Line level notch filter

[Josephjcole]

So here's the thing: I've got a pretty good in room peak around 80Hz. I've measured it to be around 12dB with my trusty dB meter. When measuring my speakers through my computer it becomes obvious that this peak is from the room. My room is 14'x22'x7' so I guess it is no suprise that there is a problem around 80Hz.
I've been searching around recently for solutions and it seems to me the least intrusive solution is some form of EQ. I guess the most obvious solution would be to get myself some Behringer EQ and plug it in. The only problem is that I'm a complete cheap skate. So I would like to build a line level notch filter (if passive is possible all the better). The only problem with that is that I'm a bit slow when it comes to electronic circuits. I've studied enough passive cross-overs that I'm starting to understand how they work, but I don't think I can design any kind of line level notch filter. The only thing I've found through googling is this: http://www.linkwitzlab.com/filters.htm#7. Which is great since it has a number of different line level circuits. However it does not have any of the equations for adapting these to different frequencies. Does anybody have any expierence with this or any helpful advice? Thanks.
Joe

 

[carlosfm]

You don't say what speakers you have...
Have you tried positioning them better?
Bass-reflex in the back panel, speakers too close from corners?
A pic may help.

You should consider and try lots of things before going to more radical "solutions".

 

[Josephjcole]

Yeah, sorry maybe I could have given more background info. Anyways.... I'll attach a picture. There are not many options for placement. I've tried different distances from the back wall, setting up the speakers to form a triangle with the corner as one point (this sounded best, but put the sweet spot somewhere between the kitchen table and the front door). I've also pulled them pretty far out into the room, farther than I could have lived with, still to no avail. The room is an old southern style log cabin which is made by stacking logs, then filling in the gaps with concrete (14'x22'x7'). I've had horns, and other BR speakers, all exhibit more or less the same bass issues. I'm now listening to MLTLs. I've thought of trying a more mechanical solution such as stuffing the ports to give it a more aperiodic approach, but I believe this will just cause a loss in most all bass. I like my bass, just not the 80Hz peak.
Joe

 

[carlosfm]

Make some small stands (~30cm) and lift those speakers off the floor.
Try it.
You have the BR ports too low, that can be problematic.

 

[BobEllis]

Linkwitz tells how to calculate the notch components in the sketch underneath the filter you linked to - the part called synthesis - pick R1, gain, Fo and Q, then plug and chug - carefully watching units. Q=Fo/(delta F) - see the sketch. take the value for L into one of the active inductors and calculate that part

another place that might be easier to understand is the Delta audio active filter 1 manual - http://www.delta-audio.com/Active_Filter_One/Active_filter_one_ver_1.0_dokumentation.pdf

Either way you will need to determine teh Q of your resonance, by measuring its 3 db down points (like turning Linkwitz graph upside down)

I second SL's suggestion to model your filter in something like Circuit Maker (there is a free download that will handle this easily) Using his values the notch modeled perfectly. However, using my values calculated with his formulae it was usually off a bit, but CM lets you change values and look at the resulting response. Keep a table of standard values handy as you change things, so you will be able to buy the parts you specify.

Good luck

 

[Josephjcole]

Bob thanks for the advice. I have to admit that I'm a little slow when it come to circuits. Would something like this work: http://mac.softpedia.com/get/Math-Scientific/iSpice.shtml as I'm using a Mac. If so I'll give it a try. I'm afraid I don't know how to determine Q. After finding the peak and the two -3dB points, what equation would I use?
Thanks
Joe

 

[BobEllis]

that should work - all you need to do is model a simple circuit and check the response curve.

call the 3 db down points F1 and F2, the center freq is Fo

Q= Fo/(F2-F1) - the center frequency divided by the difference of the two 3dB down points.

 

[Josephjcole]

Well, I stuffed the ports. Supprisingly small amount of difference. Slightly less bass, but same muddy, boomy sound. After thinking about it a little I might try and go ahead and build a full out equaliser. I can just imagine myself taking out the 80Hz peak only to be annoyed by a peak at 180Hz or something of the like. Maybe something like 1/3 octave bands from 30Hz to 200Hz. Who knows maybe I'll learn something. These look to be the most complete plans:
http://sound.westhost.com/project75.htm
That is of course someone can recomend a better design or kit.
Bob thanks for the advice, I downloaded "Spice" for the mac, but it looks like it might take me a little while to learn how to use.
Suggestions welcome
Joe

this is getting pretty far off topic from fullrange drivers.... moderators feel free to move me if you need/want to. thanks.