What is a Low Pass Filter

I can not fin a definition that a newbie like me can understand searching Audiogon. My Subwoofer, Sonus Faber Cremona, has 2 dials in the back. One for Low Pass Filter, switchable from 38-65Hz 6 positions and one for Gain that reads min to max.

More Info:

Thanks in advance for any help,
Just as the name implies, the filter will pass all of the frequencies below the cutoff frequency, but will attenuate those frequencies above the cutoff frequency. Generally, the slope of the filter is also specified. If a filter has a slope of 6db per octave, also referred to as a first order slope, and the cutoff frequency is 100HZ then one octave higher, 200HZ the signal has been attenuated by 6db, one octave higher than that, 400Hz, the sound will be down by 12db, and so on.
Viridian, I do not understand your response. Let me ask you this: Why would I want to "attenuate" any frequencies? I am assuming that by "attenuate" you mean that it is modifying the signal above a certain frequency?

A filter lessens (attenuates) frequencies above or below a selected cut off frequency. Loudspeakers use filters to selectively limit the frequencies sent to individual drivers. A subwoofer input could employ a low pass filter to allow only bass frequencies to be sent to the woofer cones. As the name implies, a low pass filter lets low frequencies pass through unchanged.
Otherwise, your subwoofer would also be receiving midrange signals full force, which would make it much easier to hear its location. Your ears would know that it is not blending as well as could be with your main speakers, as it should 'disappear' when set up well. And as far as a sub trying to do high frequences like cymbals, well, that's really funny to hear.

Hope this adds to your understanding.
I am told that different filters will alter the quality of the sound and that, even though a filter is screening out inappropriate frequencies, there can be a perceptable altering of the final product that you might or not not like.

This is just one adjustment in your overall system.
Is the sub on the link your subwoofer?
i hope that you didn't pay the hefty 3500$ for a subwoofer
and you don't even know what is the purpose of the low pass filter .. :p
Another way to look at it, is that some drivers are designed to handle high frequencies (tweeters), some are designed for miidrange (smaller to medium woofers) and some are designed for low frequencies (larger woofers, subwoofers). Filters or crossovers ensure that only the appropriate signals are being sent to each driver. You don't want mid and high frequencies heading to your subwoofer...hence the need for a low-pass filter. Similarly, you don't want low frequencies heading to a tweeter.


Low pass filter means only low frequency signals are sent through and high frequency signals are filtered. Opposite is true for a high pass filter. Your low pass filter is adjustable to allow a range of frequencies 38-65 hertz to be set, anything above is filtered out. Using the lowest 38 hertz setting will only allow that frequency and lower to be seen by the subwoofer, generally 20 hertz the lowest possible to humans. By increasing it more low signal will hit the subwoofer and most likely produce not necesarily more bass but more bass signal attenuation. The gain will allow you to separately adjust the level of the sub over your overall preamp level. To start leave the gain in "0" or neutral and try adjusting the filter to your listening tatses. My guess/taste is a lower setting for HT and higher for music.
To answer your second question, attenuate does not simply mean to modify; it specifically means to reduce. The value of reducing frequencies above a given selected frequency is to keep the midrange frequencies out of your woofer. There are several obvious reasons for this. A large cone has a high mass and may not be able to respond quickly enough to produce middle frequencies without many distortions, including those of time and amplitude. The dispersion of a large driver is not sufficient in the middle frequencies, though horn enthusiasts may argue this. Having a small midrange driver and a large woofer covering the same frequencies will surely change the frequency response of the system and having two sources for the same frequency that are not physically close will also create destructive and additive interference between the two wavefronts, just like pitching pebbles into the pond.
Thank you all for your responses. I am still a bit confused. Specifically One question: Does the Low Pass Filter completely filter out anything above the Low Pass Filter’s freq setting much like the PrePro's Sub cross over? Or does it reduce it somehow?
If it completely filters out anything above the Low Pass Filter’s freq setting than I have I been missing out on 15 freqs all this time? Since I was told that most people have the PrePro’s crossover at 80hz, that's what I did. And if the PrePro is sending all information 80hz and below to the sub and the sub filters out anything above the Low Pass Filter settings, than I have been missing all those freqs?
Also, Why would my sub highest Low Pass Filter be 65hz? Don’t they know that some people like sending the sub up to 80hz?
It seems that there should be a rule that the PrePro’s crossover can not exceeds the Low Pass Filter’s freq setting.