[Russtafarian]

I just read Kal Rubinson’s review of the Bag End E-Trap at Stereophile.com.

http://www.stereophile.com/musicintheround/708mitr/index1.html

The E-Trap is a 10” powered woofer in a box with two microphones and a tuning circuit.  You find a bass peak in the room, put the E-Trap in the pressure zone, and tune it to the frequency of the peak.  The E-Trap “hears” the peak and reproduces it out of phase, resulting in a reduction of the bass peak amplitude.  Pretty cool.

After reading the review it occurred to me that I’ve got enough gear lying around to try this out for myself.  I’ve got a powered sub, a digital eq, a measurement mic and a mic preamp with a phase switch.  I know from previous room measurements that I have resonance peaks at 38 hz and 71 hz.  Here’s how I’m thinking of approaching this.   

• I put the “bass trap” sub in a corner and connect it to the eq, mic preamp, and mic.  The mic sits on top of the sub in the corner.
• I bandwidth limit the sub to 100 hz and under so I don’t have to deal with microphone feedback at higher frequencies.
• I tune the eq to give me a 10 db narrow Q boost at 38 hz.
• I turn on the sub and slowly turn up the mic preamp until I find the microphone feedback threshold, then back the preamp down about 6db.
• I play a 38hz tone through the stereo system and experiment with gain, phase, eq boost and Q until I find a range of settings that reduces the bass peak at the listening position.
• If this works I’ll try adding a 71hz filter if that frequency is still a problem.

Here’s a question.  Should I try to flatten the response of the “bass trap” sub in the room with the eq before adding any tuning filters and engaging the mic?  I’m thinking no since the response of the “bass trap” sub will be accounted for when setting the filter phase, amplitude and Q.

Anyone else tried this?  Any suggestions?

Russ

[PSP]

Hi Russ,
First, let me emphasize that I'm not a professional and I could be way off base here, but... I would give a lot of thought to (a) keeping your microphone fairly close to the correcting subwoofer to so that the most accurate correction is applied and (b) minimizing vibrational coupling between the microphone and the correcting subwoofer.  If vibrations from the correcting sub make the microphone diaphragm "shake, rattle, and roll" it will be very difficult to get an accurate correction signal that can be sent to the sub.  So, maybe you can mount the mike on a floor stand (or clamp it to a nearby wall?) rather than stand it on the sub enclosure... or maybe you could enclose your sub in 4 feet of concrete... 

Good luck, and let us know how it all turns out!
Peter

[Russtafarian]

Good point Peter.  I'll figure out a way to keep the mic close but acoustically isolated from the woofer.

[Ethan Winer]

Here’s how I’m thinking of approaching this.

Nice thought, but I believe it's more complicated that that. This type of bass trap works by sending a canceling wave back toward the oncoming source. So at the minimum you need to reverse the polarity somewhere along the way.

There's also a fundamental problem with this type of trap. I've never tested the Bag End E-Trap, but as far as I can tell it has some of the same problems as room EQ. For an absorber to be effective it must be suitably large. Room treatment is all about surface coverage, so if a room has, say, 800 square feet of surface area, you need to cover some meaningful percentage of that surface. If you stick a small loudspeaker-based bass trap in one corner, it won't do much even if its absorption is 100 percent because it's just too small.

To solve this, the active circuitry is set up to offer more than 100 percent absorption. A device like the Bag End works by playing a countering signal into the room. If 100 percent countering isn't enough, it can be set to more than 100 percent. But then the improvement becomes localized where the test microphone was placed. So a large resonating peak will be reduced at that location, but likely made worse elsewhere.

Again, I haven't had the opportunity to actually test the Bag End trap, but simple physics suggests this is what would have to happen.

--Ethan