Basics of crossover points

Crossovers & frequencies:

Just something quick I wrote about the basics of crossover points & slopes:

Let's start simple with crossover filters, high pass (HPF), low pass (LPF) and bandpass. Basically they are filters that divide up the frequencies and allow/pass the proper frequency to go to the correct speaker (driver), that they are meant to play.
High Pass means only frequencies above the crossover point, or sometimes referred to as the cutoff frequency, are played. For example, a 3000 hz high pass will only play frequencies above 3000 hz. Low Pass means only frequencies below the crossover point are played. For example, a 80 hz low pass means only frequencies below 80 hz will be played. BandPass plays frequencies between two points by utilizing both a high pass and low pass in the same filter network. So, for example you could have a midrange driver only playing 80 hz to 3000 hz.

Next are slopes.
Slopes refer to the numbers, 6 decibel (db),12db, 16db and 24db, are the most common. What they actually mean is "X" decibel drop off after the cutoff frequency PER OCTAVE. An octave is double or half a given frequency. So for example let's take a 3000 hertz high pass filter. (Which is common for tweeters). The frequencies do not just stop playing below the 3000hz point, they roll of at a certain db ( attenuated ).
So a 12 db slope, it will mean a 12 decibel dropoff of output (sound) at 1500 hz (one octave lower) (half) and 24 dbs by 750 hz, etc. The higher your slope number determines how sharp your cutoff is. 48 db is like a cliff and 12db is a gradual slope. Now understand that a 3 db gain is double the output, and 3 db loss is half the output.

Now an example for a sub.
Let's say the low pass filter is set at 80 hertz for a sub. Which means only frequencies below 80hz get played. But the frequencies ABOVE 80hz don't just stop being played, they will "rolloff" at a certain db slope. So say u put a 6db slope on the sub, (crossed over at 80hz) This means all frequencies above 80hz will gradually roll off. So one octave above 80hz is 160hz (double) will be 6 decibels DROP of output in sound, or the power has been reduced by 6db or 75% and each octave higher, reduces the power by an ADDITIONAL 6db. So basically 6 decibels less as loud. A 3db drop of output is usually heard as only 2 or 3 clicks less on the volume dial on the headunit. And 10db drop of output is around half as loud.


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This is the very basics on how crossover points act and play a role in tuning. I won't go into phase and time alignment etc. I see Erin had a good tuning thread going here. Also on DIYMA too:) hopefully this helps someone.

Cheers