Please 'Boom' Responsibly As most of you have noticed, the noise ordinances have become much tougher lately. Most of this is due to idiots, yes IDIOTS, who drive through residential areas with their windows down while their system is playing at full power. To make things worse, the music they listen to has all sorts of foul language that's not suitable for small children, (who may be playing outside). There are even a few people, who are even beyond idiot status, that play their systems at full power through residential areas after 10:00 PM (when many people go to bed). I don't believe that this type of behavior is good for the industry. If the fines get too stiff, people will stop buying large systems. If this happens, more people will get out of car audio (who wants a mediocre system). People get interested in things because they're exciting. A deck and four 6.5" speakers are not going to interest many of the younger car audio enthusiasts. If car audio enthusiasts keep annoying more and more people, the fines will keep getting tougher. All of this will only reduce interest in the equipment that fuels the industry. If you want to listen to your system at full volume, get out on the highway where there's little chance of bothering anyone. When you get to a red light, turn it down. If the only thing attractive about you is your 'system', you have some work to do. Bottom line... Think about what you're doing. Think about other people. It's not the end of the world if you have to turn the volume down for a little while.

Passive Crossovers:
A passive crossover has no active filters as were used in the electronic crossover. It uses coils (inductors) and capacitors to cause a rolloff of the audio level. The simplest types use a single coil OR capacitor connected in series with the speaker. If a capacitor is used, it will reduce the lower frequencies getting through to the speaker. If an inductor is used, it will reduce the higher frequencies getting through to the speaker. In this configuration, the rate of rolloff is 6dB/octave. In a more complex configuration combining both coils and capacitors on each speaker, a steeper rolloff is obtainable.
The diagram below shows how capacitors would be used/connected in a high pass passive crossover system.

passxohi.gif

The diagram below shows how coils would be used in a low pass passive crossover system.

passxolo.gif

The diagram below shows how the coils and caps would be used in a tri mode system.

passxovr.gif

Put your mouse over the crossover component values on each side of the graph to see how the slope slides as you change the crossover point. You should also notice how the coils roll off the high frequencies and the capacitors roll off the low frequencies. The straight line is the audio signal level with no crossover components in series with the speakers. The crossover points are roughly based on a 4 ohm speaker. The first time through, you should hold your mouse over each value until the image loads completely.
This demo shows how the crossover point shifts when a 47 microfarad capacitor is used with different speaker loads.
Speaker Impedance
1 ohms
2 ohms
4 ohms
8 ohms
 Flat response > no crossover

Calculations and review

This diagram shows the rolloff characteristics of 3 different passive crossovers each of which consist of a single coil (of different values) creating the rolloff. The vertical lines are one octave apart and horizontal lines are 3dB apart. The red line shows the frequency response of a filter with a 200hz crossover point. The green line has a crossover point of 600 hz. The purple line has a crossover point of 1600hz. The rate of rolloff is only 6dB/octave.

passrllo.gif

This section shows the math involved in selecting the proper coil for your particular speaker system. This is the formula needed. lopsfmla.gif

Impedance = The rated impedance of the speaker
Pi=3.14....
Frequency = The desired 3dB down (crossover) point
Inductor = Value of proper coil - value given in henries (multiply by 1000 to convert to millihenries)

For this example, let's choose 4 ohms as the speaker's rated impedance. We will use 200hz as the desired crossover point.
Value of the coil=4/(2*3.14*200)
Value of the coil=3.18 millihenries

The output level of a 4 ohm speaker with a 3.18 millihenry coil in series with it, would follow (approximately) the red line. If it were a 4 ohm resistor, instead of a speaker, the frequency response would follow the line exactly. A speaker has inductance in it's voice coil which would make finding the exact response difficult but this equation will get you close to the desired response.

This diagram shows the rolloff characteristics of 3 different passive crossovers each of which consist of a single capacitor (of different values) creating the rolloff. The red line shows the frequency response of a filter with a 3200hz crossover point. The green line has a crossover point of 1200 hz. The purple line has a crossover point of 400hz. The rate of rolloff is only 6dB/octave.

passrlhi.gif

This section shows the math involved in selecting the proper capacitor for your particular speaker system. This is the formula needed.

hipsfmla.gif

Impedance = The rated impedance of the speaker
Pi=3.14....
Frequency = The desired 3dB down (crossover) point
Capacitor = Value of proper capacitor - value given in farads (multiply by 1,000,000 to convert to microfarads)

For this example, let's choose 4 ohms as the speaker's rated impedance. We will use 1200hz as the desired crossover point.
Value of the capacitor=1/(2*3.14*1200*4)
Value of the capacitor=33 microfarads

The output level of a 4 ohm speaker with a 33 microfarad capacitor in series with it, would follow (approximately) the red line. Again, if it were a 4 ohm resistor, instead of a speaker, the frequency response would follow the line exactly. A speaker has inductance in its voice coil which would make finding the exact response difficult but this equation will get you close to the desired response.
Check your math here.
High Frequency Impedance? Ohms
Low Frequency Impedance? Ohms
Crossover Frequency? Hertz
Capacitor value = Microfarads
Inductor value = Millihenries
Note: If you cannot find the correct value for a crossover, you can combine different values to make up the proper value.
 
This is how the series/parallel configurations effect the values of the components.
  • Coils in series: Add the values together. It works like speakers and resistors in series. Two 4mh coils in series is 8 mh.
  • Coils in parallel: The total inductance is less than the smallest coil's value. If the values are equal, simply divide the value of a single coil by the number of coils in parallel. Two 4mh coils in parallel is 2mh.

This program calculate the value of 2 parallel connected inductors.
Data Input:
Value of inductor 1? millihenries
Value of inductor 2? millihenries
Data Output:
Value of inductors in series: millihenries
Value of inductors in parallel: millihenries

  • Capacitors in series: The total value is less than the value of the smallest capacitor. If the values of all of the caps are equal, simply divide the value of a single capacitor by the total number of caps in series. Two 10uf (uf=microfafads) capacitors in series is 5uf total.
  • Capacitors in parallel: The total value is the value of the individual capacitors added together. Two 10uf capacitors in parallel is 20uf total.

This program calculate the value of 2 series connected capacitors.
Data Input:
Value of capacitor 1? microfarads
Value of capacitor 2? microfarads
Data Output:
Value of capacitors in series: microfarads
Value of capacitors in parallel: microfarads

This diagram shows 2 different ways to cross subs at 100hz. All of the subs are 4 ohms. 100hz crossovers
Capacitor Voltage Ratings
All capacitors have a maximum voltage rating. Capacitors used for crossovers are no different. Capacitors rated for use at 100 volts are OK to use with virtually all car audio amplifiers. For amplifiers rated to produce more than 2500 watts into 4 ohms, you will need higher voltage capacitors. 50 volt caps are good for amplifier rated up to 600 watts into 4 ohms. There is a calculator on the 'damping factor' page of this site which will calculate the voltage needed to produce a given power into a given load. Use capacitors rated higher than the voltage given by the calculator.
Polarized vs Non polarized Capacitors
Most capacitors are non polarized (bi-polar) but there are a few that are polarized and have a positive and a negative terminal. Polarized capacitors should never be used for audio transmission (or coupling). The most common and least expensive capacitors used in crossovers are 'electrolytic' capacitors. Electrolytics can be found in both polarized and non polarized versions. When you buy electrolytic capacitors for use in passive crossovers, make sure that they are non polarized. Other capacitors used in passive crossovers such as Mylar, polypropylene and polyester are non polarized.
For more information on higher order crossovers, go here.
The following demo shows how the crossover's output signal level changes with a change in input frequency or a change in the crossover point. The output sine waves indicate power output. A sine wave half as tall as the original indicates that the speaker would receive only half power at that frequency and crossover point. Click HERE to make it fill this window.

You should remember:
1.A passive crossover requires no external power source to operate.
2.A passive crossover uses caps, coils and resistors to reduce the signal level above and/or below a certain frequency.

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