Car Audio Basics - Equalizers & Crossovers

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Equalizers give you the capability to fine tune your system. It is virtually impossible to get speakers to reproduce sound perfectly. In a multiple speaker system things are even more complex because the different drivers interact with each other. With an equalizer you can boost or cut certain frequency ranges to tailor the overall sound to whatever you desire. Usually you go for more accurate reproduction and then add some bass for a more "dynamic" sound.

An electronic crossover takes a full range signal and divides it into different frequency ranges. The most common types are 2-way or 3-way. A 2-way crossover divides the frequency range in half at some cutoff frequency. All signals below the cutoff frequency are routed to a low pass pre-amp output and the rest are sent to a high pass output. These outputs can be connected to amps to dedicate those amps to producing only those frequency ranges. A 3-way crossover is similar but splits the signal into 3 parts. You can get a single box that has an equalizer and crossover. Electronic or active crossovers act on pre-amp level signals. They use the pre-amp level output of your head unit as input and their outputs go to your amp(s). By doing this you keep the amp from trying to amplify frequencies that you do not want (like high frequencies for a sub amp). On the other hand, a passive crossover acts on the signals after they have been amplified, they are connected after the amp and before the speakers. Usually these are just simple high pass or low pass units. You connect a high pass crossover to a speaker to block bass to that speaker. Some people call these bass blockers. You use a low pass crossover with a woofer so it only plays "lows."

Number of Bands in the EQ: The number of bands in an equalizer tells you how fine an adjustment you can make. A 10 band equalizer breaks up the audio range into 10 parts and you can adjust the levels of any of them. The Q of an equalizer tells you how wide a range each adjustment makes. Let us say a specific band is labeled as 100 Hz. A high Q high equalizer will only boost or cut frequencies right around 100 Hz and not really affect signals at say 70 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 bad for fine tuning. A 30 band equalizer is great for making specific adjustments and tailoring the sound exactly how you want it. A tool called an RTA (real time analyzer) is used in setting those equalizers. It gives the system a flat signal (pink noise) and shows the user what the system returns. The user adjusts the equalizer until the RTA shows the desired response. The desired response is rarely flat because a flat setting results in dull, bass shy sound that is hard and edgy. Working with an experienced installer is key here.

Slope of the Crossover: When any crossover splits the frequencies it is not a hard split. At the crossover frequency in a 2-way crossover both outputs will have this frequency in the output albeit at a lower level. How fast the crossover transitions from one output with rising frequency to another is called the slope of the crossover. There are many interchangable terms for crossovers. A 1st order crossovers transitions at 6dB/octave or 10dB/decade. A 2nd order one will transition twice as quickly. For tweeters a minimum of a 2nd order crossover should be use in order to prevent the tweeter from seeing any bass frequencies. 4th order crossovers are common and digital crossovers of any order are possible but expensive. Use at least a 2nd order crossover to be safe. For an example of tweeter safety, say we want to use a high pass crossover frequency of 2kHz. With a 1st order crossover (or filter) at 1kHz the level is only down by 6dB and only down by 12dB at 500Hz. 500Hz is way too low for a tweeter to play so this will probably cause the tweeter to distort or blow up. Using a 2nd order filter would have the output down by 24dB which would be a signficant improvement. 3rd and 4th order filters are even better but expensive. Also analog crossovers change the phase response so try wiring your tweeter out of phase to see if it makes the sound better or worse and leave it the way it sounds better to you.

Crossovers can also be made to have different types of response near the crossover point. Butterworth filters have smooth but slow response. Chebychev filters are quicker but have some overshoot. Details of these filters is beyond the scope of this page.