[BrianP]

I've got the rest of the room acoustics under good control -- RTA shows pretty flat response from the listening position, EXCEPT for an 8dB peak at 80Hz. No surprise -- the room is 14 ft. wide, which is almost exactly one wavelength at 80Hz. This peak is there with all speakers, no matter where I put them in the room.

So, should I be thinking corner bass traps or parametric EQ to knock it down? How would I go about figuring out the least expensive DIY bass traps that would work at this frequency? Alternately, what is the least expensive EQ that wouldn't screw up the rest of the range with added noise or distortion?

[richidoo]

Notching it (EQ) would be easiest. An active analog line level notch filter could be placed before the amp. This article will give you some ideas, and there are other filter circuits on the net. The downside is the opamp buffer can damage the sound on the rest of the signal, especially the sense of space and life. It's OK in the low frequencies if you are only notching the bass drivers, but keep diy opamp circuits away from tweeter and midrange drivers. The notch is a complex filter so the insertion loss is too high to do it without a buffer amp.
More info: http://focus.ti.com/lit/an/sloa093/sloa093.pdf

If you have digital source like Squeezebox or computer media player like foobar, EQ plugins can do the notch with minimal degradation. Commercial EQ components could be inserted, like Rives Audio PARC, or a pro audio EQ like Manley. These maintain excellent sound quality but are expensive.

Unless you build tuned bass traps, you will be using broadband traps which should be as thick as possible. But a tuned trap is the ticket if you know exactly which freq you want to kill. I think that's what I would try if I couldn't do digital EQ. I would build my own based on Ethan Winer's plans. If you want to keep the electronic signal "pure," then acoustic is the wtg.

[Jeffrey Hedback]

BrianP,

I would actually suggest acoustically attacking 80Hz first and then apply subtractive parametric EQ.  80Hz is almost ALWAYS a "winnable war" (encouragement!).  It would be helpful to give a full dimensional description of the room (L, W, H and unique factors...opening, etc.)  A plan view diagram is best.  It is then posisble to input your factors into a quality room mode calcultor and see what modes are going on in the 80Hz range and what plane they are occuring in.  Then appropriate "traps" can be placed in those locations.  The vertical corners will always help, but you may have a mode best treated in different junction or location.

I'll check in on this thread occasionally and gladly lend a hand as possible.

[bpape]

Are you using a single stereo pair or mains plus a sub?  If a sub, and xover is at 80, you can try reversing phase on the sub to introduce a null at that frequency without adding anything to the room or electronics.

I'll agree with Jeff that 80 Hz is doable.  The trick is to see where it's coming from.  Could be the width, could be a combination of things.  I'd play just an 80 Hz tone and do some head poking to see where it's building up to determine if corners will be of value or not in terms of frequency response.

Bryan

[Ethan Winer]

80Hz is almost ALWAYS a "winnable war" (encouragement!).

Yes, when straddling corners thick bass traps have peak absorption around 100 Hz, and good traps can easily improve 80 Hz. So I'd try that before EQ for several reasons:

* Bass traps improve peaks and nulls at other frequencies too.

* Bass traps will reduce ringing as well as flatten the response.

--Ethan

[BrianP]

Are you using a single stereo pair or mains plus a sub?  If a sub, and xover is at 80, you can try reversing phase on the sub to introduce a null at that frequency without adding anything to the room or electronics.

Done that with satellite/sub configurations, and it works well. But right now I'm running some 12" 3-ways that go flat to 25Hz (thanks in part to another big room mode at 25Hz), and don't need the sub. So I'll look into building some DIY bass traps tuned to 80Hz and see how that works. Time to do some research.

[srlaudio]

I haven't seen any 12" 3 ways that are flat to 25 hz.  Maybe a stand alone 12" Bag End Infrasub would do that.  I can send you a 30 hz tone (sine wave) at zero level, if your amplifier and speakers can reproduce that accurately, you are fortunate!  Then you have to worry about all the stuff in the room "singing along" even though they don't know the tune lol......

[oneinthepipe]

I haven't seen any 12" 3 ways that are flat to 25 hz.

Salk SoundScape 12 is -3 dB at 18 hz, -1.5dB at 19 hz, -0dB at 22 hz, if I recall correctly.

[BrianP]

I haven't seen any 12" 3 ways that are flat to 25 hz.  Maybe a stand alone 12" Bag End Infrasub would do that.  I can send you a 30 hz tone (sine wave) at zero level, if your amplifier and speakers can reproduce that accurately, you are fortunate!  Then you have to worry about all the stuff in the room "singing along" even though they don't know the tune lol......

The speakers in question have an f3 of 34 Hz, measured anechoic (ie. in the back yard). BUT there is a powerful mode at 24Hz in my living room, which wreaks havoc when stimulated by my flat-to-18Hz Velodyne sub (rattles doors, windows, dishes in the cupboards). With these speakers, it just extends them down nicely.

Which raises another question -- using bass traps, can I get rid of the 80Hz hump without completely killing the one at 24Hz, which is actually benign with these particular speakers?