Introduction to Crossovers
Ever wondered what all that “high-pass”, “low-pass”, “band-pass” buzz was all about? We clear a few things up with regards to crossovers in car audio.
Frequencies
To understand crossovers first we must understand frequencies.
Imagine a speaker resting at its zero point or middle position. The cone of the speaker then moves up to a maximum position then down, past the zero point, to a minimum position before returning back to the middle. This is known as one cycle. As a speaker moves in and out it compresses and expands the air around it. In doing this it creates sound waves, which are then perceived by your ears as sound.
If a speaker completed 1 cycle in a single second it would play a 1hz frequency or note. If the speaker were to move through 50 cycles each second this would play a 50hz tone. If a tweeter were to go through 12,000 cycles (which is not uncommon) in a second then this would play a 12khz (kilo hertz) tone.
An average person can hear frequencies from about 28 Hz to 18,000 Hz. However, an average song will generally only range from about 35Hz to 15,000 Hz.
Crossovers
Where do crossovers fit into all this? Different kinds of speakers are specially designed for reproducing different frequencies. Subs for example are designed to move massive amounts of air. Their cones move in and out relatively slowly and have large extension capabilities (large distance between maximum and minimum positions). The mass of a subwoofer would be unable to complete the necessary amount of cycles to compete with a tweeter. Instead it would sound heavily distorted and probably cause itself some damage. To change direction 30,000 times each second requires a very lite weight but very strong cone like that of a tweeter. If a tweeter were fed low frequencies from an amplifier, its cone would reach its maximum extension and sit there waiting to go to its next extension. This would quickly burn the tweeter out.
The job of the crossover is to send the correct frequency range to the correct speaker by filtering out the unwanted frequencies. For each speaker these crossovers can be either:
- High Pass
Allow passage of frequencies that are higher then that stated by the crossover - Low Pass
Allow passage of frequencies that are lower then that stated by the crossover - Band Pass
Specify both an upper and lower frequency. Allows frequencies between those stated can pass
There are two types of crossovers. Active crossovers and Passive crossovers
Active Crossovers
Active crossovers separate frequencies at line level such as that delivered by RCA cables. This is therefore done before an amplifier, amplifies the signal. These crossovers are normally either placed between the source unit (head deck/unit) and the amplifier, built directly into the amplifier itself or built into a source unit. They will normally offer some kind of adjustment where an installer can change the cut off frequency and/or simply change the filter to either high pass, low pass, band pass or off (full pass).
Passive Crossovers
Passive crossovers go inline between an amplifier (remember this could be the amplifier in your head unit) and a speaker or speakers. Crossovers that are packaged with component speakers are an example of this. These crossovers offer some extra electrical resistance to the amplifier, therefore stealing some of the power meant for the speakers. Additionally these crossovers are normally preset and are not easily adjustable. The benefits of passive crossovers are that they allow multiple speaker types to run of a single amplified channel. For example a single full pass channel may run to a 3 way passive crossover. The crossover will take the signal and for each of the three speaker types that are connected, filter out the inappropriate frequencies, then send the signal on to the speaker.
Crossover Slopes
Rather then cutting off frequencies all together, crossovers effectively make it harder and harder for frequencies outside the range to pass through. If drawn on a graph this would show a gradual decent thru the frequencies as opposed to a complete drop off. The ability to make it harder for these frequencies is measured in decibels per octave or db/octave. The most basic crossovers (a simple single capacitor or inductor) are rated at 6db/octave. They increase by 6db from there eg 12db/octave 18db/octave. It is important to then realise that if someone was to run a two speaker system with 6db/octave crossovers, a high pass set at 100hz on one speaker and low pass set of 100hz on the other, the sound would peak at 100hz. The reason is because the two crossover slopes oppose each other and for a certain range of frequencies, both speakers would be reproducing the same sound.
Decisions!!
Passive crossovers are supplied with speakers that require them when purchased new. If speakers are purchased 2nd hand then passive crossovers meant for similar applications or adjustable active crossovers should be used for this application.
Active crossovers offer a whole lot more options because an installer can often adjust crossover frequencies. There is no perfect setting for any given speaker, instead it is generally a matter of preference and system type.
Most lower end amplifiers have switches to choose between low pass, high pass or full pass (off/all pass). The frequencies that are preset in these options normally range from about 80hz to 150hz.
Preset Frequencies
- Subs
A sub should definitely be run off a channel or channels switched to low pass. - Multi-way Speakers
6″x9″s, coaxial speakers or any speaker that has multiple cones or speakers attached, should normally be run on full pass to reach its full potential. These speakers can however be run on high pass without having any adverse effects. - Components
Midrange speakers (mids) and components (mid and tweeter combination) can be run on either full or high pass filters. These speakers will generally handle more power without distortion when run on a high pass filter. - Tweeters
Tweeters, which are wired on their own, without a mid, but with a passive crossover in line, can be run on full pass, but should be run on high pass. Tweeters that do not have passive crossovers should not be directly attached to an amp unless the amp, or other active crossover, has the option of setting a very high crossover point.
Adjustable Frequencies
- Subs
Most subs are designed to play frequencies below 120hz. Generally the smaller the sub the higher the frequency that it can safely handle. Its also important to remember that bass starts to become directional (can here which direction its coming from) at frequencies above about 70hz. When building an audio system for sound quality its essential to ensure that all sound appears to be coming from the front of the car. - Mids
Midrange speakers are as their name suggests designed to play middle frequencies from about 75hz to 3khz. Generally the larger mids are designed to play lower frequencies while the smaller ones are best at higher frequencies. They can however be played at almost any frequency so long as they are not being distorted. - Tweeters
Tweeters are designed exclusively to play high frequencies about 1.5khz and above. Again larger tweeters can reproduce lower frequencies then smaller tweeters.
