Parametric or Graphic Equalization…That is the question
Mobile audio is a personal thing. When we’re listening to the stereo while cruising down the highway just shy of the speed of light, we’re in our private domain; our "space" so to speak. All of the different shapes, angles, and materials used in the design and construction of our "space" will effect the sound of the music. Soft, padded materials absorb sounds while harder materials reflect sounds. Speaker placement is a manufacturing afterthought. The results are almost always negative. Vehicles are just not designed for the accurate reproduction of music. Crossover points, speaker placement, and level setting can only do so much. These adjustments cannot compensate for all of the negative effects the car interior has on the music we listen to.
Once all other aspects of your system design have been addressed, the next best way to get your favourite music to sound as close as possible to what the artist (or recording engineer) intended you to hear, or to tailor it to your own particular taste, is by the use of a precision equalizer. Their ideal function is to modify the sound of an audio system to counteract the negative changes produced by the listening environment (whew!). In our case, this would be the vehicle we installed our sound system into.
The end result of correctly using equalization in an audio system will be vastly improved sound quality. The way in which the two common equalizer types function however, is slightly different.
Equalizers, or EQ’s as we’ll call them from now on, are signal processors. They take the incoming music signal, modify it, and send it out to the crossover(s) and amplifier's). They are usually categorized by the number of segments or "bands" they have. Each "band" of an EQ effects a certain segment of the audio frequency spectrum (typically 20Hz to 20,000Hz). EQ’s can range from a simple 1 band, all the way up to 20, 30 or even 60 bands.
To keep things simple, individual EQ bands, both graphic and parametric, consist of both high-pass and low-pass filters. They can actually be thought of as a type of band-pass crossover. The high-pass point allows frequencies above it to be effected, while the low-pass point allows frequencies below it to be processed. The level (Boost / Cut) or relative volume of each band can then be adjusted as desired.
Graphic EQ’s are so named because the position of their slide controls (if so equipped) form a graphic or visual representation of the desired results. The modified frequency response is then referred to as the "curve" of the system. This makes them relatively easy to use.
Graphic EQ’s have fixed frequency bands; the center frequency of each control cannot be changed. They are usually classified by the spacing of these frequency controls. Typically one, one-half, or one-third octave.
The frequencies above and below the center frequency that will be effected by any individual control is called its bandwidth or "Q". Typically, the "Q" of each band will change with the amount of boost or cut applied. The more boost you apply to an individual band, the narrower (more "peaky") the Q will become. Lower amounts of boost will effect a wider area. This results in smoother overall frequency response. The bandwidth on some graphic EQ’s does not change with boost or cut. This type of EQ is called "Constant Q".
When tuning a graphic EQ, the problem frequency needs to be at (or at least close to) the center frequency of one of the bands to allow a correction to be made. The more bands the EQ has, the greater the ability to correct problem areas.
Parametric EQ’s provide for the adjustment of three parameters and have three controls: Frequency, Gain (boost/cut), and Bandwidth (or Q). This arrangement can provide the ultimate in EQ control and flexibility.
The frequency control allows the user to select the center frequency of the particular problem area. There can be many different configurations for this control. Depending on the design and number of individual bands, this control can be adjusted over a large frequency range or, if the EQ has many bands, control will be limited to a smaller range. Move this control to the center of the area needing correction.
The second control varies the bandwidth or Q. It dictates how wide or narrow the corresponding frequency range will be. A high Q results in a narrow frequency range. A low Q represents a wider range of control. It should be adjusted to compensate for the size (or width) of the area of frequencies causing a problem.
Finally, we have the gain control. It sets the relative volume (boost or cut) of the parameters chosen by the above controls. Use it wisely (and sparingly).
The Big Picture…
Achieving sonic nirvana in a vehicle can be the mobile audio equivalent of the search for the Holy Grail. It’s a difficult job at best. In our homes it’s much easier. Engineers have absolute control over the design of home speaker systems. They can optimize performance through correct speaker placement and crossover design. The result should be ideal speaker-system frequency response. Speaker engineers have a pretty good idea of the type of area their speakers will be used in. They don't about road noise, ignition systems, or the blown, big-block, mobile substitute for manhood next to them at a stoplight. In a vehicle, we have no parallel surfaces or the luxury of placing speakers anywhere we please. We have many odd angles, differing surface materials, and consistently bad speaker placement.
To the average mobile audio enthusiast, the setting of a precision equalizer could seem like a difficult task. IT IS! Without the use of a pink noise source (a test signal consisting of equal energy at all octaves. It sounds like the static between radio stations) and real-time analyzer (RTA) it is very difficult to correctly set any serious EQ. An RTA will "look into" the frequency response of your system and show you the problem areas. Still want to do it by ear? Let’s look at this another way. You’re driving down the road listening to your favorite CD and out of nowhere you say to yourself: Oh my, my system is down 4dB at 2.7kHz! I don’t think that very many normal human beings would disagree with me that this scenario is not likely to happen. If you don’t know where the problem(s) exists in your system’s frequency response, how can you fix it? The RTA is specifically designed for this job.
The main idea when setting your EQ is to end up with a smooth frequency response. If you are not specifically competing in a competition, DO NOT EQUALIZE YOUR SYSTEM TO A FLAT LINE! This will result in an exaggerated high frequency response and a perceived lack of bass. Try setting a gently sloping curve with the bass above the zero line, and the high frequencies sloping downward to a little below the line. This will sound more natural. Try to remove the "peaks" before filling the "dips". This will allow the most efficient use of amplifier power. Every 3dB of boost requires double the amplifier power to make it happen. You can see that a light touch on the boost control is preferred.
Once a basic curve is set with the RTA, it’s time to whip out your favourite CD’s and do some listening. The following adjustment is to be made with the most sensitive tool in your arsenal: YOUR EARS. Only by listening to familiar music, and with a little help from the Equalization & Tuning Chart, you can either smooth out your vehicle’s basic response, or drastically change it: it’s entirely up to you.
Periodically, while making adjustments, compare the curve you are creating to the system in an unequalized state. This allows you to compare your original system sound to where you’re at now. After the tweaking is done, live with your system for awhile before making any more changes.
The Bottom Line…
It all comes down to personal preference. The purpose of owning a product like this is to get maximum performance from your system so you can sit back and enjoy the incredible enhancement a quality equalizer can provide.