High Pass Filter / Low Pass Filter

[bnw]

OK, I'll be the one to admit it because I know there must be dozens of us that have NO idea...
What exactly do these things do and how are the implemented? Also what's with all the different types in WinISD?

[Geoff St. Germain]

Well, a high pass filter allows high frequencies through and filters out low frequencies. Low pass does the same with low frequencies. These filters can be of different "order" depending on how quickly the roll-off of the filtered frequencies is, higher order meaning a faster roll-off (steeper slope).

You can read some more here:

http://en.wikipedia.org/wiki/Low-pas...w-pass_filters

I couldn't find a nice, comprehensive reference so perhaps someone else will know where one is. I learned about them mostly in school, but as far as applications to acoustics I learned about then in Loudspeaker Design Cookbook by Vance Dickason.

[bnw]

Hey thanks Geoff. Thats a pretty good start. So now my question is; How and why would we apply them? Are they just to make sure a particular driver works in only a selected range of frequencies? Is this like passive equalization?

[Geoff St. Germain]

Well, the reason to apply them (in reference to a subwoofer) is like you said, to have the driver work in only a certain band of frequencies. In the case of a subwoofer there would be a low-pass so that only frequencies below what the mains are playing are sent to the subwoofer and there could be a high-pass to prevent very low frequencies that have the potential to damage a subwoofer. For instance, for vented subwoofers frequencies below the tuning frequency result in very large excursion demands which could cause the driver to attempt to move further than it can mechanically resulting in what is called bottoming which can result in mechanical damage to the driver.

[hddummy]

It should also be said that a low pass filter, when used in home theater applications, is usually implemented on the surround sound receiver. Most plate amps and dedicated subwoofer amps (ie buttkicker) have built in low pass filters. You should never have these activated or you should turn them to the highest settings so that the receiver can do it's job. If you were to use double filters, the roll off slope would be twice as steep as intended and you would have a dip in frequency responce between the mains and the subwoofer.

[bnw]

OK, so when i set my sub cutt-off to 80Hz or 120Hz, that in effect is my low-pass filter?

[hddummy]

correct....let me preface my previous statement with the assumption that your sub is connected to your receiver with a subwoofer output. If you have your sub hooked up using right and left mains output from the receiver then you WILL need to use the built in low pass filtering on the sub amp or get a seperate filter. Which method of connection you use is determined by wether or not you receiver has a dedicated sub out and how you have the bass management settings on the receiver.

[bnw]

Yes, I'm coming off my sub outputs on my receiver. Thanks for your input Matt.
And HEY, you stole what should have been MY screen name!

[hddummy]

My pleasure. Home theater is sort of a passion of mine. I would be happy to give advice on any related topic if I'm able.

You have just one other item to double check. Make sure your receiver is set to send LFE/bass material to just sub. You could also set it to go to the sub and mains if you have really good full range mains and depending on the balance of sound you want between the two.

[bnw]

Yeah, when I finish my LLT's I will. Right now I have the B&W 605's with built in powered subs for mains and I do have them set for sub/LFE pre-out. Once I get the SonoSubs finished the main's subs will be relegated to woofer duty and I'll send the deep stuff to the "real" subs.

[hddummy]

Sounds like a pretty sweet setup!!

[bnw]

Well like yourself, its a passion for me. I've waited a long time and built & bought slowly (financial constraints) but I do have what for me is now a VERY satifying set-up. Thank god for audiogon!

[Warmon]

Geoff St. Germain wrote:

..For instance, for vented subwoofers frequencies below the tuning frequency result in very large excursion demands which could cause the driver to attempt to move further than it can mechanically resulting in what is called bottoming which can result in mechanical damage to the driver.

Geoff, are the frequencies below what the driver can produce still a factor? Take the graph below. It's from a driver that has a freq response of 18-400hz and a box tuned to 25. Is there any concern in the area < 18hz?

Warmon -

[Geoff St. Germain]

That could be a problem. It is going to depend on the Xmech of the driver as well as if you're going to be playing material through the driver in the sub 18 Hz range.

[bnw]

While i'm no expert, its been demonstrated and explained to me the starting with the losses at the output source (DVD player), you'll never get there. System power, cables, gain settings, all work to protect you.

[Geoff St. Germain]

Well, that entirely depends on many factors, including the power you're delivering to the driver, the tuning frequency and the driver's excursion capabilities. IME, I've only heard of natural protection from overexcursion as a result of very low tuning, like in an LLT where the enclosure is tuned in the mid teens. Tuning a driver to 25 Hz would not qualify.

[bnw]

Yeah, you're exactly right. I missed that (25Hz). I was remembering my LLT's that'll be tuned to 11Hz... if I ever get the time to finish them!