands in the EQ, the higher the Q, so the different bands are not affected by each other. Simple bass and treble controls have the lowest Q. Equalizers with only few bands are good for making general adjustments but not helpful for the finer gradients.
Another key distinction is the degree of effect, rated in decibels, that each adjustment either boosts or cuts the center frequency it controls. A typical value is probably 10 dB, with many choices available in the 8 to 15 dB range. This means that each control will minimize or maximize the relative level of its frequency by +10 dB, or - 10 dB from the center, which is the unmodified position. Keep in mind of course, that this is a relative value. If all the controls are raised or lowered at the same time, and by the same amount, then all we have is just a large and expensive volume control.
A 30 band equalizer is great for making precise, adjust-ments. A real time analyzer (RTA) oscilloscope is used in calibrating the effect of these equalizers. The RTA produces a flat pink noise signal and shows the technician how the system responds in acoustical effect. The equalizer is adjusted until the RTA verifies the desired response. This desired response is rarely linear because most listeners prefer a more dynamic pattern, with more bass emphasis. If you must have a 30 band equalizer, then having a high degree of technical knowledge is essential to getting the most out of it..
An equalizer's capabilities are especially useful when dealing with poorly recorded, non-linear analog recordings, or if the "mix" selected by the original recording engineer does not quite suit your taste. But with most modern digital, full range, linear, recordings, the sonic limitations of the analog era, no longer mandate the use of an equalizer to acheive a high fidelity result. Many people find that as a general practice, equalizers can be left out of the system with not too much loss of control. The simple bass and treble tone controls on most receivers provide all the shaping capabilities needed most of the time.
However, many vehicles have a natural resonance frequency between 100 and 200 HZ, and road noise occurs between 25 and 200 Hz. So even an otherwise perfectly balanced system may get competition at these resonant frequencies, exaggerating them in a way that can muddy sound and mask other frequencies. If this is a noticeable problem, a graphic EQ, can be used to overcome this interference by adjusting the affected bands to compensate.