What is LPF and HPF exactly?

[Puppet Master 10+ year member]

Hi all. I am going to reset my amp so that my gain and such are right, but I am still kind of a car audio noob. So my question is, what is the LPF and HPF on my amp, and what should they be set to? I know what they should be set to is probably a hard question to answer without some specs, but if someone could educate me on what they are used for and such, I am sure I could figure it out. Thanks a lot.

Click to read more...

 

[dB-r 10+ year member]

LPF = low pass filter

HPF = high pass filter

It means exactly what it sounds like. The LPF let's the low frequencies pass through, the HPF lets only the high frequencies pass through.

LPF = crossover for subs

HPF = crossover for mids/highs

Set them to the desired crossover frequency for your application. Like say if you got some 4" highs, and they are rated for 500hz and up response then you start at 500hz for a crossover point on the HPF. If you got subs that are good from 100hz down then you start at 100hz on the LPF.

Tailor the settings to how you want your system to sound.

 

[dB-r 10+ year member]

Also, the crossovers, LPF and HPF will have a slope, usually either 12db / octave or 18db / octave. This is called rolloff also.

It looks like this.

LPF at 80hz = 0db 0-80hz*

-3db * 100hz

-6db * 120hz

-9db * 140hz

-12db * 160hz

-18db * 180hz and up...

Im sure someone who cares more about filter slopes will post and correct me, but that will give you a general idea of how it works....... Goes backwards for the HPF

 

[req 5,000+ posts]

no. not for "subs"

a low pass filter "lets low frequencies pass" and cuts high frequencies out.

a high pass filter "lets high frequencies pass" and cuts low frequencies out.

they do not STOP frequencies from playing, they attenuate (turn down exponentially) at a given decible to octave ratio.

so if you have a 1st order highpass filter (aka 4db\oct slope), set at 100hz, an OCTAVE below 100hz (an octave is 1\2 or 2x a given frequency) it would be 4db LESS than whatever is playing above 100hz. and an octave below that, it would be 8db less than wahtever is 100hz and above.

the higher 'order' or 'slope' the more decibles will be cut an octave up or down, and tighter crossover points can be set, and the higher or lower a given speaker can play safely.

 

[dB-r 10+ year member]

Like I said, someone who cares more about crossover slopes will come in here and correct me.... blah blah

 

[dB-r 10+ year member]

I think you may have succeeded in confusing the living crap out of him.....

 

[req 5,000+ posts]

good stuff as always squeak. and thank you mr bill, that all seems on the mark.
but people (even myself sometimes //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif ) wont understand what a 'crossover slope' is, unless they SEE a graph of both points, with the rolloffs. this is how they will understand, so this is my contribution.

Crossover Points Cont'd - now you all know that a crossover point is meant to limit each speakers 'bandwidth' so that each speaker can only play that certain range of frequencies, and retain a higher volume and better clarity. this is the reason we use components with passive corssovers, or an active crossover that we can select the exact points.

you should also understand that a crossover WILL NOT act as a 'brick wall' and stop the speaker from playing that frequency. a crossover is meant to limit the extenstion of the speaker so that it will not play frequencies it was not designed to play at loud volumes. this means that there will be an 'overlap' of frequencies.

for example (and this is ONLY for example, i highly doubt you will see this kind of crossover and respose curves... ever) this is a subwoofer (a GREAT subwoofer mind you //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif ) and a midbass speaker, lets say an 8" speaker.

as you can see, the point where they 'dip' together is the crossover point (please correct me if im wrong). this point (by looking at the graph) is 80hz. this is normally where people set thier subwoofers and midrange - its practically a 'rule of thumb' to try about 80hz on your frist try, and go up or down from there depending on if YOU LIKE IT OR NOT. because noone can tell you what you like and what you dont like. this is 100% subjective.

now, as the speakers 'roll off', or start to play the frequencies below\above their crossover points, that frequency will get lower. here is the tricky part. since the other speaker is picking up that frequency as its companion drops it, the music wont sound like there is a giant hole (or it shouldnt if the points are set well).

Refrence Level - this is somthing that people usually dont comprehend eaither. a 'refrence level' stereo system- is a system that can FULLY REPRODUCE, at any volume level that it is capable of, 20hz->20khz with no distortion. now, it may sound easy by reading all this stuff we are teaching you, but its not qute that simple. the '0db' setting most of you see on your home theatre recievers, or stereos, refers to the *CURRENT* volume, or the standard volume, or whatever volume it will produce with how many watts the amplifiers are producing, if you turn the knob down from 0db, you will be ATTENUATING the signal through the amplifier - and thus, your system will more than not, be below 0db. for example, we have a single 10" speaker playing a 50hz tone (in and out 50 times per second), about the middle of the 'sub bass' frequencies. we are feeding it 500 watts of power in whatever box, and whatever car. when we put a microphone in the car, the car reads 100db. that is the pressure level of the air as the speaker moves in and out, its a way to measure volume (as most of you SHOULD know). now, REFRENCE LEVEL, means that EVERY FREQUENCY (or **** near it) should be able to play 100db. so our tweeters playing a tone of 5631.245hz should be able to play 100db, and likewise, the midrange should be able to play 235.654hz at 100db. this is the '0db' mark. say the system is at a volume of 20, and its hitting 96db, ALL FREQUENCIES should be able to play a 96db note. this is refrence level - all your frequencies have a 'FLAT' response curve. and this is what that looks like;

flat response (good)

versus a peaky and valley response curve (not good)

now, some people LIKE peaks and valleys in certain spots. this is fine. this is why stereos are subjective. some people cant hear distortion (sure //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif ) - and thats fine too. but 0db means at whatever volume that knob is at, an entire 20hz-20khz sweep should mic evenly. it wont be exact. but you get the point.

please feel free to add anything that i might have missed. hope it helps.

from the speaker FAQ.

http://www.caraudio.com/forum/showthread.php?t=90412&highlight=crossover

 

[squeak9798 5,000+ posts]

http://www.caraudio.com/forum/showpost.php?p=1746548&postcount=2

 

[dkmesa 10+ year member]

What is the setting in between that many amps have? Should I set it there for my EU-700s to run as midbass, rather than setting the low pass filter?

 

[squeak9798 5,000+ posts]

What is the setting in between that many amps have? Should I set it there for my EU-700s to run as midbass, rather than setting the low pass filter?

Huh?

For midbass you would want a bandpass filter...which is both highpassing and lowpassing a single driver. This is a feature some, but not really many, amps have.

But many amps also have a "full" or "bypass" setting which essentially turns off the amp's internal filter. This would only be a good setting to use if you were using an external (external from the amplifier, such as in a headunit/etc) crossover.

What most people do for midbass who don't have a dedicated bandpass filter is use a combination of their headunits filter and amplifier's filter to effectively bandpass the midbass driver.

 

[dkmesa 10+ year member]

Huh?
For midbass you would want a bandpass filter...which is both highpassing and lowpassing a single driver. This is a feature some, but not really many, amps have.

But many amps also have a "full" or "bypass" setting which essentially turns off the amp's internal filter. This would only be a good setting to use if you were using an external (external from the amplifier, such as in a headunit/etc) crossover.

What most people do for midbass who don't have a dedicated bandpass filter is use a combination of their headunits filter and amplifier's filter to effectively bandpass the midbass driver.

Ok, thanks. My 4 channel just has a full setting, so I'll just tweek it with my HU and amp filters then

 

[Kermit]

Hi all. I am going to reset my amp so that my gain and such are right, but I am still kind of a car audio noob. So my question is, what is the LPF and HPF on my amp, and what should they be set to? I know what they should be set to is probably a hard question to answer without some specs, but if someone could educate me on what they are used for and such, I am sure I could figure it out. Thanks a lot.