Understanding Car Audio Crossovers
We know a lot of enthusiasts are familiar with audio crossovers. This article is meant to educate new enthusiasts while also serving as a refresher course for veterans of car audio who might like to re-acquaint themselves with the fundamentals. It is our hope that this will not only be educational in terms of a theoretical understanding but also practical in terms of having real-world applications for your system.
What do We hear?
Humans can hear frequencies ranging from 20 to 20,000 cycles per second, or Hertz (Hz). This huge range represents an unattainable feat for a single speaker. Since no driver can successfully cover all those frequencies with quality and quantity, specific speakers specialize in specific ranges. Most commonly, big and powerful subwoofers handle the low bass, tweeters take care of the highest notes and midranges reproduce the frequencies in between. In order for these specially designed drivers to perform their best, they must receive only the range of frequencies for which they are designed to operate. That’s where crossovers come into play.
For music reproduction, the name of the game is moving air, and low frequencies need to move a lot of it. If the intensity or loudness is to be kept constant, each time the frequency halves the cone or dome of a loudspeaker must move four times farther. So, all things equal, for a specific loudness, the bigger the cone diameter, the less excursion necessary to reproduce that loudness. The downside is that the bigger the cone, the heavier it is (and therefore the more inertia it has).
So as frequency goes up, say 5,000Hz for a 12” woofer, it simply won’t be able to start and stop fast enough to reproduce that frequency. It will try nonetheless, giving rise to several kinds of audible distortions. In order to avoid that, one should filter out the high frequencies. On the opposite end of the frequency spectrum, if a tweeter can start and stop 20,000 times per second without losing its breath, it certainly can move only 50 times. The problem is that in order for its small dome area to create any usable output, it should move several feet front and back, of course destroying itself while trying. That’s why it is so critical to filter out low frequencies from tweeters—in order to keep them alive from over-excursion. In between these drivers, midranges suffer from both problems at both extremes of the audio band.
Filters
Coming to the rescue, the crossover filters out the high frequencies before they reach the subwoofer, the low frequencies before they reach the tweeter, and so on. A crossover point between two speakers consists of two filters, a lowpass and highpass, typically crossing over at the same frequency (but not necessarily). Of the three different types of filters, highpass gets rid of lower frequencies, lowpass gets rid of higher frequencies, and bandpass gets rid of frequencies above and below a specific range. Regardless of type, a filter has two areas: the pass-band and the stop-band. The pass-band is where the filter is not filtering—it’s the working region of the driver. The stop-band is where attenuation takes place.
A filter can be described by three different properties: the crossover point, the slope and its “Q”.
Crossover Point
The crossover point or crossover frequency is determined as the frequency at which the level has already decreased 3 decibels (dB). It represents the beginning of the stop-band. Also known as the half-power point, it’s the level difference at which, on average, human beings start noticing a change in level.
Slope
The slope represents the area of the stop-band, and is commonly expressed in multiples of 6dB of attenuation per octave (i.e., doubling or halving of a frequency). In audio, we typically use four different attenuations: first-order filters, which attenuate 6dB per octave; second-order filters, 12dB per octave; third-order filters, 18dB per octave; and fourth-order filters, 24dB per octave. It is possible to create higher slopes, but fourth-order filters usually are more than enough. Filters, besides attenuating, basically produce two other negative side effects: they alter phase and transient response.
Changes in phase can be understood as a small delay of the signal. It’s not an absolute delay, like seconds, but a delay in degrees with respect to the wavelength at the crossover frequency. It is said that first-order crossovers shift phase 90 degrees; second-order, 180 degrees; third-order, 270 degrees; and fourth-order, 360-degrees.
Common wisdom would dictate that fourth-order filters remain in phase since they shift a full cycle and that second-order filters could be fixed by changing the polarity of one of the drivers. However, that is an oversimplification. Too many other things take place in the equation for physics to work this simply. Transient response, the second negative side effect of filters, can be explained as the ability of a driver to obey what the amplifier is telling it to do. The higher the filter order, the more compromised the transient response is.
“Q”
The “Q” of a filter is the same “Q” used in enclosure design and parametric equalizers. Known as the “figure of merit,” this calculated quantity describes resonance. It can be defined as center frequency divided by bandwidth, at -3dB points. Different filter “Qs” describe the shape of the “knee” of the roll-off response, near the crossover point, and have been named for the engineer who first mathematically described them, such as Butterworth (very common) with a Q of 0.707 or Linkwitz-Riley with a Q of 0.49.
Passive vs. Active
In addition, filters can be passive or active. An active circuit needs the power to work while a passive one doesn’t. Nowadays, active filters can be found in almost any amplifier or can be added to a system as external equipment with signal level connections. Active filters generally are used before amplification while passive ones are used after. The most common passive filters in a system come in component speaker systems. These are typically those little boxes containing capacitors, inductors, and resistors that are connected between the amplifier and the corresponding speaker.
Let’s go a little deeper into some specific areas and learn how to get the most out of them.
Subsonic Filters
A subsonic filter is a highpass filter working at a frequency typically between 20 and 40Hz. To be clear, we should state that “subsonic” doesn’t denote operation at a speed slower than that of sound, or 1,130’ a second (at sea level). For example, commercial airplanes usually fly at subsonic speeds, never reaching MACH 1 or the speed of sound. (In contrast, the Concorde used to be a supersonic airplane because it could fly at speeds much higher than the speed of sound.) In crossovers, a subsonic filter is not called “subsonic” because it travels slower than the speed of sound! Therefore, a more appropriate term would be “infrasonic,” which means at a frequency below the known standard of human hearing, or 20 to 20,000Hz. (The opposite would be “ultrasonic,” like the waves doctors use to check fetuses in pregnant women.)
Infrasonic
A subsonic filter gets rid of very low frequencies. Difficult to reproduce, deep frequencies can be damaging to smaller woofers. Subwoofers using ported enclosures could also benefit from this type of filter as they typically “lose control” below the tuning frequency of the enclosure. As the frequency played through the subwoofer drops below the tuning frequency, the potential of speaker failure increases dramatically.
Do you remember your aunt’s old turntable on which she would play soft music? Despite the lack of low-frequency content, the woofers would move fiercely. They almost leaped out of the enclosures, but this extra movement produced no audible sound. Cheap turntables are well-known for producing mechanical noises in the range of 3 to 30Hz while spinning. To solve this, many turntables included subsonic filters in order to eliminate those harmful low frequencies.
Advantages of Subsonic Filters
Now, we’re well beyond low-quality turntables and talking here about car audio. What can we gain from subsonic filters? Something we touched on earlier, let’s elaborate here: that is, the relationship between the tuning frequency of a bass reflex enclosure and its power handling. At the tuning frequency or resonance frequency of the port, the woofer “stops” (i.e., drastically reduces motion) while almost all of the sound comes through the port.
This allows the system to accept lots of power, basically until it reaches the thermal limit of the driver, with almost no distortion. I say no distortion because if the cone doesn’t move, it doesn’t depart from its linear range. This represents a very advantageous section of that working range. But below that same tuning frequency, the opposite scenario takes place. You could say that the port “unloads,” producing canceling waves (i.e., out of phase with respect to the woofer’s output) and behaving more like a simple hole in the box.
This causes the driver’s motion to rapidly go out of control and easily reach the maximum allowable excursion. Therefore, the selection of tuning frequency becomes critical. Too low a frequency would protect the driver and allow for a lower frequency cut-off but would not allow for extra power handling and impact in higher notes. Too high and you produce higher pressure levels and control in more convenient zones (40 to 80Hz) but with some risks if lower frequencies are present.
This is where subsonic filters come into play: to tolerate higher tuning frequencies of bass reflex enclosures, allowing a much more aggressive sound and higher power handling by eliminating the risk of woofers bottoming out of their baskets! The chosen frequency is directly related to the tuning frequency of the port. A good starting point is around 10 or 15Hz below the port’s tuning frequency (always double check it by trial and error while playing loud music).
If you read the preceding explanation, you should understand that it doesn’t officially apply to sealed boxes, but everything is possible. Just remember that any filter also affects phase, the transient response and ultimately the overall sound quality of the system. A filter can also include some amplification before its stopband, giving rise to sixth-order vented boxes, but this would be a theme for another article.
We select the best crossovers for you here
How To Set Crossover Frequency For Car Audio System
We’ll look at active and passive crossovers, and the techniques involved when using only our ears and some familiar recordings to make adjustments. We’ll also discuss using an RTA to help tweak the crossovers, followed by using our ears for final system tweaking.Be sure to read the previous article and review it.
Make absolutely sure you’re familiar with all the features of the system components you’ll be using. You should know what all of the controls on the amplifiers, head unit, passive crossovers, and any other components do before attempting to perform any serious system tuning. All too often (and believe me, I’ve done it myself), we jump right in and try to figure things as we go. Sometimes we’re successful, but often, we spend a lot more time “figuring” when it would have been much quicker and easier to read the manual first!
RTFM – Read the Factory Manual!
Once you acquire a lot of experience, it’s easier to take some shortcuts. But even if you’re totally familiar with the components and the type of system being tweaked, it’s a good idea to follow a standard procedure. The results will be much more consistent and you’ll work faster, too.
Tuning a Basic System Using Basic Techniques
Let’s look at a relatively simple system using a basic head unit, a 2-channel amp connected to passive crossovers for the midrange/tweeter speakers, and a single-channel low-frequency amp with a subwoofer. After ensuring all the connections are made correctly as recommended in the manuals, we need to make some initial crossover settings before turning the system on.
With passive crossovers, set any switches or jumper terminals to the reference or neutral position. We’ll try the different positions for these later, but for now, we need a starting reference. On the amplifiers, set the gain control all the way down and neutralize any bass contouring or equalization controls.
On a basic system like this one, the only crossover settings you’ll need to work with will be between the subwoofer and midrange frequencies. This active crossover can be inside the head unit with a dedicated subwoofer and high-frequency signal output channels. Or, it may be crossovers as part of the input stage of one or both of the amps.
The specifics depend entirely on the components you have in the system, so it’s impossible to give very precise details here. Wherever the crossover controls are, try setting the lowpass crossover for the subwoofer and the highpass crossover for the mid/tweeter amp somewhere between 70 and 90Hz. If the crossover slopes can be adjusted, set both the low- and high-pass slopes to the same setting, probably 12 or 24dB per octave. Again, these are just starting points.
Gain Structure
Now, it’s time to set the system gain structure for all the components. Setting the gain structure is a very critical step, but I’d bet over 90 percent of all aftermarket car audio systems built don’t have the system gain structure optimized. Setting crossovers and gain structure go hand-in-hand. One definitely can affect the other.
When the gain structure is set properly, the system will achieve maximum output levels with minimum noise and distortion; the system will be reliable and you will have total usability of all controls, including the ability to turn the volume control all the way up without fear of blowing something apart. It’s not that difficult to do and it doesn’t take much time, but it does require some special techniques and a couple of special tools.
Unfortunately, it will require a separate article to fully explain gain settings. For our purposes here, we’ll concentrate on using the gain controls to set the relative levels of all the channels so the overall frequency response is correct.
Critical Listening
Now you should do some critical listening. For our basic system, the first step is to set the level of the subwoofer(s) relative to the higher frequency speakers. You’ll need to have some very good quality recordings whose sound you are very familiar with when played on a high-quality audio system.
Using an MP3 version of a compressed, super bass-heavy synthesized recording wouldn’t be a good choice. Find a few lossless tracks or CDs that are cleanly recorded, dynamic, and aren’t mixed with the sub-bass much louder than the rest of the music. Then spend some time listening to them on a good home audio system so that you’ll have a reference on which to base your car audio system tweaking.
While listening to the system at a moderate level, adjust the gain levels of the subwoofer amp and the high-frequency amp until the overall balance between the sub and higher frequencies sounds about right. Not too boomy and not thin-sounding—just a good overall balance.
Adjust Crossover Frequency
Next, move the crossover frequency up and down to see if changing the crossover point makes a difference in how the system sounds. As discussed in the last article, the actual crossover point can be set anywhere in the useable overlap frequency range between the subwoofer and midrange speakers.
With the midrange speakers, the manufacturer will have stated the lowest frequency that the speaker is rated to be able to reproduce. However, the actual useable midrange low-frequency limit in your system will probably be different than what is in the manual. A good technique for determining the lowest useable frequency limit for the midrange speakers is to turn the subwoofer(s) off, then listen to the low-frequency response of the mids while varying the crossover point set.
Listen to the bass guitar, kick drum, and other instruments as the crossover point is varied from higher to lower frequency. You’ll hear significant changes when turning the crossover point down from say around 125Hz to 100Hz. But at some point, you’ll notice that the low-frequency response sound doesn’t change when the crossover point is changed. That is the lowest point at which the mids can be relied on to reproduce the music. But in any case, this lowest midrange crossover point should never be set lower than the lowest manufacturer-recommended crossover frequency for the midrange speakers.
Adjust Crossover Slopes
Next, make adjustments to the crossover slopes, if your crossovers offer this feature. Try steeper slopes, shallower slopes and, if possible, you can even mix it up using different slopes on the lowpass and highpass sections. For example, 24dB/octave on the subwoofer and 12dB/octave on the midrange. While this may be a questionable technique in home audio circles, it can be very effective when working in the acoustically hostile environment we have to deal with in a car or truck. Also, keep in mind that steeper slopes usually work better than shallow slopes, more often than not.
Also, try splitting the high- and lowpass points. For example, set the subwoofer lowpass crossover at 60Hz and the highpass point at 80Hz. This is a great technique to use in many systems, because most vehicle interiors will have a natural acoustical response peak somewhere in the 50 to 100Hz range, and splitting the crossover points can help improve the response. At this point, listen for changes in peaks or dips int he frequency response in the crossover range.
If, say, a bass guitar goes through a scale from lower to a higher frequency and you hear some notes much louder than others, adjust the crossover points so that the transition becomes smoother. This is essentially a way of performing equalization by way of the crossovers which can be very effective for tailoring the frequency response.
Polarity
You can also try reversing the polarity of the subwoofer(s) and see if the frequency response in the crossover range improves or degrades. Quite often, because of the acoustical nature of where we must mount speakers, and the physical characteristics inside of the vehicle, the sound quality of the system can be significantly changed by reversing the polarity of different speakers.
Remember that even though the crossover might be set at 80Hz, the subwoofer will still be reproducing frequencies above 80Hz and the midrange speakers will be reproducing frequencies below 80Hz. Hence, the sound from each will interact and may combine constructively or destructively at the listening position. If you hear a significant dip or peak int he crossover range, try reversing the subwoofer polarity and see whether it sounds better. Keep the setting sounds best.
Lastly, remember to consider setting the crossover point higher if it doesn’t negatively impact how the system sounds. This will help keep the lower frequencies out fo the midrange speakers and let the subs do more of the high-power work in the lower frequency range, which can improve overall system power handling and reduce distortion at higher output levels.
More Advanced System Tuning
Everything we’ve discussed so far also applies to set higher frequency crossovers and more advanced systems. Even if you are using all active crossovers, multiple subwoofers, dedicated midbass speakers, etc., the aforementioned techniques still apply. Let’s walk through a few additional examples and details to better understand what we’re talking about.
When setting crossovers between midranges and tweeters, always remember to set the highpass crossover on the tweeter no lower than the manufacturer’s recommended crossover frequency. Set it any lower and turn the volume up too high and you may see “magic smoke” from your tweeter.
When working with higher frequency speakers, I recommend tuning only one channel at a time because we want to be able to hear how the sound from the midrange and tweeter from each side combine. Turn on only the left or right channel and listen to those speakers. A track with an only spoken voice can be very revealing here because we’re all familiar with how people sound when they talk and it’s very easy to hear problems in this mid to upper-frequency range with spoken voice recordings.
If you’re using passive crossovers, try the different switch or jumper settings available. Listen for a coherent voice, smooth frequency response, etc., in the crossover range. Try all of the available setting combinations. Also, try reversing the tweeter polarity. This might not make a significant difference, but occasionally it will improve the sound, depending on where the tweeters are mounted relative to the midrange.
With active crossovers on the midrange/tweeter crossover, you have more control and can try varying the crossover point, splitting the high and lowpass crossover points, vary the slopes, etc. Again, tweak only one channel at a time.
When you’re happy with the sound for one channel, match the settings and polarities on the other channel and listen to the entire system. You may want to experiment further at this point, but make sure to document what you get at every step so you can get back to this point if future changes don’t work out.
Using an RTA for Crossover Setting
An RTA can really help with the initial crossover setting if you have access to one. A 1/3-octave RTA will work fine for crossover setting, but a higher-resolution version, like a 1/12- or 1/24-octave RTA will help for critical equalization.
When using an RTA for crossover setting, start with the base settings we discussed earlier, use a pink noise disc, set the RTA response to a slow setting for a long average of the readings and remember to sweep the microphone slowly around where the listener’s head will be as we discussed in the last article.
Now, focus on the crossover frequency range on the display while you make changes: crossover frequency, gain levels, slopes, splitting the high and lowpass points, reverse polarity on speakers, etc. Look for the smoothest frequency response possible, but be careful not to overdo it while trying to get the RTA curve perfectly flat. Remember, in the end it matters how it sounds to your ears not how the dots on a visual display line up. And when tweaking the midrange-to-tweeter crossover with an RTA, again work with only one channel at a time.
Don’t forget to try reversing polarity on the speakers on either one side of the crossover or the other, especially if there is an apparent deep hole in the frequency range near the crossover point that does not go away by changing the other crossover parameters. The hole may be caused by the destructive combination of the higher and lower frequencies from the different speakers at the listening location. Reversing polarity may improve the response or it may not. But give it a shot.
As always, your ears are the final judge! Document your settings and then go back through everything while listening to the system, make some changes, and see if you can improve it further. Look at the RTA as a time-saving tool to get you deep into the game quicker than using just your ears. But also remember if the RTA curve looks good but it sounds bad, trust your ears. You won’t be driving around looking at an RTA – you’ll be listening to the music.
Wrapping It Up
Document your settings as you go. Don’t be afraid to set everything back to zero if you get to a point that just isn’t working. Take breaks, or even put it away until tomorrow if it’s just not coming together. If you’re not pressed for time, take your time. It will make a difference.
Of course, you’ll make further changes to match your personal taste over the course of several months, but these techniques will help you get to a point where it’s close and you’ll know what you’ve got. Then, it’s a whole lot easier to make it sound like what you want it to.
Read our article 5 Best Car Audio Crossovers
